Home Forums Reloading Varmint Rifles Case dimensions v pressure

When using brass from comercial supplier verses that from military use there is a difference (usually) in the thickness of the brass and therfore there is a difference in capacity, which in turn effects the max powder volume. Now the question is this - If a comercial case (thinner brass=more capacity) can run for example x fps with y bullet, which is getting on for a max load and velocity, will the thicker brass case ie less capacity be able to run the same velocity and pressure with a little less powder. In other words does the capacity dictate possible velocity and pressure limits or does pressure dictate velocity because of capacity.
Or more simply, can I expect to get the same velocity from the slightly smaller capacity of the thicker brass military case as I can get from the thinner comercial case at the same pressure with slightly less powder.

Close enough that small differences are lost in the noise. The general rule is that a larger case will allow - and require - more powder to reach the same pressure and so allow a velocity increase at the 1/4 rule level. See



The question comes up in a fairly clear context in cartridges like the 7x61 Sharpe & Harte which was magnumized by the maker as the 7x61 Super and all the variety of improved cartridges. One confounding factor is judging pressure by pressure signs with distinctly different brass - that is case expansion can be about the same at quite different pressures or quite different at the same pressure so not much can be concluded.

Well, it depends.
Sometimes the extra space allows for more powder, and the extra powder "outweights" the loss of space and give you more velocity.

In other instances they dymanics favor the increased pressure in the more confined space.

For a long barrel, with an over bore cartridge, the thinner commercial brasses, such as Winchester will typically yield the best velocities for the pressure.

Shorter barrels, straighter cases tend to favor the dymanics of the heavier brass.

Of course that's just for velocity....no saying which brass a rifle may favor for accuracy....

basically, if the outside dims are the same but the inside dims are smaller due to thicker brass, the same load of powder will give higher pressure in the case with the thicker brass. All other things being equal. However, this can be mitigated by the length of the leade, and the seating of the bullet and the burn rate of the powder.