Relation between case fill and pressure?
- Thread starter harry_x1
- Start date
More powder = more bang = more pressure -> Generally, if all other variables are equal.
However, if your case fill is too low, given that rounds are usually fired while horizontal - there can be a case where the primer flash can go above the settled powder and ignite way more powder that when the case is properly filled - which also leads to a dangerous pressure situation.
However, if your case fill is too low, given that rounds are usually fired while horizontal - there can be a case where the primer flash can go above the settled powder and ignite way more powder that when the case is properly filled - which also leads to a dangerous pressure situation.
Without going into problems with low fill percentages you (or someone) could probably tweak an internal ballistics program with varying case volume to see chamber pressure.
From Hodgdon Loading Data:
22 Nosler, same case length but more volume than a 223
A STARTING load for the 60gr SIE HP with Accurate 2015 is 23.6gr, 2811fps, 48,500 psi.
The loaded length is longer @ 2.260" than the length 2.120" given for the .223.
With the 223 the MAXIMUM listed load with Accurate 2015 is20.6gr, 2950fps, 47,800psi.
ERROR READING TABLE: 223, 60gr SIE HP is 22.6gr 2941fps, 55,000psi
The smaller volume of the 223 uses less powder, gives a higher velocity BUT a a higher pressure.
From Hodgdon Loading Data:
22 Nosler, same case length but more volume than a 223
A STARTING load for the 60gr SIE HP with Accurate 2015 is 23.6gr, 2811fps, 48,500 psi.
The loaded length is longer @ 2.260" than the length 2.120" given for the .223.
With the 223 the MAXIMUM listed load with Accurate 2015 is
ERROR READING TABLE: 223, 60gr SIE HP is 22.6gr 2941fps, 55,000psi
The smaller volume of the 223 uses less powder, gives a higher velocity BUT a a higher pressure.
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My QL CD is still couple of weeks from delivery...is there any other online program that I can use?
In the example given above, the 223 load is likely a very high percent of fill and the 22N much less fill.
I load two different cases for the 22N.
22 Nosler with Dogtown brass and 22 Nosgar, necked and bumped 6mm Hagar brass.
The Hagar is heavier and holds about 1.2 grains less water (bumped and trimmed).
Reducing the charge about 1 grain (Staball 6.5) in the 22Nosgar gives about the same velocity.
3000fps with Berger 85.5 @ the muzzle, and about 2500fps @ 300 yds, and 2040fps @ 600 yds.
Don't know the pressures.
I load two different cases for the 22N.
22 Nosler with Dogtown brass and 22 Nosgar, necked and bumped 6mm Hagar brass.
The Hagar is heavier and holds about 1.2 grains less water (bumped and trimmed).
Reducing the charge about 1 grain (Staball 6.5) in the 22Nosgar gives about the same velocity.
3000fps with Berger 85.5 @ the muzzle, and about 2500fps @ 300 yds, and 2040fps @ 600 yds.
Don't know the pressures.
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The optimum scenario is close to max fill, that creates 100% burn rate for your cartridge. Too hot of a powder and you don’t get the fill ratio. Too slow of a powder and you don’t get 100% burn.
I've loaded a .243 Win with RWS brass, and PMP brass. PMP brass is much thicker so it has less case capacity than RWS.
Sizing die, mandrel, primer, bullet, powder type, powder charge, seating depth - all the same.
The RWS brass gave me velocity around what the powder manual said it would.
The PMP brass was around 150 fps faster. I need to reduce the load by around 2 full grains with PMP brass to get the same velocity as with RWS.
Jager
Gold $$ Contributor
It's an interesting question. But there's a pretty direct relationship between powder quantity and fill ratio. If you exclude powder... you're pretty much stuck comparing different cartridges. Which introduces a whole bunch of other factors.
If you're trying to compare fill ratio changes in a single, given cartridge (without changing powder charge)... the only way to achieve that is to change powders to something with a different bulk density. But if you do that, you're almost certainly also changing the burn rate.
Kinda like whack-a-mole at the loading bench.
Many of us consider fill ratio to be a very important factor... but more in the context of accuracy than in chamber pressure. It's pretty easy hit whatever fill ratio you want to target... just by selecting the right powder.
Broadly speaking, I don't think there's much of a relationship between fill ratio, all by itself, and chamber pressure.
If you're trying to compare fill ratio changes in a single, given cartridge (without changing powder charge)... the only way to achieve that is to change powders to something with a different bulk density. But if you do that, you're almost certainly also changing the burn rate.
Kinda like whack-a-mole at the loading bench.
Many of us consider fill ratio to be a very important factor... but more in the context of accuracy than in chamber pressure. It's pretty easy hit whatever fill ratio you want to target... just by selecting the right powder.
Broadly speaking, I don't think there's much of a relationship between fill ratio, all by itself, and chamber pressure.
Gordons reloading tool
You can run a parametric powder search in both applications in which you set the parameters you want to meet. Also, not only will case volume change in various makes and lots of brass but the same powder and lot will burn differently in every rifle and the powder model Ba factor will need to be calibrated for each rifle, casing and bullet change
BTW, the most accurate load I ever found in my factory 243 with a 100 gr Hornady interlock was 35.5 gr Of IMR-3031 and it filled the case 77%. This load shot in one hole all day.
BTW, the most accurate load I ever found in my factory 243 with a 100 gr Hornady interlock was 35.5 gr Of IMR-3031 and it filled the case 77%. This load shot in one hole all day.
Look up BBT P-MAX
It has jived nicely with actual results and plays well with solids.
Hang on! If you change the case-size so that a particular weight of powder has a different fill-ratio, you have changed the case volume by definition, in other words you've altered the combustion chamber volume. That changes pressures, and it has nothing to do with fill-ratio.
As several people point out, different makes of brass for a single cartridge design often do this as a result of their construction and charges need to be adjusted as a result. Yes, fill-ratio changes with different case make volumes, but that's a side-effect and with the possible exception of compressed ball-type powders resulting from the change don't affect pressures. Traditionally, 100% fill-ratio or a shade under was regarded as desirable to promote consistent burn behaviours and pressure build-up, but Bryan Litz and his Applied Ballistics llc testers found no evidence (through ES/SD values) that 100% or nearly so levels performed better than others several percentage points lower in one of his Modern Advancements in Long Range Shooting books.
Wow, you're jumping into the deep end early.
As pointed out earlier, same amount of powder, lower fill percentage means more case capacity.
The direct answer to your question is the peak pressure will go down for powders and charge weights reasonably applicable to your application. With the larger case, you'll need to add powder to restore peak pressure. The larger case, using more powder to restore the peak pressure to previous levels, will have a higher velocity because the larger starting volume and increased powder mass will maintain a higher average pressure in the barrel.
For the same barrel length, increasing the mean pressure doesn't have nearly as strong effect on the velocity as the peak pressure. Rifle bullets fired from high performance cartridges typically acquire about 1/3 their final velocity by peak pressure. This is the core of the overbore problem. You can partially offset it by using more of a slower powder in the larger case, but you're still working on the average pressure.
QL is not a reloading manual. With the 33XC you're working with, it's a bit like a high performance motorcycle. It'll launch you over a cliff as cheerfully as it'll take you down the road. It will provide a lot more warning than the internet that you're doing something stupid if you pay attention. A lot of the information out there on the 33XC is somewhere between irrational exuberance and really stupid. The core fallacies seem to be maximum velocities give maximum performance and the bullets will operate well at any pressure the case will take. With the Tubb brass and jacketed bullets, it's straight forward to demonstrate that neither is true but you'll have to do your testing at over a mile to see it. The Tubb brass will hold primers after repeated firings at eye watering pressures. When your copy of QL arrives, use 0.35 as the weighting factor, 12,000 psi for the start pressure and fill in the rest of the required data from your ammo. Adjust the start pressure to match the velocities for your ammo. By the time you're to 16-18,000 psi for the start pressure to get the velocity to line up, the barrel is either done or you've entered a feedback loop. The start pressure drives up the peak pressure which drives up the start pressure which drives up the peak pressure, ....... At that point, you're exiting the exuberance phase. The first time you do that, you're going to be very disappointed with the velocities.
The velocities need to be recent because with overbore ELR cartridges they will rise until the barrel dies if you maintain constant jump. With a cartridge as overbore as the 33XC, you'll need to adjust the load every box of 50. My first one was good for 300 rounds before the performance beyond a mile deteriorated noticeably. 100 yard accuracy was done by 450. What kills the extended long range performance isn't the loss of precision, it's the contribution of the barrel to BC variation and it's impossible to discern inside of a mile. That was my first barrel with the cartridge and I viewed it basically as a lab rat. Early on, I shot a lot of high pressure rounds in 10 shot strings using double base powders to generate performance data to select bullets with.
On barrel life, there is no point in trying to preserve it. You can extend it some, but why? A barrel's life should be judged on what you learned from it or how many hits it provided. Test it like you intend to use it. If you're going to shoot a 10 shot course of fire, shoot 10 shot groups and evaluate velocity spreads from a 10 shot perspective. If that seems like it's going to hurt too much, move back to a 300 Norma or 300 PRC to do your rites of passage. That's not intended as a slam.
If you buy, read and internalize the latest Litz book, you'll save yourself a lot of time and components.
Ned Ludd
Silver $$ Contributor
There are some important caveats/considerations here. What is not changing in any of these scenarios is the volume of the chamber itself, which remains constant. In the scenario of using different brands of brass that actually have different case wall thickness, the final internal volume (i.e. the pressure cell volume) would be different as the two brands cases with differing wall thickness expanded fully to fit the chamber. Higher pressure would result from igniting the same weight of powder in the case with thicker walls (i.e. smaller internal volume). This is somewhat of an over-simplification as there could also be some expansion differences as described next. I think the prediction for this scenario would most likely be greater area under the pressure curve for the case with thicker walls (i.e. lesser internal volume).
A slightly different scenario would be the difference between virgin brass and fire-formed brass of the same manufacturer/Lot. Because the wall thickness would be the same (unlike the scenario with different brands of brass that have different case wall thickness), both virgin and fire-formed brass of the same manufacturer/Lot would ultimately expand under the influence of 50-60K psi to reach the same final pressure cell volume, regardless of the initial difference in internal capacity between virgin and fire-formed brass. So the difference in this scenario would not be the final pressure cell volume, but the amount of energy required to expand the two different states of the brass (virgin versus fire-formed) to fit the chamber. The final pressure cell volume would be the same. I'm not exactly sure how this would affect the pressure curve (i.e. area under the curve/peak height), but part of the difference would certainly be due to the extra energy necessary to fully expand the virgin brass to fit the chamber. That is not quite the same as the first scenario where the primary difference would be due to an actual difference in the pressure cell volume.
A slightly different scenario would be the difference between virgin brass and fire-formed brass of the same manufacturer/Lot. Because the wall thickness would be the same (unlike the scenario with different brands of brass that have different case wall thickness), both virgin and fire-formed brass of the same manufacturer/Lot would ultimately expand under the influence of 50-60K psi to reach the same final pressure cell volume, regardless of the initial difference in internal capacity between virgin and fire-formed brass. So the difference in this scenario would not be the final pressure cell volume, but the amount of energy required to expand the two different states of the brass (virgin versus fire-formed) to fit the chamber. The final pressure cell volume would be the same. I'm not exactly sure how this would affect the pressure curve (i.e. area under the curve/peak height), but part of the difference would certainly be due to the extra energy necessary to fully expand the virgin brass to fit the chamber. That is not quite the same as the first scenario where the primary difference would be due to an actual difference in the pressure cell volume.
USMCDOC
Silver $$ Contributor
Gordon's Reloading Tool and it is free..
100% correct, to my knowledge.
I think one of those caveats is that the burn rate of powder is itself sensitive to pressure. So on the one hand, burn rate drives pressure. On the other, pressure drives burn rate. There’s a feedback element to this that can make it difficult to generalize a bit.
So while it’s true that the larger case with the same charge will have less PEAK pressure, it’s not just because of the case volume in terms of PV=NRT. The time and burn rate aspect of this is critical too.
When the primer flashes and powder ignites, pressure will rise within the case until the case expands and bullet releases (think of this as “combustion chamber #1.” But then the bullet briefly pauses in the rifling as pressure rises far enough to engrave the bullet and obdurate it to the bore. The “pause” while pressure builds is “combustion chamber #2”. The volume of both of these “chambers” is relevant to pressure. Jamming bullets in the rifle raises peak pressure because it essentially merges the two separate chambers and their pressure rises into one event with faster burn (which means more peak pressure for the same area under the curve. This is also (IMO) why longer throats lower pressure— they allow for more expansion before “chamber #2” is formed in addition to lowering the engraving/obdurating force required because the bullet has a flying head start at the rifling.
The case volume matters because it determines the initial slope of this pressure rise by slowing or accelerating the initial rate of combustion.
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