If you check out The Muzzleloading Forum, and maybe you already have, you'll find all kinds of great information on the use of patched round balls. There are several guys posting over there with some serious field experience with PRB's and it's surprising just how effective they are with what most would consider minimal powder charges.
The PRB has been slaying critters pretty effectively for centuries. Other than an arrow, no projectile has been in use longer.
My 62 caliber caplock likes everything from 50 grains of 2F to 120 grains of the same with a PRB. At 50 grains it's a doll to shoot and I've no doubt it would do great on deer. At 120 grains it's all I want to stand behind. I've yet to take any game with it but got drawn for a late season muzzleloader moose tag this year. Hopefully I'll get time to hunt it. Last time I got drawn for it I wasn't able to go.
Here's a link to the Muzzleloading Forum.
Muzzleloading 
