I just got my 1/12 twist re-barreled 500 S&W back, 4"-barrel, muzzle brake to finish at final length of 5", and I've got another that'll put it just sub 6" if I find it's worth it.
The smith tested it with his load, 520gr hardcast went 1470fps for just a nudge shy of 2500 ft/lbs
Fits in all my holsters for the original 4" version it was factory, muzzle brake just sticks out a hair.
For reference, the 340gr +p+ 44 mag is about 1500 ft/lbs of energy, has a 264 sectional density and can only be shot in a few guns. Good load for a handgun no doubt.
If I shot for the same ft/lbs of energy this 500 would kick about like a 38 special and still run that 520gr bullet which has a 296 sectional density, just shy of the 300 they like to see for African dangerous game.
We'll see where I end up, hoping to keep the 2500 ft/lb load in the gun for the big game, maybe see about something a bit more reduced for practice.
The faster twist is so these stabilize well and the smith got some great accuracy with them, all looks well, I'll run some 600gr rounds as well to see, I want to run some hillbilly penetration tests and test straight line penetration by using some angled material to try and deflect the path, see if it stays straight.
Been felling trees the past couple days, had a big 8-10" diameter branch fall on me about 2hrs ago, that was a real hoot...