Scale grains to match barrel length?

[physikal]

I am shooting a Thompson Contender G2 Rifle with what I believe is an 16" or 18" barrel. The load data I am looking at used a 24" barrel to test with. The loads call for 31-33gr of H4895 powder. How would I scale that to work safely, if at all, with my shorter barrel?

Thanks!

[Brian Pfleuger]

You don't need to change the load for barrel length, at least from a pressure standpoint. You will simply have more muzzle pressure and somewhat less velocity. Depending on the powder and charge, the increased muzzle pressure may result in increased "rocket effect" recoil.

You may need to adjust the charge in order to achieve best accuracy.

[physikal]

Roger that! Thanks!

[Unclenick]

Yuppers! The bullet is typically between one and two inches into its travel down the bore when the pressure peaks. That's why the shorter barrel doesn't affect peak pressure. It's long over with by the time a bullet leaves the muzzle. The obvious exception is snub nose revolvers. Particularly in magnum loadings, the bullet can sometimes exit in the middle of the pressure peak. That can cause muzzle velocity to be erratic because shot-to-shot variation causes the exact location of the bullet at the peak pressure to move around a little.