Case Volume Effect

[Tiratore]

When changing brass brands ... If a new batch has an internal volume 3% less than the old batch, how much should the powder charge be reduced to have the same peak pressure? What would be the change for the same muzzle velocity?

Tiratore

 

[savagedasher]

When changing brass brands ... If a new batch has an internal volume 3% less than the old batch, how much should the powder charge be reduced to have the same peak pressure? What would be the change for the same muzzle velocity?

Tiratore

3% would be a good start . Larry

 

[Cowtownup]

Using data from my straight 284 and Quickload....

66.85 case volume with 55 grain H4831 = 57,833 psi @ 2809 fps

If I drop case volume 3% to 64.85 just as in your scenario...

64.85 case volume with 54 grains H4831 = 57,851 psi @ 2802 fps

Therefore, a 3% drop in case volume on a 284 Winchester will need a 1.8% drop in powder charge to maintain similar pressure and velocity. Lots of variables here, I just threw something quick together to help you along....

 

[savagedasher]

Using data from my straight 284 and Quickload....

66.85 case volume with 55 grain H4831 = 57,833 psi @ 2809 fps

If I drop case volume 3% to 64.85 just as in your scenario...

64.85 case volume with 54 grains H4831 = 57,851 psi @ 2802 fps

Therefore, a 3% drop in case volume on a 284 Winchester will need a 1.8% drop in powder charge to maintain similar pressure and velocity. Lots of variables here, I just threw something quick together to help you along....

Then 3% is still safe on a 284 what is the amount on a ppc . And wouldn't that change with a different powder say faster or slower burn rate ? Way to opean ended question for me . Larry

 

[Twoboxer]

When changing brass brands ... If a new batch has an internal volume 3% less than the old batch, how much should the powder charge be reduced to have the same peak pressure? What would be the change for the same muzzle velocity?

Tiratore

Have both your "new" brass and your "old" brass been fired out of your chamber and trimmed to the same length? If not, there may be no difference at all.

 

[Tiratore]

Yes, fired cases in the same rifle trimmed to the same length.

One source says peak pressure is inversely proportional to case volume to the base of the bullet. Furthermore, the same source says pressure is proportional the 4th power of charge.

Since peak pressure doesn't occur when the bullet in the case, I find the statement hard to believe. Hence my question.

Tiratore

 

[Twoboxer]

. . . One source says peak pressure is inversely proportional to case volume to the base of the bullet. Furthermore, the same source says pressure is proportional the 4th power of charge.

Since peak pressure doesn't occur when the bullet in the case, I find the statement hard to believe. Hence my question.

Tiratore

Assuming identical bullets are the same distance from the lands in each case, all the factors besides chamber volume remain the same.

If two cases with the same external dimensions have 3% different volumes, the difference can only be due to thinner case walls. Which makes the chamber larger. As the bullet moves out, that "larger" chamber is always behind it, and all else remains the same. The pressure created from the same powder charge must be lower out of the larger chamber.

Since physics is involved and only one variable is changed, the pressure change should be proportional to something lol. Whether that's the 4th power of the charge or not is beyond the training or memory of this old, 1-year physics major lol.

 

[ireload2]

Check out the ideal gas law for a good approximation of what the pressure, and temperature relationships are.
At very small increments calculus says the results will be approximately linear.

The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law and Avogadro's Law.[1] The ideal gas law is often written as

PV=nRT,

where:

P is the pressure of the gas,
V is the volume of the gas,
n is the amount of substance of gas (in moles),
R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
T is the absolute temperature of the gas.

It can also be derived microscopically from kinetic theory, as was achieved (apparently independently) by August Krönig in 1856[2] and Rudolf Clausius in 1857.[3]

 

[Tiratore]

Yes, but -
Peak pressure does not occur when the gas volume is the volume of the inside of the case. Some model that includes bullet movement and gas production from solid powder would be required, hence the Quickload numbers should provide a more correct relationship.

Thanks to cowtownup.

Tiratore

 

[Ned Ludd]

If you have QuickLoad, let the program give you a good working value based on a 3% decrease in case volume>>>adjust charge weight to give you the same barrel time (not necessarily the same muzzle velocity) at the reduced case volume.

If you don't have access to QL, the easiest approach is simply carry out a charge weight test (fine increment) starting perhaps 4% lower and working up. Unfortunately, you're dealing with responses that aren't necessarily linear, and therefore don't always correlate on a 1:1 basis.

 

[SheepDog]

If a case has 3% less internal volume then to get the same pressure with everything else the same you need to add about 3% more powder.
The problem you run into is that adding that additional powder will cause more velocity because there is a longer push on the bullet.
If it is important to keep the same velocity then use the same amount of powder. Even though the pressure is reduced the bullet will have very close to the same velocity.
For most shooters this problem is resolved by sorting cases by their volume. (after firing and uniforming them)

 

[Ned Ludd]

If a case has 3% less internal volume then to get the same pressure with everything else the same you need to add about 3% more powder.
The problem you run into is that adding that additional powder will cause more velocity because there is a longer push on the bullet.
If it is important to keep the same velocity then use the same amount of powder. Even though the pressure is reduced the bullet will have very close to the same velocity.
For most shooters this problem is resolved by sorting cases by their volume. (after firing and uniforming them)

I don't think so. Adding more powder to a smaller case volume (pressure cell volume) will increase the pressure significantly, not decrease it. Keeping constant velocity/pressure with changing case volume requires charge weight to change in the same direction as case volume. Unfortunately, as I stated above, the responses are not linear. They may seem to be linear if the change increment is very small, but as the increment gets larger, the values will move farther and farther away from a simple linear response. This is exactly why extending the freebore on a chamber, which facilitates seating a longer bullet farther out of the case so as to retain more useable case volume, allows us to achieve the same velocity at lower pressure.

Above is an example of some predictions I ran using Quickload. I started with a well-characterized 90 VLD/H4895 load I shoot in one of my .223 F-TR rifles.

The values for case volume (Lapua brass), charge weight, pressure, barrel time, and velocity are shown in the (first) data column marked as "100% Case Vol.". These data have been calibrated so that predicted and actual velocity match exactly by adjusting the burn rate factor, Ba.

In the "97% Case Vol." (second) data column, I reduced both the case volume and the charge weight (shown in red) values by 3%. You can readily see that the predicted values for pressure, barrel time, and velocity are lower than the parental values in the "100% Case Vol." column, suggesting that a 3% reduction in charge weight over-compensates for the 3% reduction in case volume.

In the (third) column (also marked "97% Case Vol."), I kept the 3% reduced case volume, but increased charge weight until the barrel time (23.18 gr, shown in blue) matched the value in the parental load "100% Case Vol." column. As expected with comparable barrel times, the predicted pressures in the two loads are also comparable (56,965 psi versus 57026 psi). However, predicted velocity is almost 20 fps slower. Further, the predicted charge weight required to maintain the same barrel time with the 3% reduction in case volume was 23.38 gr, only a 2.2% reduction in charge weight.

It may seem like a difference in charge weight of 0.8% is trivial (2.2% decrease versus 3.0% decrease), but in reality it may not be trivial at all. In this specific example, a 0.8% difference amounts to only 0.2 gr. However, in a case with 2 to 3 times greater capacity than the small .223 Rem case, the actual difference in charge weight required to maintain a given pressure following a change in case volume (i.e when switching lots of brands of brass) could actually be much larger. Notice that a 3% decrease in charge weight concurrent with a 3% decrease in case volume reduced the predicted velocity by almost 40 fps, certainly more than enough in this particular load to move it significantly outside the optimal window.

The magnitude of the difference will depend a number of things, including type of powder, case volume, case volume differential, and where the original load actually was on the pressure curve relative to MAX pressure. In the end, you're still going to have to determine empirically by actual testing where the new load will shoot. Dropping the charge weight by an equivalent percentage as the decrease in case volume is certainly a reasonable approximation, although to be absolutely sure, I might go even a bit further for safety's sake. In a small case like the .223 Rem, that extra little bit won't mean much extra effort when re-optimizing charge weight. It might be a little extra work in a much larger case, but still not a huge deal. The bottom line is that reloading parameters don't always behave in a linear fashion and a program like QuickLoad can be very useful in make prediction for non-linear functions. In cases such as the OP's where the difference in case volume is relatively small, you can probably get away with treating it as if it was linear. If the differential is larger, definitely err on the side of caution.

 

[Barrel Nut]

........
P is the pressure of the gas,
V is the volume of the gas,
n is the amount of substance of gas (in moles),
R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
T is the absolute temperature of the gas.

Moles are the new prairie dogs.

 

[Tiratore]

thanks, gstaylorg.
You have confirmed my suspicions, and you are pushing me toward buying a copy of Quickload. If we only had a convenient way of measuring chamber pressure, directly or indirectly ...

 

[Ned Ludd]

thanks, gstaylorg.
You have confirmed my suspicions, and you are pushing me toward buying a copy of Quickload. If we only had a convenient way of measuring chamber pressure, directly or indirectly ...

IMO - it's worth every penny, several times over. I find it to be a very useful addition to the overall set of reloading tools. However, keep in mind that no computer program can account for every variable and store every possible piece of data to make calculations and predictions. You still have to shoot your loads, measure velocities, and see what happens to groups on the target in order to make the final determination. For me, QL doesn't replace that process, it just facilitates and expedites it.