I have a sporterized P1917 in 308 Norma Mag that is going to be re-bored into a 358 Norma Mag. The barrel on it at the moment is a 26" heavy profile monster.

I want to chop it down to 20" and thread it for a can (suppressor). Before anyone asks I have a can that supports the diameter and that kind of gas volume and pressure. The reason for the 20" barrel is I am right on the edge of enough shoulder to support the can at 22" or more. The diameter at 20" has enough meat due to the taper; dropping to 19" would be even better. However I have chopped magnums to 19" before. It wasn't great; muzzle pressure skyrockets inversely to projectile velocity. It becomes a silly endeavor, and you really, really don't want to shoot a 18" 300 WM without a can on it. Talk about loud!

So the question is has anybody run a chrono against legit 358 Norma loads in a 20" barrel? I have run Quickload calculations and both the muzzle pressure and velocities look okay. I kind of expected that as the expansion ratio and velocity loss is going to better with a .358" bore compared to a .308" bore.

The goal of this endeavor is to have a 4,000 ft/lbs energy, medium caliber, heavy for caliber bullet that is silenced, light enough to carry, and still okay to shoot prone.