GRT - What Am I Doing Wrong?

[Dr. Strangelove]

Evening all,

I downloaded GRT the other day on my OH’s laptop (I’ve got a MacBook!). It installed fine and I think I’m putting in all the correct parameters. However, the days it gives me seems very off in terms of pressure and velocity being very high.

Load data - .308, 47.81gr water (per @borbal’s method), 45.8gr CFE223 (0.3gr over the Hodgdon min charge), 175gr SMK, 24.5” bbl, 2.005” case length and 2.800” OAL. Real world MV of 2680fps average over two 5 shot groups.

The QL data from a mate was 5fps off my measured velocity (Magnetospeed) compared to real world data:



P-Max, with the same input parameters, gave a higher pressure and MV. However, within 100fps from my real world MV so no complaints as a reference tool!



Then GRT comes into play… Showing 95.7k PSI and 2,935fps. I even checked the CFE bulk density was correct on GRT and it seemed to be!




Where am I going wrong with GRT? I’ve heard good reports from other people who’ve used it and found it to be fairly accurate!

Cheers,



Dr. S

 

[scrumbag]

I would say the issue is probably CFE-223 isn't well calibrated for GRT is one issue. (When you select the powder look at the little coloured bar)




Also for a 308W, case capacity looks awfully low. (Your value 47, default is ~56)

I think also you have mixed up powder space and case volume...



If you look in the "expert" view, you see eff. case volume which if you set case volume to 56 gr of H2O gets you very close to powder space in PMax

That would be where I start.

Scrummy

 

[Alantoo]

47.81gr water (per @borbal’s method),

I think also you have mixed up powder space and case volume...

Yes I think so too. I measured my .308W Lapua cases as 56.17gr

I had noticed that volume measuring difference between P-MAX and GRT before...

If you hover over /highlight the categories in GRT you get an info pointer e.g.




Incidentally my GRT is running directly on my old MacBook Pro using Crossover....

 

[borbal]

As noted above, the problem was that the "case volume" of 47.81 grains H2O was entered into the wrong field in GRT. The "case volume" in GRT is the volume to the case mouth.

In P-Max, the "powder space" is the space in the case behind the loaded bullet (with no powder in the case) and in GRT, that is called the "eff. case volume". You can see that in Scrumbag's example above, the eff. case volume is 47.68 which is only a tenth of a grain away from what you measured.

The reason for the confusion is that actually, what all these simulator programs need to know, is the "powder space" as defined above. Quickload and GRT get at the powder space by calculating the volume of the bullet that is inside the case, and then taking that away from the case volume - which the volume to the case mouth. But to able to do that, you need libraries of the dimensions of all the bullets on the planet, and libraries of all the case capacities of every case made - which is a HUGE amount of effort.

However, both QuickLoad and GRT encourage you to measure the case volume for your particular case. It turns out that measuring the powder space is really just as easy, and it is better to measure that directly than infer the powder space from the case volume as is done by QuickLoad and GRT. So, that is what P-Max asks you do to - and it saves me a great deal of effort in gathering all that data on bullets and cases.

 

[Dr. Strangelove]

Thanks @scrumbag, @Alantoo and @borbal! It would appear I didn’t RTFQ and input the wrong variable for the case volume.

Will try again later and see how I get on!

 

[Dr. Strangelove]

Turns out it’s a lot happier with the correct data!



Next question, perhaps best aimed at @Laurie: has anyone used RS60 in .308? I’m after a European, temperature stable powder to get me up to the 2750fps mark with 175gr SMKs. Both GRT and QL seem to suggest it’ll work fairly well in that regard:

 

[308tikka]

has anyone used RS60 in .308?

Yes. 190-215gn projectiles in suitable chambers.

But you might find it difficult to get hold of.

You may also find N150/550 useful as a european powder.

 

[Laurie]

Yes. 190-215gn projectiles in suitable chambers.

Just so - and in very specialised small primer brass loads at that in extreme-freebore match chambers. Not what I'd regard as a suitable powder for 175s (or even 308 Win at all) in normal circumstances. Internal ballistics programs such as QuickLOAD/GRT show RS60 as suitable (and often producing highest MVs) for almost every cartridge it seems including many 223 Rem combinations. Reload Swiss itself doesn't give any loads data for the powder and describes its faster burning RS50 and 52 as 'universal 308 Winchester powders' which is a pretty fair description.

Next question, perhaps best aimed at @Laurie: has anyone used RS60 in .308? I’m after a European, temperature stable powder to get me up to the 2750fps mark with 175gr SMKs. Both GRT and QL seem to suggest it’ll work fairly well in that regard:

I've no personal experience of RS60's temperature stability, but many Americans who have describe it as quite the reverse. (Americans know it under its other name, Alliant Reloder 17.) Viht N150 is an excellent choice for this application as @308tikka says, or N550 if you're willing to trade barrel life for some extra muzzle velocity. You need a powder funnel with a few inches of drop tube and a slow angled pour to get enough of either into the 308 case, especially N150. N550, as per all Viht N500 series powders has recently been upgraded to improve temperature stability. Depending on whose gossip you believe, N150 may also have undergone such an upgrade too, but Viht hasn't said so officially as yet - if that is the case. RS50/52 are very much Nitrochemie's alternatives to this pair. I've used RS52 in the past with good results in 308 and didn't see any signs of temperature sensitivity in our conditions even after (deliberately) leaving a box of 308 match ammo loaded with it it in a closed car in direct sunlight for a couple of hours.

For the 175 in 308, the extremely long in the tooth Alliant Re15 is probably the best all-rounder, better still its twin that retailed a lot cheaper, Norma 203-B - same thing, different tin and label - from Eurenco Bofors in Sweden. Sadly, there are two problems. Re15/203-B is very temperature affected, although that won't affect most shooters in this country. Secondly RUAG Ammotec-UK having reintroduced the Norma range into the UK presumably expecting an easy windfall from REACH-induced shortages has now given up after less than three years and isn't importing any more. The new Alliant Re TS 15.5 (also from Eurenco Bofors) has been designed to put the temperature stability issue right and may be a ballistically ideal answer to your needs as it's a slightly slower burning variant of Re15 that is reported to give better temp stability than the benchmark H. VarGet. However, there are three likely issues with it: 1) it has yet to get here ('soon' we're told); 2) Alliant Re powder prices in the UK (steep); 3) I imagine it'll be another hen's teeth job if it takes off here as it has in the USA. (Americans complain that any and all Alliant rifle powders are unobtainable in the US now and have been for some time!)

I can't comment on GRT's default values, but QuickLOAD's are well out for RS52 and N550. Pressures / MVs are heavily under-predicted for both grades, so that potentially excessive charges are shown as safe, and if using default Ba values, an initial maximum test loading ceiling producing at least 5,000 psi under that desired should be chosen. ie for a 60,000 psi cartridge like 308 Win, only load up to a predicted 54,000 psi or so, 55,000 maximum and see what the chronograph reports. Note too, that both have a tendency to see pressures suddenly 'spike' with small charge increments if over-loaded, so need extreme caution as maximum charge weights are approached.

 

[I. Farticus]

This is why I use HPS-TR to load my ammo. One wrong calculation and I could be 'nil by mouth' for a month...

 

[Muir]

I have been finding love with D073.6 in 308. Had my short barreled Ruger out using Nosler 175 RDF and they were punching ragged holes at 100 from a bipod. Might be a little 'peaky' at the top end but everything is when you talk heavies being pushed to the limits. US Military's "Special Purpose" load with a 175 gtrain Sierra (over IMR 4064) is at 2650fps so I shoot for that.

Might not suit your needs but it's a Euro powder and it seems to work.

This data below is from the U.S. "Shooter's World" site. ~Muir

308 Winchester Lapua Brass
175 gr Sierra HPBT
OAL 2.810"
Starting load & Velocity 41.0 grns @ 2,560 fps
MAX load and velocity 44.8 grns @ 2,722 fps and 61,465psi

 

[Sharpie]

Reload Swiss itself doesn't give any loads data for the powder and describes its faster burning RS50 and 52 as 'universal 308 Winchester powders' which is a pretty fair description.

I would agree with that too. Been using RS50 (since before RS was officially imported, Commonside's TR140, still got 2 kilos left). Excellent in normal bullet weight 308 loads, and normal .223 (e.g. 55 gr bullets).

However, both QuickLoad and GRT encourage you to measure the case volume for your particular case. It turns out that measuring the powder space is really just as easy, and it is better to measure that directly than infer the powder space from the case volume as is done by QuickLoad and GRT. So, that is what P-Max asks you do to - and it saves me a great deal of effort in gathering all that data on bullets and cases.

I would concur with that that.

Too much bamboozlement in building up case and bullet databases that may or may not be correct, based on some samples measured a while ago. So simple to measure what you actually have.

The same goes for powder. As Laurie has identified, QL can get things wrong too. The idea is that the QL "man" measures powder properties in scientific apparatus. Presumably he has a massive bunker full of every known powder that he has tested, once. Or uses some other figures, maybe from the powder manufacturers themselves (they surely ought to know, and have the equipment to do so themselves, since they actually make the stuff).

Nevertheless, as Laurie has identified, QL can get it wrong in some circumstances, where real world tests (AKA the use of a chrono. the only practical tool available to ordinary shooters) do not accord with simulations. Very few have direct access to pressure testing equipment, however in the UK it is readily available from the Birmingham Proof House, for I think a very modest fee. In the past I have had some dialogue with them about doing some tests, but not actually taken the step.

Outwith that, we are pretty much left trying to read the runes by looking at case heads for pressure signs, maybe measuring case head expansion with a micrometer, opinionating on which makes of brass are tougher than lesser ones and so on.

People like HPS, and Edinburgh rifles, use this for their proofed ammunition. Edinburgh Rifles will even tell you their recipes for their factory ammo, show you their test results, so that you can assemble your own reloads using the "copper" bullets that they would like you to buy as components instead. With some confidence.

I think the idea with GRT is that the "community" feed back real world measurements that are then used to refine it's powder models, hopefully a process of continuous improvement. I don't know the details of that, or whether a change-log is given to alert a user that one of their simulations may now give a different result to that which they have previously been relying on.

ISTM that many US powders can be sourced from many places. Even switched about but supposedly the same. There also appears to be an ongoing monopolisation of many, under one owner, which may, or may not, be healthy for the consumer.

As far as I am concerned, I prefer to have some idea of who, and where, my powder comes from. Which ATM means from Sweden, Switzerland, the Czech republic, Belgium, France etc. By their makers' names.

 

[Laurie]

ISTM that many US powders can be sourced from many places. Even switched about but supposedly the same. There also appears to be an ongoing monopolisation of many, under one owner, which may, or may not, be healthy for the consumer.

A simple rule of thumb on this issue is that NO EXTRUDED (ie tubular) POWDERS ARE MANUFACTURED IN THE USA irrespective of brand name. The only powder factory left in the US that makes rifle powders for handloaders is the former Olin Industries plant in St. Marks, Florida, now part of the General Dynamics Corporation's energetics division, and it only makes 'ball type' or so-called 'hybrids' like Hodgdon Hybrid 100V that use ball powder manufacturing processes. (Winchester and Hodgdon ball/'spherical' and Alliant Power-Pro grades are all from there, also for Americans some of the 'Accurate' brand grades.)

For government orders, there is a ball powder plant at Radford, Virginia, but AFAIK none of its rifle powder output goes to the handloading or commercial ammunition manufacturing markets.

Otherwise:
most IMR powders are imported from Canada (General Dynamics Valleyfield Quebec plant), a couple from Australia.
Hodgdon brand extrudeds from Australia (Thales/ADI Mulwala, NSW).
All Alliant rifle powders are imports bar the 'Power-Pro' ball powders, mostly from Eurenco Bofors, Sweden; a minority from Nitrochemie Wimmins Switzerland (the RS manufacturer).
Ramshot ball powders (Eurenco P B Clermont in Belgium).
Shooters World both ball and extruded (renamed Lovex grades from Explosia a.s. in the Czech Republic).
Norma (Eurenco Bofors, Sweden).
Vihtavuori (Nammo Vihtavuori Oy, Finland).

Hodgdon has become the dominant US supplier / company owner, but note that it doesn't make a single pound of the stuff itself. Everything is contracted in from the two General Dynamics Corp's plants, ADI Australia, and now with its takeover of Western Powders (Ramshot and Accurate grades) P B Clermont in Belgium, plus yet again General Dynamics St. Marks and Valleyfield. Combined ownership sees a plus of spreading testing / loads development; loads data printing and online resources; bottling and distribution etc., over more products and Hodgdon does that very well. The downside is a single dominant domestic US supplier - I'm surprised that the US anti-trust laws haven't kicked in, but there are still three major non-Hodgdon suppliers in play in the US, four if Shooters World is now big enough to be called 'major'.

Alliant doesn't make any powders either, rifle grades being mostly sourced in Europe, a minority only (ball grades) in the US.

 

[nCognitos]

From another thread, some links in case you don't have them already

This is the link to the GRT channel on YouTube, which is where I did my learning https://www.youtube.com/channel/UCEdG4IMYRhSA4Tc09hUKN_A

This is the link to the Discord server where you can ask questions and participate in the discussion Join the GordonsReloading Discord Server!

Because I use bullets that are not in the database, I have been measuring them carefully and adding them to my user database, then checking MV. I let GRT do all the seating depth and volume calculations and I aim for PMax -15% but I am not trying for max MV, I am trying to reproduce service rifle loads that work with the iron sights.

One of the bits of GRT that has taken me a long time to get my head around is the powder chart showing bomb measurements and the calibrated model to represent dynamic behaviour in a rifle. RS did share a lot of information with GRT, however, I make no further comment on it as it's not a powder I use.



There certainly seems to be a different correlation for RS than for Ramshot, given the curve for TAC



From the user manual

"After adaptation and calibration, the mathematical representation does not necessarily correspond to the original measurement data, since the propellant charge behaves dynamically differently than in the static closed vessel. Without a closed vessel measurement, it is possible to create a reengineered model using ballistic data including pressure equipment directly on the barrel over a calibre spectrum and bullet weights.

The closed vessel measurement is therefore one of the most important initial data for a powder model. As a rule, powder manufacturers perform this measurement for research and quality control purposes.

The value Ba given in the powder data is the start of this curve, which is composed of two or more sections. In contrast to the other software, GRT has a three-step representation of the burn-up behaviour which accounts for multi-base propellant behaviours and partially for additives of propellants nowadays like anti-copper-fouling and temperature stabilizers etc."

Again, not suggesting GRT is any better or worse than any other programme but I do like the way everything is explained and I can see the benefit of crowdsourcing user data to check back against the models.

 

[Sharpie]

A simple rule of thumb on this issue is that NO EXTRUDED (ie tubular) POWDERS ARE MANUFACTURED IN THE USA irrespective of brand name. The only powder factory left in the US that makes rifle powders for handloaders is the former Olin Industries plant in St. Marks, Florida, now part of the General Dynamics Corporation's energetics division, and it only makes 'ball type' or so-called 'hybrids' like Hodgdon Hybrid 100V that use ball powder manufacturing processes. (Winchester and Hodgdon ball/'spherical' and Alliant Power-Pro grades are all from there, also for Americans some of the 'Accurate' brand grades.)

For government orders, there is a ball powder plant at Radford, Virginia, but AFAIK none of its rifle powder output goes to the handloading or commercial ammunition manufacturing markets.

Otherwise:
most IMR powders are imported from Canada (General Dynamics Valleyfield Quebec plant), a couple from Australia.
Hodgdon brand extrudeds from Australia (Thales/ADI Mulwala, NSW).
All Alliant rifle powders are imports bar the 'Power-Pro' ball powders, mostly from Eurenco Bofors, Sweden; a minority from Nitrochemie Wimmins Switzerland (the RS manufacturer).
Ramshot ball powders (Eurenco P B Clermont in Belgium).
Shooters World both ball and extruded (renamed Lovex grades from Explosia a.s. in the Czech Republic).
Norma (Eurenco Bofors, Sweden).
Vihtavuori (Nammo Vihtavuori Oy, Finland).

Hodgdon has become the dominant US supplier / company owner, but note that it doesn't make a single pound of the stuff itself. Everything is contracted in from the two General Dynamics Corp's plants, ADI Australia, and now with its takeover of Western Powders (Ramshot and Accurate grades) P B Clermont in Belgium, plus yet again General Dynamics St. Marks and Valleyfield. Combined ownership sees a plus of spreading testing / loads development; loads data printing and online resources; bottling and distribution etc., over more products and Hodgdon does that very well. The downside is a single dominant domestic US supplier - I'm surprised that the US anti-trust laws haven't kicked in, but there are still three major non-Hodgdon suppliers in play in the US, four if Shooters World is now big enough to be called 'major'.

Alliant doesn't make any powders either, rifle grades being mostly sourced in Europe, a minority only (ball grades) in the US.

I think we missed out at least one more, the people who make MAXAM powders in Spain. I think they are part of the Dynamit Nobel group who make Vectan in France. Both of which are huge suppliers to the British shotgun cartridge manufacturing industry, but also make rifle powders, spherical and extruded.

Then I think there is a Polish operation, but not doing stuff for the civilian market. Could be wrong about that.

I had a mild interest in trying out MAXAM's stick rifle powder, when it appeared at about £50/kilo, but now it is priced at much the same as everything else, so lost interest.

This stuff: Maxam GDB 111



@Muir might be a little surprised at how much of USA stuff is actually manufactured in Europe. As well, of course, as Australia.

Regarding Hodgdon, some history, and an excerpt from their mission statement: A rather gushing article.

An Official Journal Of The NRA | Hodgdon: The Inside Story

“Hodgdon Powder Company operates following Biblical principles to honor God. Our Mission is to provide quality products and services in a manner which enhances the lives of our employees, families, customers and our communities. In doing so, we will deal with integrity and honesty, reflecting that people are more important than dollars and our purpose is to bring credit to our Lord Jesus Christ.”

And some pertinent reloading tips:

Choosing a Powder for a New Cartridge

Simple Trick for Monitoring Pressure of Your Rifle Reloads

Which Data Do I Use When My Specific Bullet is Not Listed?

What Velocity is My Handload?

I am also somewhat bemused by pressure testing methods. I thought that was all done and dusted, A case drilled with a hole in a specific place, matched up to a test barrel with a Kistler piezo transducer, charge amplifier, signal conditioning and processing, recording etc. The way Birmingham Proof House do it, for example. Also SAAMI and CIP.

Or, for NATO EPVAT testing, where so so many rounds must be tested for every batch assessment, drilling cases and lining them up just-so is not practical, so the pressure transducer is fitted in the test barrel, just beyond the chamber. Different numbers (higher) because the transducer is further forward, not in the chamber.

Then there a funkier ways of doing it, as it seems Lyman do. I honestly have no idea how their method works, using a "conformal transducer" but you can see a little on this vid. at about 13 minutes in.

Some of these methods can also give a dynamic trace of the pressure rise and fall as the bullet speeds down the barrel. But this is not their primary purpose.

 

[Alantoo]

However, both QuickLoad and GRT encourage you to measure the case volume for your particular case. It turns out that measuring the powder space is really just as easy, and it is better to measure that directly than infer the powder space from the case volume as is done by QuickLoad and GRT

When measuring the powder space by filling the case through the flash hole, how do you allow for the void in the primer?

You video seemed to show the water being brought just to the top of the flash hole. Does P-MAX add a specific amount for the primer void?

Or is the 25mg /0.4gr -ish not significant?

Alan

 

[Sharpie]

When measuring the powder space by filling the case through the flash hole, how do you allow for the void in the primer consistently?

You video seemed to show the water being brought just to the top of the flash hole. Does P-MAX add a specific amount for the primer void?

Or is it not significant?

Alan

I'll chip in here.

Yes the volume of the primer pocket is significant. You should either blot it dry before the second weighing, or brim it full then correct. I had a bit of a ding-dong on another thread with a couple of people who didn't think it much mattered, using the other method, filling the case from the mouth then applying a correction according to the seating depth of the bullet, i.e. the length of shank inside the case.

I think the flash-hole fill, with the bullet in question seated at an arbitrary reference depth, close to that actually being used, is the superior method. However it requires at least the case neck to have been re-sized to retain the bullet. Then you have to pull the bullet out again before reloading, which I don't think is a great idea.

My instinct is that the important measurement is that of a fireformed case, not a FL resized one. Which makes it a bit more complicated, if you don't have a neck sizer die.

For normal work, particularly if wanting to sort all your brass by internal volume, apparently some do, I'd suggest simply measuring them using the neck fill method, straight out of the rifle after firing, primers left in place as the plug. Assuming that they were all trimmed to the same length before the firing.

The correction for the amount displaced by the bullet when seated is trivial, if considered to be a flat based cylinder. If instead it has a boat tail, well some might worry about that, but I don't.

Dimensions are here, page 36 refers: https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

You can very simply calculate the extra water mass taken up by a brim-full primer pocket. In the case of a large rifle pocket it is about 1.13 grains of water. Small rifle, about 0.77 gr. Of course a primer is a little bit hollow inside. I'm guessing, perhaps 30% of it is a void.

But simpler to just blot it out of the pocket so it is up to the base of the flash hole, but no more.

If you are worried about the tiny additional volume inside the primer itself, I suppose you could weigh a spent primer (dry), then ad a drop or two of water until the inside is filled, level off, weigh again. But I rather think that you would be chasing stuff that really doesn't matter at all, nor is likely to be relevant to any simulation.

 

[Alantoo]

I'll chip in here.

Yes the volume of the primer pocket is significant. You should either blot it dry before the second weighing, or brim it full then correct.

Dimensions are here, page 36 refers: https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

You can very simply calculate the extra water mass taken up by a brim-full primer pocket. In the case of a large rifle pocket it is about 1.13 grains of water. Small rifle, about 0.77 gr.

But simpler to just blot it out of the pocket so it is up to the base of the flash hole, but no more.

Thank you for jumping in but I don't think you have understood the reason behind my query.

A brimful primer pocket, or water just filling the flash hole is not representing the volume accurately, which was the point I was making.

The primer void I was referring to is the void within the primer i.e. the volume of the primer cup less the volume of the anvil...not the void within the primer pocket.

As I posted above I had just weighed a couple of fired large rifle primers complete with their anvils and filled with water. There was 25mg of water which converts to 0.386gr. I didn't use rinse aid and there may have been a bubble under the anvil but I couldn't see one.

Without the anvil I weighed 32mg of water in a large rifle primer cup which converts to 0.494gr.

My post was a query to @borbal as to whether P-MAX added an aproximate/nominal figure to allow for that unmeasured primer void when filling through the flash hole and bringing the water level to the top of the flash hole/base of the primer pocket....and whether that 0.386gr was significant.

I used a fired case with the primer left in when measuring for GRT in order to include that void around the anvil in the primer as it is part of the total volume.

Alan

 

[Alantoo]

Without the anvil I weighed 32mg of water in a large rifle primer cup which converts to 0.494gr.

Just re-weighed that with a slight crown and it was 38mg of water...the first was slightly under with a meniscus so average would be 35mg 0.54gr

Alan

 

[borbal]

My post was a query to @borbal as to whether P-MAX added an aproximate/nominal figure to allow for that unmeasured primer void when filling through the flash hole and bringing the water level to the top of the flash hole/base of the primer pocket....and whether that 0.386gr was significant.

You are asking whether a third of a grain - less than 1% of the powder space - is significant.

You can experiment with P-Max to see how much of a difference it will make. Take a typical 308 Win case with 50 grains H2O of powder space behind the loaded 155 grain bullet, loaded up with 45 grains of N150, which will fill the case up (100% loading density). Bear in mind that this is a stiff working load with a predicted maximum pressure of around 50,000 psi and a muzzle velocity from a 24 inch barrel of around 2680 ft/sec. But we are not on the hairy edge of a load which will blow the gun up by adding one more kernel of powder.

How much difference would there be to the maximum pressure and muzzle velocity if the powder space is reduced by 1% to 49.5 grains of water?The pressure would increase by about 1000 psi, and the muzzle velocity would increase by about 10 ft/sec, in round terms.

To put this in perspective, these are the sort of variations you would expect to see from shot to shot for a load like that anyway. If you have a standard deviation of 10 ft/sec in your muzzle velocity (which is quite typical), then statistically, any change in the loading conditions is not significant unless it results in a change in muzzle velocity of at least two standard deviations. So, in terms of how much change a variation of 1% in the powder space will have, it is "down in the noise" in terms of any real effect. It is not significant in a meaningful sense.

It is also worth putting a small error in measured case capacity in the context of other things that might affect the pressure and muzzle velocity. As I have said before, what these simulator programs really want to know is the free space in the case of the loaded round. That is, how much space is there for the powder gasses to expand into before the bullet starts to move off up the barrel. QuickLoad and GRT calculate this space from the measured case capacity to the case mouth. They then subtract the volume that the bullet takes up when seated in the case, and they also subtract the volume taken up by the powder itself to arrive at a value for the free space in the loaded round.

There is an error budget then. First, the error in the case capacity, which we have just been talking about. Then there is the error in space the bullet takes up in the case. How accurate are the bullet dimensions in the bullet library compared to the bullets you are actually using? And in considering how much volume the powder is occupying, how accurate is the powder density used in the program?

The powder densities from the powder manufacturers are given to two significant figures, so the error there might be 2% typically.

The critical thing to check with your bullet is that the length of the bullet is the same as the length quoted in the bullet library. If this differs by 0.01", that could easily lead to an error of half a grain H2O in the calculated free space.

I would maintain that measuring the powder space in the case directly is inherently more accurate as you are then not concerned how accurately the program accounts for the volume taken up by the seated bullet. You are taking out one of the potential errors in the error budget. But I would also say that it is not worth getting too hung up on whether the space in the primer should be accounted for. The error introduced by taking this into account, or not, is not significant.

 

[Sharpie]

Just re-weighed that with a slight crown and it was 38mg of water...the first was slightly under with a meniscus so average would be 35mg 0.54gr

Alan

Sorry if I rambled on a bit and rather missed your point. Except for the last three paragraphs.

But here is another idea of how to estimate much void space there might be inside a seated primer without the use of water:

Model a large rifle primer as a cylinder, made of brass. Density of brass is about 8.5 g/cc (depending on alloy composition).

Diameter say 5.4 mm, length say 3.2 mm. If that was solid brass it would weigh about 623 mg. Of course you could measure your own primers instead of using this estimate.

Weigh an actual fired primer. Subtract that from 623 mg. The difference is the mass of brass that represents the empty space inside. Correct that for brass density (i.e. divide by 8.5). The result is the volume of air space inside. I.e. the same as water weight. Since 1 mm^3 of water weighs 1 mg.



Or, lets turn it around:

You have measured 35mg of water capacity, though that is for the cup only, not including the anvil. So, that, to me, suggests that you might discover that if you simply weighed a fired primer without the anvil it would come in at around 326 mg. If so, you might conclude that fired primers are about 50% hollow inside, including the firing pin indentation. but not including the anvil.

For your anvil-in figure of 25mg instead, I estimate the fired primer would weigh about 411mg. 34% hollow.

What do yours weigh ?

Somewhere earlier, or maybe on another thread, I made a SWAG that the figure for a complete fired primer might be about 30%. I think that might be about right.

Is this important ? Simply do a sensitivity analysis. Run your simulator of choice with, and without, correction for void space inside the primer. Let us know how the Pmax and muzzle velocity changes, and whether you think it is likely to make a real world difference.