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Discussion Starter #1 (Edited)
I hope this is the right sub-forum...

The basic question is what role does case shape play in chamber pressure and choosing the right powder?

Here's the long version and there are two parts to my question:

Consider two cartridges with identical case capacities (my limited observations are coming from working up loads for my .358 WSSM wildcat).

1) It seems to me that if the powder burns in more or less in a spherical pattern that a short fat powder column would burn faster than a long thin powder column, and that the shorter fatter powder column would build pressure faster than the cartridge with the longer thinner powder column.

In my observations, my .358 WSSM has identical measured case capacity to the .358 Winchester at 46.8gr of H2O. Using .358 Win load data, I'm not generally able to get to max loads before seeing pressure signs (hard bolt lift). I will then back off half a grain and fire a few rounds to make sure there are no pressure signs. I'm also measuring case head diameter to 0.0001" to make sure it does not change.

This observation (pressure signs before max load is reached) leads me to my assumption #1, and it would be a bad thing to ignore the elastic limits of steel and brass. I'm also observing that my muzzle velocities are a good indication that I am getting similar max chamber pressure to the published load data after correcting for a 4" difference between my barrel and the test barrel, and usually doing a bit better than 25fps/inch lost in barrel length. In one case I am matching MV of the published load regardless of the shorter barrel. This might be expected owing that many people believe the short fat powder column is more efficient.

Due to this faster, more efficient burn, would I be correct to say that I could get even better performance in the .358 WSSM by going to an even slower powder than what might be recommended for .358 Win?

2) My second question is probably more academic. I am an engineer and I have discussed this with my B-I-L who is also an engineer who agrees. Pressure is force per unit area, and assuming that the laws of physics are the same in a rifle chamber... I made a rough calculation of the surface area of both rifle cases in question (.358 WSSM and .358 Win). I don't have the numbers in front of me but the difference in surface area was not insiginificant. If P=F/A then F=P*A and the overall force on the steel of the chamber of the .358 WSSM would be less than that of the .358 Win, effectively making the WSSM rifle "stronger" and able to handle a higher chamber pressure using the same steel.

Is this (in addition to the the thickness of the brass case) what allows the .243 WSSM to have a SAAMI limit of 65kpsi while the .243 Win, which has a nearly identical case capacity, has a limit of 60kpsi?
 

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Is this (in addition to the the thickness of the brass case) what allows the .243 WSSM to have a SAAMI limit of 65kpsi while the .243 Win, which has a nearly identical case capacity, has a limit of 60kpsi?


My guess on this particular question in your post would be that the design of the chamber and cartridge is not what makes it stronger (im sure the thick brass might come into play), but the manufacturer designed those particular actions to withstand those pressures in order to get the published velocities, dont worry somone more knoledgable will be a long soon ;).
 

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I think it is the thickness/strength of the brass (along with surface area) that defines the difference between the 243 WSSM and 243 Winchester, not the other way around. Most of the actions both of these cartridges are chambered in are capable of withstanding substantially more than 65kpsi, irrespective of chamber surface area. The case is supposed to be the weakest link in the chain, by design. Also, there is a limit to the ductility of brass, and 65kpsi is really pushing it. That's why you haven't seen that number go any higher, even though rifles could easily be built to withstand significantly higher pressure. 65,000psi has been the upper limit for decades and it won't go appreciably higher, regardless of case length or design.
 

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Going to have to agree with broom-jm on this one.... also believe it's the case thickness and-or alloy. Doubt the case shape has anything to do with pressure ratings and don't believe that the rifle actions are any different.
 

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Kludge,

Several complicating factors occur in the comparison you are making. For one, a short, fat case, not including the bullet, has less distance from the primer at its extremes than a long, skinny case of the same water capacity does. Thus, a flame front can move throughout the short fat case faster. For that reason it is common that a slower powder be used in the short, fat case, than in the long, skinny one.

Efficient: .257-40mm Grenade





Inefficient: .50-08:





Another element affecting things is what the QuickLOAD internal ballistics program calls the weighting factor. This is the percentage of the powder charge that gets pushed down the bore with the bullet at firing. Since the flame and pressure start at the rear, this portion of the powder that chases the bullet is harder to ignite, delaying total powder ignition and moving the pressure peak further along in the time line. This weighting factor gets larger as the bullet gets closer to the diameter of the case body. QuckLOAD defaults to numbers in the 70% range for straight wall cases and in the 50% range for most bottleneck cases. But even among those the actual percentage will be smaller as the case gets wider relative to the bullet.
 

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OK, Nick...help me understand how what you just wrote correlates to the actual cartridge in question. The 243WSSM case has been necked up to shoot a 35 caliber bullet. Are you saying that the short-fat powder column is having to chase that big bullet down the bore? Is it safe to say that the mass, or moment of inertia, is greater with the larger, heavier bullet, resulting in higher chamber pressures? OR, does the larger bore area, in this case, mitigate that rise in pressure?

Fundamentally, is it the BRASS that determines what pressure can be tolerated, or the firing chamber the brass is fed into?
 

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I am not an engineer (but I translated for them on TV when I was at NASA!). One factor that determines the maximum pressure a cartridge is rated for is the age of the rifles chambered for it. The 30-06 has been slowly but steadily downrated over the decades because there are rifles for it over a century old now. Rifles are not cheese and do not get stronger with age. That's why the 270 has a higher pressure ceiling than the '06: the oldest rifles factory chambered for it are 35 years younger. One other example is the 280 Rem, which originally was chambered in pump rifles as well as bolt actions. The pump was considered weaker and so the round has a decidedly lower maximum SAAMI pressure rating. That's why the wunderkind cartridges of the past few years can have very high pressure ceilings: no rifle for them is older than the round is.

As to shape and such, what Nick says is (to the best of my non-technical background) spot on. I have a much lower appreciation of Quickload than Nick does, having personally seen it predict things that were wildly off. But the facts are that a short, fat powder column is ignited much more thoroughly and in greater percentage than a long thin powder column. In bottleneck cases, there is probably a plug of powder that is compressed almost to a solid behind the bullet. That plug burns (if at all) only on the exposed rear surface as it goes down the bore like a wad behind the bullet. A short powder column and a bullet seated to the base of the neck minimize or eliminate that effect, making the round more "efficient" at burning all the powder. That's just one effect of case shape, and there are probably many others.
 

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Rocky, the 30-06 loadings have increased over the years, not decreased. The original 30-06 loadings was about the same speed as a 308 today


As issued in 1906 the rimless cartridge held a 150-grain spitzer, flat-base cupronickel jacketed bullet with 2700 fps muzzle velocity. In 1926, to improve machine gun effective range, the bullet was replaced by a 172-grain 9-degree boattail design with the same 2700 fps at the muzzle, designated the "Ball, caliber 30, M1." The velocity was reduced for a time to 2640 fps, but in 1938, as the gas-operated Garand came into service, the specs returned to the flat-base 150-grain loading, called the "Ball, caliber 30, M2" round. It was the M2 that accounted for most of the ammunition expended in World War II.

http://www.olive-drab.com/od_firearms_ammo_30-06.php
 

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Discussion Starter #9
Nick, great drawings and explanation.

jm, Not pretending to be an expert here, so see if this makes sense. If I were to take the same powder charge behind two equal weight (mass) bullets, one is a .243 bullet and one is a .358 bullet, the force on base of the .358" bullet - at the moment of ignition - will be grater than the force on the .243 bullet by the ratio of the sqaures of the bullet diameters (.358^2/.243^2) = 2.17. So the .358, from F=ma, will start its initial acceleration down the bore at 2.17x the acceleration of the .243 bullet.

However due to the size of the bore, the pressure is dropping (or not rising) much faster in the .358 bore since the volume of the bore is also 2.17 times as big, but we obviously can't fit 2x the powder to match the pressure curve of the .243.

So to compensate we make the bullet heavier. If we make it 2.17x as massive we will get identical acceleration at the moment of ignition - but we still haven't compensated for the volume of the bore, the pressure is still going to drop faster. To get the same pressure curve we have to compensate for both the area of the base of the bullet and the volume of the bore with an even heavier bullet. At this point there's now no way to match the velocity of the .243.

And we still haven't taken into account that the bullet friction in the bore on a .358 is going to be roughly .358/.243 times what it is in the .243 bore, which will further oppose the force of acceleration. Can we ever match the .243 velocity? Sure but we need to increase the case capacity to account for the extra energey needed - think .50 BMG, a scaled up .30-06.

To your question, yes, the large bore does mitigate the pressure rise - this is how it was done with black powder for a long time.

As far as whether it's the brass or the chamber that limits chamber pressure, I think it has to be both, but I think if we could lock the breech the same way we do on a big gun (which don't use cases) it would be all chamber. My reasoning: The brass is always going to stretch more easily than the chamber. If the chamber can fully support and prevent the brass from stecthing past the point of elasticity, it will always spring back enough to be extracted. But at some point the chamber will strecth and the brass will strecth with it. Then when the pressure is gone the chamber returns to its pre-stretched size but the brass cannot, the elastic limits of the case were exceeded and the case is stuck in the chamber. The chamber allowing the brass to stretch further than it can spring back is the definition of over pressure (assuming that the chamber/breech can prevent things like primer blowout, launching the bolt toward your face, etc.). The chamber will hold until it's tensile strength is exceeded

To me it explains the large shank Savage barrles.
 

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With my non-degreed engineer thinking, there was a published article some years ago when the "short, fat" cartridges were just gaining popularity, that the fatter case with the sharper shoulder allowed more efficient powder burn while the projectile was still more or less in the case neck because the sharper shoulder held up the unburned propellant longer in the case.

This caused my pulse to beat rapidly and I rushed to my local gunsmith to have a Ruger M77 (tang safety) rebarreled and chambered for the proprietary 7mm Dakota cartridge, which is a .404 Jeffry case shortened and necked to .284. With a 24" Shilen barrel and slow burn rate powders (VV N170, RL-25) and 175 gr flat base bullets, there doesn't appear to be any unburnt powder leaving the bore off the bench.
 

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Jwp475,

I think you may be confusing psi, as determined by Piezo transducer, with psi, as determined by copper crusher. Today they are distinguished as psi and cup. Before the non-conformal Piezo transducers were developed, the result of copper crusher tests were reported as psi because it was believed they were measuring accurately. When better instrumentation became available, it revealed copper crushers typically ran about 15-20% low in the .30-06 pressure range. The military persisted in the fiction that copper crushers gave real psi for a couple of decades after commercial ammo makers wised up. The military didn't wise up until the mid-1990's. This is why you see military 7.62 NATO specs from then and earlier giving peak pressures of 50-55,000 psi (depending on the specific round), when SAAMI specified .308 Win at 62,000 psi. A lot of folks thought that meant commercial loads were unsafe in military guns because of the pressure difference, when, in fact, there was no difference in the upper limits if you measured them the same way. And, in reality, military ammo, which is loaded right up close to rated pressure while commercial makers are often conservative, is typically actually the warmer of the two.


Jason,

The brass is the seal, but the guns vary in how they support it. The Springfield '03, for example, leaves a short unsupported area under the case head, so you can run higher peak pressures safely with the same brass in, say, a Remington or even a Garand action than you can a Springfield before the heads blow out and destroy the magazine area and the stock.

Probably a more common example is brass sticking in light weight revolvers before any kind of pressure sign appears on the brass. Cylinders are often quite thin at the outer diameter of the cylinder, and sticking happens when the steel, which is more elastic than brass, stretches beyond the elastic limit of the brass, then returns to shape over top of the fattened brass. That traps the brass, making extraction sticky. Brass can usually spring back about a thousandth of an inch after a single fireforming, so this indicates the steel is stretching more than a thousandth. It is possible in some instances to actually burst a revolver cylinder without a lot of primer flattening or head flow. The gaps for the head are tighter in revolvers than in many other guns. So it is an example of the steel design being critical to containing pressure together with the brass seal.

As to the powder pressure, consider a cylinder that is open at one end and another that is closed at one end except for a small hole. We put the same amount of the same powder in both and ignite both. Which sees the higher pressure? Obviously the one with the small hole represents better containment. In the case of the cylinder with a hole, if you elongate that while maintaining the same volume, its diameter eventually reaches that of the hole, and you are back to an open-ended cylinder. And these are just fluid (propellant gas) flow considerations. We haven't stuck a bullet in yet. When we do, the same degree of resistance to the expanding gases requires the same sectional density to get the same pressure to apply the same accelerating force per unit bullet area. So, given the diameter difference and ignoring the fluid dynamics, the wider bullet has to be heavier in proportion to its cross sectional area just to break even on inertial resistance to acceleration.

As a result of the above, when you look at large diameter and relatively straight cases, like the .45-70, you typically see faster powders used than you do for same volume bottleneck cases with lighter bullets. You have to add the case shape's fluid constriction resistance to the bullet's inertial resistance to figure out what the powder is building pressure against.


Rocky,

QuickLOAD's modeling method works well, but obeys the old computer programming adage, garbage-in, garbage out. I have yet to find a situation where, if the input data is truly correct, the program does not do a pretty good job of reflecting reality. Where reflecting reality can get difficult is when powder lots change significantly or when when firing dynamics aren't quite conventional.

An example of the first of the above would be the complaints about lot consistency long range shooters have had in the past with Varget. QuickLOAD's models are built mainly by testing ADI powders. The models for Varget and AR2208, which should be the same because it's the same powder in different packages, were not the same in earlier QuickLOAD versions (they are in the current version). That reflected what Mr. Broemel bought to test at the time he first did it. As a result, lots of advice to use the AR2208 model for current Varget appeared, and that seems to work until Broemel updated the Varget model.

An example of the second sort of problem has come up on another forum where a member is using Vihtavuori N310 to propel a 200 grain LSWC. VV's databook shows a maximum charge of 4 grains giving 892 fps from a 6" tube. QuickLOAD predicted 896 fps and about 17,900 psi. Pretty good agreement. Change the tube down to 5" and it's about 867 fps. But this fellow has worked up to 4.5 grains and is seeing only 780 fps from a 5" barrel. That should correspond to just about 9,000 psi.

Why is his reality so different from not only the QuickLOAD prediction but also from VV's measured data? Well, he's using a 5" Wilson match barrel. The match barrel has a longer, more tapered throat than a standard 1911 barrel. Apparently that soft lead bullet is being unseated and driven into the throat by the primer before the powder comes fully up to pressure (and which may be a magnum primer; haven't heard back on that yet). So the powder is building pressure against a moving bullet. That's harder to do than when you have good start pressure, so the system behaves as if the case volume were almost twice what it actually is. This can happen with small case volumes and a lubricated lead bullet without a tight roll crimp.

That kind of stuff are just examples of the kinds of pratfalls you have to watch out for, but note that you can encounter them either with QuickLOAD's predictions or with manual data. It also illustrates that chronograph feedback is pretty important to getting the most out of the software. There are now a number of write-ups appearing online for how to use the program more effectively. It's not entirely user friendly, but I don't think it ever pretended to be and I don't think the input parameters can be simplified enough so it is. Attempting to do that brings you to something more like Load From a Disk, which works, but only with some powders that you can't adjust or add to, and only within a limited range of percent case fill that does not include compressed loads. That sort of limitation will still let a lot of folks use the software who might get tangled up in QuickLOAD's more comprehensive set of arguments, so I think it serves a useful purpose. But if you want more complete control over what you're doing, you have to educate yourself about the dynamics or at least do chronograph validations to stay out of trouble. There's just no getting around that.
 

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This is the percentage of the powder charge that gets pushed down the bore with the bullet at firing. Since the flame and pressure start at the rear, this portion of the powder that chases the bullet is harder to ignite, delaying total powder ignition and moving the pressure peak further along in the time line.
Rocky Raab said:
But the facts are that a short, fat powder column is ignited much more thoroughly and in greater percentage than a long thin powder column.
I have a vague memory of Elmer Keith's writings (in the '50s about work in the '30s?) on the .333OKH that had a long thin tube soldered(?) to the flash hole to move the primer flame to start the powder burning at the base of the bullet and progress backward. Apparently it didn't work as imagined, but based on these quotes, is it an idea to be tried again with the slower burning powders available now?

Just wondering. . .

The Old Guy
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Jwp475,

I think you may be confusing psi, as determined by Piezo transducer, with psi, as determined by copper crusher. Today they are distinguished as psi and cup. Before the non-conformal Piezo transducers were developed, the result of copper crusher tests were reported as psi because it was believed they were measuring accurately, when better instrumentation later revealed they typically they ran about 15-20% low in the .30-06 pressure range. The military persisted in the fiction that copper crushers gave real psi for a couple of decades after commercial ammo makers wised up, and didn't stop until the mid-1990's. This is why you see military 7.62 NATO specs from that time giving peak pressures of 50-55,000 psi (depending on the specific round), when SAAMI specified .308 Win at 62,000 psi. A lot of folks thought that meant commercial loads were unsafe in military guns because of the pressure difference, when, in fact, there was no difference in the upper limits if you measured them the same way. And in reality, military ammo, which is loaded right up close to rated pressure while commercial makers are often conservative with their loads, is typically actually the warmer of the two.

Not confusing the two at all, just going by the 150 grain load in the 30-06 being 2700 FPS in the begaining and is 2900+ today
 

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I'm not talking about velocity, but SAAMI pressure limits. They have been lowered for the 30-06 over the decades. Better powders are what permit it to get higher velocity at lower pressure than when it was introduced. Back then, a powder about like 3031 was the "slow" powder until 4895 was developed specifically for the '06. Even now, you can't get much velocity with 3031 before you hit the pressure ceiling.

Elmer's flashtube idea was one place where his creative juices dried up. The tubes burned through in no time; it was extremely difficult to get powder in the case without filling the tube as well; and an empty tube took up so much case volume that it was like shooting reduced charges. In the end, it was a dumb idea.
 

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Ditto on pressure vs. velocity. The modern powders keep doing more with lower peak pressures by delaying the peak until the bullet has moved further and creating more total gas that keeps pressures higher toward the muzzle. So, bullet acceleration is higher after the peak than it is with faster powders. Hodgdon's load data, for example, has a 150 grain bullet at 3005 fps using H380 loaded to 45,900 cup, while Winchester 748 at 46,000 cup only gets to 2,810 fps with all other variables the same in 30-06.

Remember, it is the average pressure during barrel time and not the peak pressure that determines velocity.

One other factor that erases half the velocity difference with the old loads is that commercial load velocities are measured at 15 feet from the muzzle while the military uses 78 feet from the muzzle for rifle rounds. For a flat base 150 grain bullet, that accounts for almost 50 fps of difference.

From Hatcher's Book of the Garand:

M1906 ball, 150 grain bullet, 2700 fps, Pyro DG powder at 50,000 cup MAP
M2 ball, 152 grain bullet, 2805 fps, IMR powder at 50,000 cup MAP

(Note: I changed Hatcher's psi to cup to reflect the equipment they used for pressure determinations.)

M1906 ball was the WWI load. M2 was developed in the late 1930's as a National Guard substitute for M1 ball. If these rounds had been measured by a modern commercial maker at 15 feet, one of my ballistics programs says they would have come out as:

M1906, 2742 fps
M2, 2860 fps

So, by the time WWII broke out, powder was already getting up to some of the velocities we have today, but still needing higher peak pressure to do it.
 

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I didnt read word for word all the other's response's so I might be redundant on some points. Short fat case and powder columns , I belive started with the BR crowd in the 22 and 6mm BR, Theory of such, I believe is to obtain a more consistant pressure curve due to more consistant or even ignition, aiding accuracy. Don't know how one would prove or disprove however.

Brass can add to the strength of the receiver, but to what degree is hard for us handloaders to deterime without pressure testing equipment. I think brass life is more the factor for us handloaders with thicker stronger brass.
I think we all know and agree the each firearm is it's own boss when it comes to max loads. I think why you are not reaching max with the one, is likely due to on or both the chamber dimensions are tight, or the barrel is tight. Or both. If you still have room in the case going to slower powders is certainly an option I would investigate. I have done this for many that I have. One reason is I tend to get better accuracy with full cases and more often than not I get more velocity.
example,,,,
I have a 93X74R in a tight chambered Encore, from a Douglas blank, also noted for being tight on dimensions as well. I started with the Nosler 250 BT and their load data with RL-15. I had sticky issues at the starting load and running 100fps under max velocity. I switched to H4350, with roughly same results. On to H4831 and 66 grs, not much better, Still wanting to see, I went outside any data using my last charge weight 4831, and went to RL-22. I got to the max charge weight listed for H4350 of 68 grs. with RL-22. I had no pressure signs or issues, yet my velocity ended up about 200 fps faster than book. Neither 4831 or RL-22 are listed for a 250, Accuracy was running 1/3 to 1/2 moa on a bad day, and that is more than I hoped for. Some day I may experiment with slower powder yet, as I still have room.
I might can account for some of the speed due to a longer throat, but likely moreso the tight specs on the chamber itself, and I'm sure I'm above the spec'd 40,000cup, but I know the Encore will handle it. And after 5 loads, brass that is noted for short life, is still going with tight primer pockets.

Dave
 

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Trapdoor action. ;)
 

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2) My second question is probably more academic. I am an engineer and I have discussed this with my B-I-L who is also an engineer who agrees. Pressure is force per unit area, and assuming that the laws of physics are the same in a rifle chamber... I made a rough calculation of the surface area of both rifle cases in question (.358 WSSM and .358 Win). I don't have the numbers in front of me but the difference in surface area was not insiginificant. If P=F/A then F=P*A and the overall force on the steel of the chamber of the .358 WSSM would be less than that of the .358 Win, effectively making the WSSM rifle "stronger" and able to handle a higher chamber pressure using the same steel.
I agree with this.

Is this (in addition to the the thickness of the brass case) what allows the .243 WSSM to have a SAAMI limit of 65kpsi while the .243 Win, which has a nearly identical case capacity, has a limit of 60kpsi?
No way! I believe the issue is that you switched over from talking/thinking/calculating about equal force resulting in lower pressure, but then asking if that's why pressure could be higher. :)
 

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Frontal ignition does work. It is widely used in artillery rounds. The problem is that with small cases the tube itself occupies a disproportionately large % of the potential case volume requiring a larger case to offset the lost capacity. It also complicates case design, case charging and for our purposes depriming. The question becomes whether it is worth the hassle.
 
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