As previously mentioned, get the cylinder throats checked and fixed. It will help the jacketed bullet loads shoot more consistently too.
Any bullet from 250-300grs. with a flat face of decent meplat will flatten the whitetail easily.
Powders from Unique, CFE Pistol, Blue Dot, Longshot, 2400 and 4227 all work well in the Colt. Load them into the 1000-1100 fps range and practice, practice and take your "target" load hunting with you.
Loading and getting proficient with heavily loaded jacketed bullets is more expensive, requires more discipline and just isn't necessary to keel over a deer. Of course if that's what you want, by all means go ahead. Sometimes you just need to do it to see for yourself!
In that case, H110 isn't the only powder, but it's the most established with the best data. Heavier bullets work better, 300-340 grains.
