So then...
Could one figure any powder charge that's safe for a cast bullet to be safe for a copper jacketed bullet of the same mass?..but not the other way around?
>That's probably true, but (theoretically speaking here) it's possible that you'll need data specific to jacketed bullets for handgun cartridges out of rifle length barrels to ensure that the bullet makes it all the way out of the bore.
Although that was never found in the whole history of metallic cartridges. Decades ago, one of the magazines decided they wanted to stick a bullet right at the muzzle end. They got 40' of barrel and a .25 Auto. Fired a round and noted where the bullet stopped. Don't remember, but they cut off 1/4 to 1/2 the distance from the muzzle to where the bullet stopped, and the bullet traveled further down the barreatto the muzzle. IF there was enough pressure to get within a foot of the muzzle, the bullet came out of the barrel. I believe they had about 20' of barrel with a .25 Auto and the bullet exited that barrel.
Even the .22 LR is still gaining velocity out of a 20" barrel, although the .22 Win Mag appears to reach peak velocity in a 17" barrel.