Powerbelts are slightly undersized so they go down easy, gas seal is achieved by the plastic insert on the bottom. Since the plastic does not encapsulate the bullet there is no plastic residue. As you've stated, they are devastating on deer. Never kept one inside but the exit hole is large unlike the 50 cal hole with most other maxis.
Renegade Flintlock 50, 1:48: 245 PB HP 100gr FF Goex, 3"@100
Lyman GPH Flintlock 50, 1:32: 295 PB HP 100gr T7 FFF, 3-4"@150
With the Lyman and T7 seems, I can shoot as many shots as I want without cleaning and no difficulty loading. The most I've done at 1 time is about 20.