How fast does a 50 cal PRB need to be for deer?

[Tb54]

My reply to the OP’s question is more on the theoretical than the practical—-

If a well placed, subsonic, 30gr., .22 short can reliably kill deer, (and it can) then a subsonic PRB from a muzzleloader will have no problem. Shot placement is what brings home meat.

 

[8 BORE]

How fast does a 50 cal PRB need to be for deer?

twice as fast as the deer

Fast enough to penetrate the vital organs.