Effect of OAL on Pressure

[neutron619]

Hello chaps.

Quick question for the reloading crowd. I took a couple of ladders to a BDS range day today.

I've been trying H335 under a 110gr SP in .308 and Reloder 15 under a 150gr SP.

I couldn't get close to maximum loads at all. I was expecting that there might be a point before maximum where pressure signs started appearing, but I was barely above the starting loads when I started seeing pressure signs (brass flowing into the extractor recess the most obvious, some stiffness in the bolt also). In spite of the pressure signs, the way they were shooting made them feel like they were light, mild loads - certainly not "maximum pressure" loads.

Both lots were loaded within the limits of the data, above the minimum OAL (I don't have a comparator / measure-to-ogive gauge yet). The 110s at 2.580" and the 150's at 2.725". For both, I could seat the bullet further out of the case, though probably not to max OAL for the cartridge in the case of the 110s.

What I'd like to know is whether a difference of a tenth of an inch or two seating depth is likely to make much of a difference to pressure? I.e. if I've got another 3-5gr before I get to maximum load, should I seat further out of the case and try another ladder to see if I can get further towards maximum or is it not likely to make much difference as to how far I can go?

I should mention that one load of the 150's looked very promising (one ragged hole at 100m) but that the rest of them were mediocre with respect to accuracy. None of the 110's performed particularly well, so I either need a new approach or to try a new powder.

Thanks for any thoughts / pointers you can give.

Adam.

 

[cambsman]

Hi. Have you modelled the loads in a simulation programme? If not, if you want to send me full details of loads then I am happy to run them thru the Quickload software programme. Will give you an idea on pressure etc etc. Might help gain some insight. Not perfect as cannot replicate your specific gun but might help.

 

[Muir]

Hi. Have you modelled the loads in a simulation programme? If not, if you want to send me full details of loads then I am happy to run them thru the Quickload software programme. Will give you an idea on pressure etc etc. Might help gain some insight. Not perfect as cannot replicate your specific gun but might help.

It's a kind offer but what would that ascertain? Not much I think, because quick load is a generic ballistic guide, much as reloading data is a guide. The data contained in load information is what the provider got on that day, with that LOT of powder, from their barrel, cut with their chamber. It will never transfer straight across to your rifle and your loads. My 7mm-08 topped out with Varget at 42.5 grains -which is starting load on Hodgdon's site, but a mid range load in Hornady's book. This is why there are constant reminders to "START AT MINIMUM LOAD AND WORK UP" because, "minimum" sometimes ain't, and maximum is sometimes never approached. It just works that way. When you start getting pressure signs you're pretty much done but that's OK. Unless it's being caused by something amiss in your loading technique, you are probably still getting the performance listed but using less powder to get there.

As to relieving the pressure signs by seating out a bit. Probably won't help a measurable amount. Even if it did, what would you do then? Continue to add powder to try to match your loading data? You'd soon be right back in the same boat.~Muir

PS: I had once a 17 MachIV that blew primer out of the case with starting loads. My best load was 15% below starting load.

 

[deeangeo]

As a rule of thumb deeper seating with a given load will increase pressure while seating a bullet further out will reduce it.
However, while I have always adhered to reloading manual data as a general rule, I have a situation with my current loads that allows me to exceed by .2gn the max charge shown with a load that provides constant good accuracy without any visible external pressure indications on the fired cases.

Indeed these cases have now been loaded and annealed to provide further longevity with what are still very good useable cartridge cases. You may anyway find that bullet manufacturers data and propellant manufacturers data are different..... - But if you exceed max data listed, go into this territory with great care. ATB

 

[neutron619]

Thanks all for your responses - much appreciated.

To cambsman: Thank you for your offer to put them through quickload - out of curiosity if for no other reason, I'll send you a PM later today with the load details.

To Muir: I take your points and if seating depth won't make that much of a difference I probably need a new powder. The range between max and min was (I think) about 6-7 grains but I was getting pressure signs after 1.5gr above starting. You asked me if I'd try to match loading data - actually I would. I only had four loads, 0.5gr between them, to test with a 1.5gr range and none of them were good. It would be good to try and generate some more possibilities though, with a whole tub of powder sitting on the shelf with no purpose to it at this point. If I could get back within safe pressure, I can get two more loads to try for every grain extra I can get in the case. I could cross my fingers that there would be an accurate load in those and try to develop it. I'm not sure I'm explaining myself very well, but hopefully you get the idea.

To deeangeo: Thanks for your reply. I think I'm too much of a newbie at this to be anything other than conservative / cautious, so I won't be straying into above-maximum territory any time soon. I guess I am curious though - I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr - does that mean I also get the velocity associated with 50gr? I don't have access to a chronograph, but I'm curious.

Thanks again guys.

Adam.

 

[Muir]

Thanks all for your responses - much appreciated.

To cambsman: Thank you for your offer to put them through quickload - out of curiosity if for no other reason, I'll send you a PM later today with the load details.

To Muir: I take your points and if seating depth won't make that much of a difference I probably need a new powder. The range between max and min was (I think) about 6-7 grains but I was getting pressure signs after 1.5gr above starting. You asked me if I'd try to match loading data - actually I would. I only had four loads, 0.5gr between them, to test with a 1.5gr range and none of them were good. It would be good to try and generate some more possibilities though, with a whole tub of powder sitting on the shelf with no purpose to it at this point. If I could get back within safe pressure, I can get two more loads to try for every grain extra I can get in the case. I could cross my fingers that there would be an accurate load in those and try to develop it. I'm not sure I'm explaining myself very well, but hopefully you get the idea.

To deeangeo: Thanks for your reply. I think I'm too much of a newbie at this to be anything other than conservative / cautious, so I won't be straying into above-maximum territory any time soon. I guess I am curious though - I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr - does that mean I also get the velocity associated with 50gr? I don't have access to a chronograph, but I'm curious.

Thanks again guys.

Adam.

Adam,
I just re read your original post and I was struck by the fact that you re getting pressure signs with two different powders and two different bullets. And you say that it seems to be mild despite the 'signs' you're getting? That indicates that something else might be the problem. Are you using the same LOT of brass for both? FL resized? ~Muir

 

[cambsman]

It's a kind offer but what would that ascertain? Not much I think, because quick load is a generic ballistic guide, much as reloading data is a guide. The data contained in load information is what the provider got on that day, with that LOT of powder, from their barrel, cut with their chamber. It will never transfer straight across to your rifle and your loads. My 7mm-08 topped out with Varget at 42.5 grains -which is starting load on Hodgdon's site, but a mid range load in Hornady's book. This is why there are constant reminders to "START AT MINIMUM LOAD AND WORK UP" because, "minimum" sometimes ain't, and maximum is sometimes never approached. It just works that way. When you start getting pressure signs you're pretty much done but that's OK. Unless it's being caused by something amiss in your loading technique, you are probably still getting the performance listed but using less powder to get there.

As to relieving the pressure signs by seating out a bit. Probably won't help a measurable amount. Even if it did, what would you do then? Continue to add powder to try to match your loading data? You'd soon be right back in the same boat.~Muir

PS: I had once a 17 MachIV that blew primer out of the case with starting loads. My best load was 15% below starting load.

Muir. It was only meant as a data point. As I said in my post the software won't model his specific rifle etc etc

 

[Muir]

Muir. It was only meant as a data point. As I said in my post the software won't model his specific rifle etc etc

I understand. I guess what I was trying to say, and said badly, is that the OP needs to deal specifically with what is happening in his rifle because his results to date are not in accordance with any data set. Didn't mean to be curt.~Muir

 

[cambsman]

No problem!

 

[Sharpie]

I wonder if because the loads showed pressure signs whether they are actually "out-performing" the data. I.e. I'm getting the pressure they expect with 50gr of powder with a load of 45gr

That doesn't sound correct. To be getting pressure signs at 10% below max load published in a reputable manual is unusual.

First thing I'd do is check your scales. If you are using a beam scale, make absolutely sure you are setting it up correctly. There are some models where it is easy to confuse the settings.

If you are using a digital scale, did you check and if necessary re-calibrate it with a test weight ? And did you check the tare of the pan hadn't drifted during the loading process ?

If you don't have a test weight (you absolutely need one if using digital scales), then try weighing a bullet. They are normally quite close to the stated weight. E.g. a 50ish grain .224 bullet, if your scales won't cope with your .308 bullets.

Pull one of your loads and re-weigh the powder, to be certain.

I am very fussy, and use two digital scales. I weigh powder using one that resolves to .02 grains, then check again on a coarser scale with 0.2 grain resolution. It only adds a few seconds.

I also weigh the completed cartridges as a final check. You need digital scales to do these checks. A beam scale is great for weighing defined powder charges, but pretty much useless for weighing anything else.

And every time I weigh another charge I check that the empty pan still reads zero on the scale. They can drift during a session.

Even if I used a beam scale, I would also check every load with a digital one.

Next question, are you using .308 brass or 7.62 ? If the latter, it may have a significantly smaller internal volume due to the thicker brass.

As for seating depth, unless you are pretty much filling the case with powder it is unlikely to have a strong effect, unless you are seating your bullets extremely deeply.

According to Lee manual the useful capacity of a .308 case is 3.43cc. Vmd of H335 is 0.06450 cc per grain. Reloder15 is 0.07060.

A quick look at my manuals suggests 48.0 grains of H335 under a 110gr bullet would be about max. 46.3 gr of Reloder15 for the 150 grainer. Do these numbers tally with your load data ?

I calculate that these loads would fill the case to about 90% (H335) or 95% (Reloder15), which is pretty much ideal. My gut feel is that these loads should be pretty tolerant of varying seating depth.

One other thought: are you cleaning the lube off the cases thoroughly? If the cases are greasy, or you have left the chamber oily, you might well see pressure signs at otherwise safe loads.

 

[neutron619]

Adam,
I just re read your original post and I was struck by the fact that you re getting pressure signs with two different powders and two different bullets. And you say that it seems to be mild despite the 'signs' you're getting? That indicates that something else might be the problem. Are you using the same LOT of brass for both? FL resized? ~Muir

Muir,

Yep, two different loads being tested yesterday. All in once fired RWS brass from a very reputable dealer. All cleaned, FL sized, trimmed to between 2.000"-2.005". Muron primers, if that tells you anything.

The first was the 110gr SP over 47-52.5gr of H335. The point of the experiment was to get something light, and fast and flat as possible. Small deer and fox in mind. I loaded 0.5gr steps and saw first pressure signs at 48.5gr (extractor mark and hard bolt lift). I tried one round of 49.0gr - it needed quite a pull to get the bolt up and open - I stopped there. One round of three at 47.0gr showed poor case obturation in the chamber - black oily marks all the way round the case on extraction - if anything it looked like low pressure rather than high and I suspected that the slight mark from the extractor on that case might have been the head of the case slamming into the bolt face. I may have been wrong given how quickly clearly high pressures were hit. The rounds did group as I'd expect, moving down and slightly left as the powder charge increased, in keeping with a longer point blank range and the behaviour of my moderator. All seemed relatively mild (light bullet, perhaps) except for one of the 48gr rounds which clocked me one on the collar bone. I'm confident in the load weights - they were measured carefully to the point of being anal about it.

The second load was the 150gr SP over 43-49gr of Reloder 15. This one performed more reasonably - first pressure signs at 46-47gr (same as above), stopped at 47.5 with difficult extraction and pronounced extractor groove mark. All the rounds fired seemed very mild compared to a string of Federal 150's I shot as a comparison. Again, they showed reasonable grouping with downwards / leftwards as powder charge increased. Both of the 46.0gr rounds went through the same hole - I'll be investigating that next. However, it didn't feel like I was shooting full power loads. Perhaps the bipod, or shooting prone or some environmental factor made me feel it less, but they kicked a lot less than the factory stuff I've used previously. This doesn't need to be a maximum power round - provided it wasn't a fluke and that whatever velocity the 46.0gr load gives is deer-legal and doesn't have a ridiculous drop on it, I'll be happy to use it. (Yesterday's more successful experiment was in fact a reduced load, but that's not relevant here.)

If I've missed any details you think are relevant, please let me know and I'll put them up.

Thank you for any insights you can give,

Adam.

 

[neutron619]

That doesn't sound correct. To be getting pressure signs at 10% below max load published in a reputable manual is unusual.

First thing I'd do is check your scales. If you are using a beam scale, make absolutely sure you are setting it up correctly. There are some models where it is easy to confuse the settings.

If you are using a digital scale, did you check and if necessary re-calibrate it with a test weight ? And did you check the tare of the pan hadn't drifted during the loading process ?

If you don't have a test weight (you absolutely need one if using digital scales), then try weighing a bullet. They are normally quite close to the stated weight. E.g. a 50ish grain .224 bullet, if your scales won't cope with your .308 bullets.

Pull one of your loads and re-weigh the powder, to be certain.

I am very fussy, and use two digital scales. I weigh powder using one that resolves to .02 grains, then check again on a coarser scale with 0.2 grain resolution. It only adds a few seconds.

I also weigh the completed cartridges as a final check. You need digital scales to do these checks. A beam scale is great for weighing defined powder charges, but pretty much useless for weighing anything else.

And every time I weigh another charge I check that the empty pan still reads zero on the scale. They can drift during a session.

Even if I used a beam scale, I would also check every load with a digital one.

Next question, are you using .308 brass or 7.62 ? If the latter, it may have a significantly smaller internal volume due to the thicker brass.

As for seating depth, unless you are pretty much filling the case with powder it is unlikely to have a strong effect, unless you are seating your bullets extremely deeply.

According to Lee manual the useful capacity of a .308 case is 3.43cc. Vmd of H335 is 0.06450 cc per grain. Reloder15 is 0.07060.

A quick look at my manuals suggests 48.0 grains of H335 under a 110gr bullet would be about max. 46.3 gr of Reloder15 for the 150 grainer. Do these numbers tally with your load data ?

I calculate that these loads would fill the case to about 90% (H335) or 95% (Reloder15), which is pretty much ideal. My gut feel is that these loads should be pretty tolerant of varying seating depth.

One other thought: are you cleaning the lube off the cases thoroughly? If the cases are greasy, or you have left the chamber oily, you might well see pressure signs at otherwise safe loads.

Thanks for these thoughts - I wonder if the reply I made to Muir just now will tell you anything useful?

Briefly: .308 brass; pretty sure I've got the scales set up right - I weigh a few bullets when I start to check calibration; afraid I've pulled all the (what I assumed were) hot rounds apart now; seating depth for the 110gr load was calculated from finding the base of a Barnes bullet and putting the SP base in the same place - Muir will remember the post, perhaps, as he answered it.

 

[Muir]

Muir,

Yep, two different loads being tested yesterday. All in once fired RWS brass from a very reputable dealer. All cleaned, FL sized, trimmed to between 2.000"-2.005". Muron primers, if that tells you anything.

The first was the 110gr SP over 47-52.5gr of H335. The point of the experiment was to get something light, and fast and flat as possible. Small deer and fox in mind. I loaded 0.5gr steps and saw first pressure signs at 48.5gr (extractor mark and hard bolt lift). I tried one round of 49.0gr - it needed quite a pull to get the bolt up and open - I stopped there. One round of three at 47.0gr showed poor case obturation in the chamber - black oily marks all the way round the case on extraction - if anything it looked like low pressure rather than high and I suspected that the slight mark from the extractor on that case might have been the head of the case slamming into the bolt face. I may have been wrong given how quickly clearly high pressures were hit. The rounds did group as I'd expect, moving down and slightly left as the powder charge increased, in keeping with a longer point blank range and the behaviour of my moderator. All seemed relatively mild (light bullet, perhaps) except for one of the 48gr rounds which clocked me one on the collar bone. I'm confident in the load weights - they were measured carefully to the point of being anal about it.

The second load was the 150gr SP over 43-49gr of Reloder 15. This one performed more reasonably - first pressure signs at 46-47gr (same as above), stopped at 47.5 with difficult extraction and pronounced extractor groove mark. All the rounds fired seemed very mild compared to a string of Federal 150's I shot as a comparison. Again, they showed reasonable grouping with downwards / leftwards as powder charge increased. Both of the 46.0gr rounds went through the same hole - I'll be investigating that next. However, it didn't feel like I was shooting full power loads. Perhaps the bipod, or shooting prone or some environmental factor made me feel it less, but they kicked a lot less than the factory stuff I've used previously. This doesn't need to be a maximum power round - provided it wasn't a fluke and that whatever velocity the 46.0gr load gives is deer-legal and doesn't have a ridiculous drop on it, I'll be happy to use it. (Yesterday's more successful experiment was in fact a reduced load, but that's not relevant here.)

If I've missed any details you think are relevant, please let me know and I'll put them up.

Thank you for any insights you can give,

Adam.

You're right. No experience with Muron primers at all!!

You may have hit on something when you say the ejector mark might be from the case slapping back. This can mimic high pressures easily: marking the case head and flattening primers... and it would match the progression. Smokey with lack of obturation until you reach a high enough pressure to begin slapping the cases back, marking the head and making the bolt lift a little more difficult. I'm assuming that the FL sized cases chambered easily? I wonder if a slight adjustment on the FL die would cure this? ~Muir

 

[cambsman]

I am no expert. Muir can confirm this as he has given me lots of great advice. I would, however offer the following tale. I had a problem loading 120 grn bullets for my 6.5x55 Sauer 202. Also had issues with 140 grn bullets but not quite as bad. On paper the loads I was using would be considered light. However in my rifle I was getting high pressure indicators, especially with very stiff bolt lift. I had to ignore conventional wisdom and go with what my rifle was telling me. Now have a selection of bullets/loads that work very nicely indeed. Not helpful with your specific problem but I mention it so that you remember to go with what is in front of you.

 

[Sharpie]

Thanks for these thoughts - I wonder if the reply I made to Muir just now will tell you anything useful?

Where did you get that load data ?

I only took a quick look at two manuals, but your start loads aren't far off the max. loads that I suggested.

Also suggest you check you are setting up your sizing die correctly. Since you must be FL re-sizing (S/H fired brass) , compare how far the shoulder is being pushed back, with the the position from your fired cases. If there is any visible difference, there may be a problem somewhere.

Excessive headspace due to incorrectly adjusted die might explain some things.

Using second hand brass is always a gamble. Might have something to do with this, probably not.

Genuine once-fired is easy to confirm. There will be no signs of it having been run through a die e.g longitudinal scratches and polishing. Compare the surface finish at the head and groove, it should look identical to the rest of the body.

"Once fired" that has been titivated by e.g. decapping and tumbling/cleaning so removing any evidence might be anything. Excellent Lapua virgin brass isn't particularly expensive, in 308.

 

[neutron619]

You're right. No experience with Muron primers at all!!

You may have hit on something when you say the ejector mark might be from the case slapping back. This can mimic high pressures easily: marking the case head and flattening primers... and it would match the progression. Smokey with lack of obturation until you reach a high enough pressure to begin slapping the cases back, marking the head and making the bolt lift a little more difficult. I'm assuming that the FL sized cases chambered easily? I wonder if a slight adjustment on the FL die would cure this? ~Muir

Muir,

Yes, the FL sized cases chambered easily and I can imagine the mechanism of "marking" the cases in the way you describe. One thing is bothering me though: I'm concerned about the bolt lift - it got harder with the higher loadings rather than easier as I'd expect if I was getting better obturation moving into the middle of the load weight range. I've just re-chambered some of the spent cases in my rifle: the Federal factory cases I fired extract noticeably more easily than the marked RWS ones from my reloads. I understand what you're saying about cases slapping back making bolt lift harder, but my gut feeling is against that (I recognise my distinct lack of experience).

The other confusing factor is that there were no flattened primers in any of the loads I shot. The obvious difficulty is that I'm not experienced enough to make sense all of this (hence asking) but I'd rather not make a mistake, carry on, and blow up or damage a decent stalking rifle (or myself). I wonder if some Quickload figures would be helpful in teasing all this out - if the software could show that I was only (probably) at 80% of maximum pressure, I might feel better about carrying on...

Edit: sorry - I'm not experienced enough to know how I might adjust the FL die to improve this?

Thank you again for talking all of this out. If I'm doing something stupid, I'd like to correct the error before I do something that actually goes wrong...

Adam.

 

[neutron619]

Where did you get that load data ?

I only took a quick look at two manuals, but your start loads aren't far off the max. loads that I suggested.

Also suggest you check you are setting up your sizing die correctly. Since you must be FL re-sizing (S/H fired brass) , compare how far the shoulder is being pushed back, with the the position from your fired cases. If there is any visible difference, there may be a problem somewhere.

Excessive headspace due to incorrectly adjusted die might explain some things.

Using second hand brass is always a gamble. Might have something to do with this, probably not.

Genuine once-fired is easy to confirm. There will be no signs of it having been run through a die e.g longitudinal scratches and polishing. Compare the surface finish at the head and groove, it should look identical to the rest of the body.

"Once fired" that has been titivated by e.g. decapping and tumbling/cleaning so removing any evidence might be anything. Excellent Lapua virgin brass isn't particularly expensive, in 308.

Sharpie,

The load data comes from Hodgdon's website (with the same load printed in the Lee 2nd Edition):

110 GR. BAR TSX FB H335 .308" 2.690" 47.3 3078 40,800 PSI 52.5 3358 53,900 PSI

with a conversion from the 2.690" OAL of the Barnes bullet to seat the Hornady SP to the same depth in the case.

Could you tell me about excessive headspace? I wondered if I'd trimmed the cases too short, or done something like that which would affect how the case expands in the chamber.

Thanks as always for any insights you can give.

Adam.

 

[neutron619]

Also suggest you check you are setting up your sizing die correctly. Since you must be FL re-sizing (S/H fired brass) , compare how far the shoulder is being pushed back, with the the position from your fired cases. If there is any visible difference, there may be a problem somewhere.

Ok - I think you're onto something here, but I don't know what the significance of this is.

I've compared a sized case with a fired one. The sized case has a longer shoulder than the fired one - this is obvious from even a cursory inspection. The expanded neck of the fired case is also obvious.

Relatively, the lower part of the shoulders are the same distance from the head of the case and the cases are both of the same overall length within a few thou. The top of the shoulder of the fired case is relatively lower than that of the sized case when they're stood mouth upwards.

Does this tell anyone something diagnostic?

Adam.

 

[Sharpie]

Sharpie,

The load data comes from Hodgdon's website (with the same load printed in the Lee 2nd Edition):

110 GR. BAR TSX FB H335 .308" 2.690" 47.3 3078 40,800 PSI 52.5 3358 53,900 PSI

with a conversion from the 2.690" OAL of the Barnes bullet to seat the Hornady SP to the same depth in the case.

Could you tell me about excessive headspace? I wondered if I'd trimmed the cases too short, or done something like that which would affect how the case expands in the chamber.

Thanks as always for any insights you can give.

Adam.

You cannot use data for Barnes TSX (solid copper) with a conventional jacketed lead bullet. They are very different. And Barnes copper bullets MUST be seated a long way back from the lands, to give them a good jump.

I also have Lee 2nd edition but that load is not in it. I have "110 grain Barnes X bullet. H335 start 44 never exceed 48.0 grains. 3210 fps. 60569 psi (thats huge).

Which is far lower. Your start load is nearly the max. in my manual.

Find some alternative load data.

Never ever believe just one load. Study at least three reliable sources and get a feel for what is reasonable.

Headspace is the distance from bolt face to the cartridge shoulder. Nothing to do with neck trimming. If you set up your sizing die too tight, it might be pushing back the shoulder too far, which can lead to various (some dangerous) problems.

Usually dies are cut slightly under-size (but within allowable tolerance) when bottomed out on their own make of shellholder. You need to back them off slightly to to size the cases up to the maximum dimension, which is preferable.

But as with anything in life, some dies are made incorrectly, and if you use another make of shell holder you are on your own.

Anyway, as Muir has observed, you are seeing worrying problems with two completely different loads (powder type and bullet weight), which is very concerning.

I again question the second hand brass. If it has been over-heated during processing, e.g. forced drying after a wet cleaning process, then the body and head may have become annealed, which would be disastrous, and entirely explain your symptoms. Just a thought.

 

[Sharpie]

Post a photo of the two cases side-by side.

Your description of the differences is unclear. E.g "The top of the shoulder of the fired case is relatively lower than that of the sized case when they're stood mouth upwards." seems unlikely, if I have understood correctly.