Gordons v Quickload v actual

[Kimmeridgien]

I'm using GRT OBT for .375 H&H Magnum and 6,5x55SE. I found that if you do the following:
* Weight sort your brass and do volume (H2O weight) measurement of the brass that you are using for the bullet in question
* Enter a good approximation for your barrel length including muzzle break (and probably suppressor)
* Enter the correct seating depth (i.e. total cartridge length minus the length of the bullet)
you will get a very good indication of:
* Node charge weights by use of the OBT calculator
* Which powders are especially worth considering and reasonably well their ordering
* What fill and burn ratio can be expected for each powder

For the .375 H&H Magnum you can't expect reliable simulated muzzle velocity. It's unclear whether the simulated pressure is accurate. For this caliber I have come to rely on published load data in the Norwegian book "Ladeboken", which for this caliber lists charge weights in a 2 grain range. I have found that this range contains either a full node or a half node just below max pressure. The same is probably true for Norma's published load data. I am using Ladeboken's max weights for TSX 270 grains, which is about 10% above the GRT nominal max pressure, as a reference for my load limit. No signs of overpressure. Apart from incorrect muzzle velocity simulation (which has very little effect on the node weight), I found that the ordering of the powders in terms of simulated velocity is not correct for lighter bullets. E.g. N550 gives max velocity for TSX 300 and TSX 270 in both simulation and reality that is faster than N540, while N540 is actually faster than N550 for TTSX 250 in reality (but not in simulation). You will notice that US load data is utterly useless, due to its ridiculous range from min to max.

I found that with good tool input, the GRT OBT simulation predicts the real node to within some +- 0.5 or even +-0.3 grains. This means great savings on components when working up a load. For this caliber, the full nodes are some 3-4 grains apart.

Please note that all the three requirements I am listing at the beginning of this post are essential for the usefulness of the OBT simulation. My lightest .375 H&H Magnum brass (Hornady) weighs about 15.2 gram. My heaviest (old RWS) weighs about 18.0 gram. The difference in volume corresponds to a difference in charge weight of some 2-3 grains for the same node. Similarly, seating depth directly corresponds to case volume. And total barrel length determines the time it takes for the shock wave traversal.

My experience with 6,5x55SE is also very good (but I have recently been more focused on my .375). The Swede is a much more common caliber, so its simulated muzzle velocity is probably more accurate.

You will find lots of people in forums who wouldn't know a clue if it jumped up and bit them. Any claim that there is no correlation between brass weight and volume is hogwash. Any claim that GRT OBT is flawed is a solid indicator that the person couldn't be bothered or trained to do it right.

 

[Bavarianbrit]

I'm using GRT OBT for .375 H&H Magnum and 6,5x55SE. I found that if you do the following:
* Weight sort your brass and do volume (H2O weight) measurement of the brass that you are using for the bullet in question
* Enter a good approximation for your barrel length including muzzle break (and probably suppressor)
* Enter the correct seating depth (i.e. total cartridge length minus the length of the bullet)
you will get a very good indication of:
* Node charge weights by use of the OBT calculator
* Which powders are especially worth considering and reasonably well their ordering
* What fill and burn ratio can be expected for each powder

For the .375 H&H Magnum you can't expect reliable simulated muzzle velocity. It's unclear whether the simulated pressure is accurate. For this caliber I have come to rely on published load data in the Norwegian book "Ladeboken", which for this caliber lists charge weights in a 2 grain range. I have found that this range contains either a full node or a half node just below max pressure. The same is probably true for Norma's published load data. I am using Ladeboken's max weights for TSX 270 grains, which is about 10% above the GRT nominal max pressure, as a reference for my load limit. No signs of overpressure. Apart from incorrect muzzle velocity simulation (which has very little effect on the node weight), I found that the ordering of the powders in terms of simulated velocity is not correct for lighter bullets. E.g. N550 gives max velocity for TSX 300 and TSX 270 in both simulation and reality that is faster than N540, while N540 is actually faster than N550 for TTSX 250 in reality (but not in simulation). You will notice that US load data is utterly useless, due to its ridiculous range from min to max.

I found that with good tool input, the GRT OBT simulation predicts the real node to within some +- 0.5 or even +-0.3 grains. This means great savings on components when working up a load. For this caliber, the full nodes are some 3-4 grains apart.

Please note that all the three requirements I am listing at the beginning of this post are essential for the usefulness of the OBT simulation. My lightest .375 H&H Magnum brass (Hornady) weighs about 15.2 gram. My heaviest (old RWS) weighs about 18.0 gram. The difference in volume corresponds to a difference in charge weight of some 2-3 grains for the same node. Similarly, seating depth directly corresponds to case volume. And total barrel length determines the time it takes for the shock wave traversal.

My experience with 6,5x55SE is also very good (but I have recently been more focused on my .375). The Swede is a much more common caliber, so its simulated muzzle velocity is probably more accurate.

You will find lots of people in forums who wouldn't know a clue if it jumped up and bit them. Any claim that there is no correlation between brass weight and volume is hogwash. Any claim that GRT OBT is flawed is a solid indicator that the person couldn't be bothered or trained to do it right.

Kinda interesting to read but having reread all total posts on this thread I am non the wiser what the meaning of OBT is as it was not, as far as I saw mentioned anywhere. Please correct me.
The use of three letter acronyms drives me crazy here in Germany zfr =zielfernrohr = (scope) etc. I use GRT most weeks too.

 

[Lancaster]

Kinda interesting to read but having reread all total posts on this thread I am non the wiser what the meaning of OBT is as it was not, as far as I saw mentioned anywhere. Please correct me.
The use of three letter acronyms drives me crazy here in Germany zfr =zielfernrohr = (scope) etc. I use GRT most weeks too.

Optimum Barrel Time, idea is that the projectile leaves the muzzle at the same point of each barrel oscillation in relation to the shock wave from the detonation travelling down the barrel, reaching the muzzle and bouncing back again, so that the oscillation at the muzzle is at the minimum at the point of exit of the bullet.
Funnily enough I've found that by tweaking burn rates to match observed velocity on QL and playing with the OBT charts, it does actually seem to work

Optimal Barrel Time Paper

 

[Deermanagement]

Think I've been very lucky. For Sako and Tikka rifles, I've loaded for 5 different calibles, 12 different bullet/powder combos. Laterly gone up in 0.8g intervals using good bullets with recomended powders to suit. The worst load I ever settled on was sub moa in a bullet I would likely never use and for my last 4 loads for 3 different rifles all produce less than 1/2 moa. Never been worried about velocities as a good bullet in the right place will always do a lethal job. So for hunting purposes, I don't think you need to spend too much time or money unless you're unlucky or you are not using the best quality.

The last load, 102g Yew Tree was tested at 300 yds on Saturday to drop test in the absence of actual fps, and vertical string was 5/8" whilst horizontal string was 1.625" no doubt produced from an approx 10mph crosswind at my 10 o'clock. New Norma brass, just neck sized, 100 thou off the lands with 41.7g of RL15 thrown from a RCBS chargmaster lite. So it took 18 shots to produce results, same as the 4 previous loads. Unless you are shooting competitively don't think you need to wear a barrel out or spend a fortune to load a good hunting round.

 

[Deermanagement]

Kinda interesting to read but having reread all total posts on this thread I am non the wiser what the meaning of OBT is as it was not, as far as I saw mentioned anywhere. Please correct me.
The use of three letter acronyms drives me crazy here in Germany zfr =zielfernrohr = (scope) etc. I use GRT most weeks too.

BTW....... I agree with you

 

[borbal]

* Enter a good approximation for your barrel length including muzzle break (and probably suppressor)

Are you sure about that? In OBT theory the reflections are timed against barrel length. But if you count a muzzle device the reflections would be barrel + device length, and the reflection would arrive on the node while the bullet in in free space, not at the muzzle, which is where the bullet last has contact with the barrel.

 

[Bavarianbrit]

Optimum Barrel Time, idea is that the projectile leaves the muzzle at the same point of each barrel oscillation in relation to the shock wave from the detonation travelling down the barrel, reaching the muzzle and bouncing back again, so that the oscillation at the muzzle is at the minimum at the point of exit of the bullet.
Funnily enough I've found that by tweaking burn rates to match observed velocity on QL and playing with the OBT charts, it does actually seem to work

Optimal Barrel Time Paper

Thanks, today was a school day for me. Thought I knew it all!!!!!!!

 

[jthyttin]

The use of three letter acronyms

Did you mean to say TLAs?

 

[Kevgun]

Thanks, today was a school day for me. Thought I knew it all!!!!!!!

Only my mrs knows it all

 

[Kimmeridgien]

Borbal, you have a point about whether to include muzzle break / suppressor or not. I don't know the answer, but you may very well be right.

It might also be that the muzzle break / suppressor adds to the factors behind the up-an-down movement of the exit end of the barrel but not the exit time vs the longitudinal shock wave that is OBT's domain. (The shock wave should continue into the muzzle break / suppressor and reflect at that end, which as you point out is not the same as the end of the barrel. I only have muzzle breaks that are fairly short, so I have no experience from what a suppressor would do.)

I'll try from another angle that is probably more accurate: With the help of GRT OBT simulation I have found Dan Newberry OCW nodes within +-0.5 or +-0.3 grains from the GRT OBT simulation charge weight. This narrows down the loads I test to fouler + 4x2 or fouler + 5x2 loads in 0.3 grain increments for .375 H&H Magnum. Within this range I have had no problems with longitudinal shock wave timing that would make the groups open up. (Hence, 2 cartridges per test load are sufficient.)

OCW is about finding a suitable range of charge weights where the end of the barrel changes direction (up->down or down->up), because at these points the POI differs only very slightly with a change of charge weight.

OBT is about finding points in time when the bullet exits the barrel that don't coincide with the longitudinal shockwave (bouncing forth and back) that comes from the start of the combustion of the powder (or possibly when the bullet hits the lands). This is related to group size. You can tune this by changing the seating depth (which affects the case volume and hence the charge weight, which can't be helped but is not a big deal). I haven't had any issues when using the manufacturers' recommended COL / COAL / total cartridge length.

OCW and OBT are somehow two peas from the same pod. I haven't dived further into this matter than what was necessary to use the GRT OBT tool successfully for load development.

Lancaster has already linked to the OBT paper. You will find the OCW information here:

Home

GRT is here:

start [Gordons Reloading Tool Community]

grtools.de

 

[Kimmeridgien]

I previously mentioned that the simulation gives an indication of which powders are most suitable. Although the ordering of the powders' muzzle velocity doesn't seem to be entirely correct, it reduces the powders to test to perhaps two or three. I only have access to VV and Norma, so there are not so many alternatives to simulate. Since the GRT OBT simulation is quite good at finding an approximation of the real (OCW) node, you can get an accurate real-life answer by only shooting fouler + e.g. 2 cartridges from each powder you are considering. If speed is a main concern, this will tell you which powder you should pursue first.

Speed is a main concern for many of the modern copper/gilding metal hunting bullets, because you want to ensure that the expansion will be sufficient for your hunting distance. You must typically optimise on speed for these bullets.

On the other hand, some of the lead-based bullets should have a much lower terminal velocity so that they don't over-expand or fragment. One of the latest additions to the hunting bullets is the long-range high-BC hunting bullet. As the name suggest, they are designed for optimal expansion at long range = low terminal velocity. If you use them for shorter-distance hunting (which in my case would be 100 meters +- 50 meters), you need to choose a powder and charge weight that results in such a low terminal velocity.

The simulation tools will provide valuable input when deciding which combinations to test. E.g. I have found that for TSX .375 H&H Magnum, N550 is the best for TSX 300 and TSX 270, N540 or possibly 203-B is best for TTSX 250, and it's a tie between N140, N150, and N540 for TSX 235. With the exception of TSX 300, all of these have an expected terminal velocity of at least 2200 fps at 150 meters with my Sako 85 (62 cm barrel). This is based on MagnetoSpeed v3 measurement and Norma's ballistic app. While the GRT OBT simulation for this caliber is off in terms of simulated muzzle velocity, it doesn't matter much for the usefulness of the tool. (I have come to use mainly VV powders, which is why I have considered Norma's powders only in a few cases. 203-B is "between" N540 and N550 in terms of real-world burn rate for this caliber.)

(150 meters corresponds to bullet drop within some 50 mm when sighted in at 100 meters, so up to this distance it's only a matter of aim and shoot when hunting. For the bullets I have been considering.)

 

[Deermanagement]

I am no reloading expert by any stretch of the imagination, however the idea of predicting the chances of producing an accurate load (one where bullet exits the muzzle at the point when the barrel is still and between oscillations) cannot be possible. I think this is being suggested but unless barrel contour, materials, tolerances, dimensions and the same for add on's (mods, brakes), are taken into account, all this before including the interaction between bullet tolerances, cartridge tolerance and load tolerances are included, and then simulated by the most sofisticated modelling software, that may or may not exist, rules out the idea it can be done by you or I.

I have done some 3d modelling of generally simple shapes to predict deflection under loads for various engineering models and they could only be used as a guide for design. Derterming actual barrel harmonics with limited information is impossible. I also wonder why people always refer to barrel oscillations as being "up and down"? If we were shooting rectangular barrels I could agree but round barrels will oscillate in all sorts of directions. Tell me if I'm wrong

Load up batches and test. There's no short cut.

 

[borbal]

OCW is about finding a suitable range of charge weights where the end of the barrel changes direction (up->down or down->up), because at these points the POI differs only very slightly with a change of charge weight.

I went to the link which explained OCW. It said there that Chris Long's explanation was the best on why it worked. "Chris Long, a friend and engineer has the most plausible idea as to why OCW loads work. He says that the acoustic shock wave which runs end-to-end down the barrel, repeating at around 18,000 feet per second, will disturb accuracy when it is near the muzzle."

Chris Long is vague about what kind of vibrations he is talking about, but radial vibrations and longitudinal vibrations run "end to end" at the speed of sound in steel, which is about 18,000 ft/sec.

But you are talking about "up->down or down->up" vibrations, which are transverse vibrations and they run much slower. The propagation velocity of transverse vibrations depends on the frequency, but for the first resonant mode of a transverse vibration on a typical rifle barrel the speed is about 360 ft/sec.

Is there some reason you are talking about a different kind of vibration than Chris Long?

 

[Lancaster]

I am no reloading expert by any stretch of the imagination, however the idea of predicting the chances of producing an accurate load (one where bullet exits the muzzle at the point when the barrel is still and between oscillations) cannot be possible. I think this is being suggested but unless barrel contour, materials, tolerances, dimensions and the same for add on's (mods, brakes), are taken into account, all this before including the interaction between bullet tolerances, cartridge tolerance and load tolerances are included, and then simulated by the most sofisticated modelling software, that may or may not exist, rules out the idea it can be done by you or I.

I have done some 3d modelling of generally simple shapes to predict deflection under loads for various engineering models and they could only be used as a guide for design. Derterming actual barrel harmonics with limited information is impossible. I also wonder why people always refer to barrel oscillations as being "up and down"? If we were shooting rectangular barrels I could agree but round barrels will oscillate in all sorts of directions. Tell me if I'm wrong

Load up batches and test. There's no short cut.

I'll give you a some examples here.

These are actual loads that have been worked up initially using QL and OBT tables to give a starting point, then tested and the most accurate loads input to GRT with the recorded velocity figures.
The OBT tool was then used to predict the charge needed to hit an accuracy node to find out if the tool could accurately suggest charge weights to hit a node when we already knew these were accurate and consistent loads.

.270 110gn TTSX, accuracy load is 63.7 Hunter, prediction from GRT for OBT to hit the nearest node was 63.93gn

.243 95gn Nos BT, accuracy load is 44.2 Hunter, prediction from GRT for OBT to hit the nearest node was 43.95gn

.222 52gn Berger FBHP, accuracy load is 23.4gn X terminator, prediction from GRT for OBT to hit the nearest node was 23.34gn

If it's not possible, why are the predictions so close to the real world figures?

 

[Kimmeridgien]

Lancaster:
Thank you for sharing some practical examples. My results from .375 H&H Magnum and 6,5x55SE are similar in nature.

Borbal:
Your remark and summary are correct:

OCW relates to resonant vibration of a string (such as in musical instruments or if you make a whipping motion with the garden hose). The string itself changes shape in a sine wave pattern.
The OCW method is simply a sequence of firing a set of rounds with increasing charge weight. You will typically notice that some sub-range of charge weights (perhaps two or three) will have the same POI. This is where the bullet exits the barrel when changing direction down->up or up->down. I haven't seen any simulation model for the OCW method (but it should be quite possible).

OBT relates to the expansion and contraction of the barrel tubular form. The shock wave traverses the barrel at the speed of sound of that metal or alloy (and is reflected at the ends until it dies out). You don't want the bullet to exit the barrel at the same time as the shock wave is at that very point, as it will kick the bullet in some more or less random direction. OBT is about finding the optimal time for the bullet exit.

Chris Long and Dan Newberry say that they realized that their two approaches came to the same or very similar practical results in terms of the predictions (my interpretation). I haven't had the time or inclination to try to understand why, but it doesn't matter so long as the result is practically useful.

After doing lots of simulations, with successively more accurate parameters found by firing test loads, I have found that the parameters I settled with are practically useful also for other bullets, bullet weights, powders, and case weights (<=> case volumes). This is really not so surprising. It is clear that the phenomena are subject to physical laws, even if we don't understand precisely how, e.g. regarding the shape or pattern made by the muzzle. The OBT simulations I made with GRT obviously give results that match what I get with the OCW method. These loads have not suffered from bad group sizes, so clearly they play well with OBT bullet exit optimality. Put differently: it is easy to see where the POI is on the muzzle pattern (the OCW domain), while you can probably only observe the OBT baddies where the groups open up.

Also, please note that before the scientific revolution, engineering wasn't about understanding why methods worked. It was about understanding what works and how to make practical use of this understanding. Before this there was only the so-called "brute force" method in which you tried anything that might please the Gods.

For those who would like an illustration of what you are trying to find with the OCW metod, please see the image included in the 21st post in this thread (top of the page I'm linking to):

You are being redirected...

The term "node" is actually a misnomer, as the top and bottom of the pattern are so-called "anti-nodes", but this can't be helped. The established terminology is what it is.

 

[borbal]

Borbal:
Your remark and summary are correct:

OCW relates to resonant vibration of a string ... This is where the bullet exits the barrel when changing direction down->up or up->down.

That is what you say. But the piece I quoted above, "Chris Long, a friend and engineer has the most plausible idea as to why OCW loads work. He says that the acoustic shock wave which runs end-to-end down the barrel, repeating at around 18,000 feet per second, will disturb accuracy when it is near the muzzle." is from Dan Newberry's own website. He says the vibrations are what Chris Long says they are - which is not what you say they are.

Now, I know you say you "haven't had the time or inclination to understand why", but I would appreciate it if you could at least say why you think the vibrations of concern are transverse vibrations and not the radial/longitudinal vibrations Chris Long is talking about.

 

[Kimmeridgien]

Borbal:
I'm afraid you will have to reread my posts until you understand them. I can teach you but I cannot make you understand.

 

[Lancaster]

I am no reloading expert by any stretch of the imagination, however the idea of predicting the chances of producing an accurate load (one where bullet exits the muzzle at the point when the barrel is still and between oscillations) cannot be possible. I think this is being suggested but unless barrel contour, materials, tolerances, dimensions and the same for add on's (mods, brakes), are taken into account, all this before including the interaction between bullet tolerances, cartridge tolerance and load tolerances are included, and then simulated by the most sofisticated modelling software, that may or may not exist, rules out the idea it can be done by you or I.

I have done some 3d modelling of generally simple shapes to predict deflection under loads for various engineering models and they could only be used as a guide for design. Derterming actual barrel harmonics with limited information is impossible. I also wonder why people always refer to barrel oscillations as being "up and down"? If we were shooting rectangular barrels I could agree but round barrels will oscillate in all sorts of directions. Tell me if I'm wrong

Load up batches and test. There's no short cut.

I've put up some examples of OBT in practice but wondered if this would help explain how I understand it and I could be completely wrong!!!

The oscillations up, down side to side in all directions are slower, lazier, waves that are effected by material composition, contour of the barrel etc and I believe that these are the waves that can cause POI to wander around the POA when working up a load as we try to find the sweet spot where the group forms consistently in the same place.
OBT is a much faster more energetic wave that oscillates back and forth along the barrel reflecting off the muzzle and breech at a pretty consistent speed of around 18000 fps which is unaffected by the contour, weight and composition of the steel the barrel is made of.
This wave squeezes the bore in an annular manner as it travels back and forth causing the bullet to speed up and slow down as it passes, a bit like squeezing a thin water pipe between finger and thumb which causes the flow of the water to pulse.
OBT aims to try and cause the bullet to exit the muzzle at the same point of the pulse/cycle every time you fire a shot, and this is why barrel length is a key factor when determining optimum bullet travel time along with burn rate and velocity within the barrel.
Like I say, I may be completely wrong, but that is my understanding of it.

 

[Kimmeridgien]

Thank you again, Lancaster.

The things going on in and with the barrel when firing a rifle are very complex. This much is AFAIK commonly accepted. I don't know whether it has been fully investigated and whether it is one phenomenon taking place in all rifles or whether there are more than one alternative phenomena between individual rifles/barrels. This is the main reason why I haven't spent time on trying to understand it further than what is necessary for making practical use of OBT simulations.

Regarding the barrel goings-on: Please see this link on barrel harmonics:

Barrel Harmonics Mode Shape Movies

I've settled with the "italic 8" pattern in the drawing by Dan Newberry in the link to Accurate Shooter I provided earlier in the thread. It is sufficient for picturing to myself what I am "looking for". This is the OCW domain of the topic.

To tune the group size you change seating depth, which affects the bullet's barrel exit visavi the longitudinal shockwave location that very instant. This is the OBT domain.

-

I would like to add a comment on the objections to the possibility that this could actually be real:

The OCW method really isn't much to discuss. It simply says to fire loads until you find a sub-range of charge weights with the same POI. As it happens, it matches Scott Satterlee's method of finding the nodes by shooting 1-shot loads over a chronometer looking for flat spots in the muzzle velocity graph (which is to be expected).

OBT theory has been found practically useful by so many people that at least QuickLoad and GRT have spent time and money adding this simulation feature. While this isn't proof that it is valid, it is at least a sign that it is worth considering. Yet the internet reloading forums are full of posts where people reject it solely based on their inability to imagine that it could be done. The only thing those people prove is that their powers of imagination are wanting.

Or, some people say that they have tried but it doesn't work. This is as impressive as some ninny rejecting the hammer on a conceptual level just because all he can manage is to pound his own thumb.

What must be realized is that being meticulous and systematic is key to almost any achievement.

If you invest some time in weight sorting your brass, identify the minimum and maximum brass case weights, then shoot say 5 of the minimum weight and 5 of the maximum weight, put the brass cases on your powder scales, measure the weight without water and compare it to the weight when filled exactly to the brim, you will learn the maximum and the minimum volume H2O your weapon will be working with. You can draw a straight line between these two min-max volumes and thus find the volume weight H2O for any case weight in between. You enter this volume weight H20 into your OBT simulation tool.

The weight : volume relationship isn't perfect - it's more of a "band" than a line - but as you will see there is a strong correlation. I have done a 25x5 set of 6,5x55SE brass of various brands and brass weights that clearly proves this. I will add a few more shortly. There is minuscule difference between brands. The same "band" goes through the entire lot. I have yet to understand what impact neck tension, different neck thickness, crimping, and so on might have.

I have done some similar testing on various .375 H&H Magnum brass. The early results are identical to the more comprehensive 6,5x55SE results.

Please note: I have generally found that manufacturers produce brass cases of highly different weight (= volume). It is not sufficient to restrict yourself to any one particular brand, thinking that this will let you shirk on volume measurement. You will get fliers if you do.

Put the measured volume weight H20, barrel length, and total cartridge length into your OBT simulation tool and it will give you a charge weight very close to the node you will find with the OCW method (or any other method).

If I understand Lancaster correctly, he has adjusted his powder properties to match muzzle velocity. I haven't done this, because I have mostly been developing loads for Barnes bullets, which are known to build higher pressures (as many other solid or thick-walled bullets). As I wrote earlier in this thread, the GRT OBT simulations give too high muzzle velocity by some 10%, but since I don't know whether this is a bullet modelling imperfection or a powder property variation I have worked around it by using the same max simulation pressure as the four loads Ladeboken provide for the TSX 270 bullet. The results are consistently very close to the real nodes, nonetheless.

 

[Lancaster]

@Kimmeridgien

"it matches Scott Satterlee's method of finding the nodes by shooting 1-shot loads over a chronometer looking for flat spots in the muzzle velocity graph (which is to be expected)."

Yep, sorry, rather skipped over that bit because the conversation had swung around to OBT....