OK bearbow I'll give you some tips that will likely improve your accuracy and also save you some money.
First thing is forget those pellets. You're better off getting loose powder (either real black powder or Pyrodex RS or Triple 7) and a powder measure.
Next get yourself some loose bullets. I like Hornady XTPs. You can get them in either .44 or .45 (.451" dia) caliber and some sabots by Harvester or MMP for the appropriate bullets. I'm more familiar with the Harvester and I use the 300 gr Hornady .429" bullet and the Harvester green crush rib sabot.
Start with 80 gr of powder and work your way up to 100 or 110 grains. Something in there should give you really good accuracy.
You can buy more expensive bullets but the deer or yotes won't know the difference and they'll be just as dead.