260's are WAY TOO LIGHT --- especially with that bullet. Powerbelts have always had penetration issues - but 50 cal round balls have issues as well if you hit any type of shoulder bone. You should really get up to 300 grains and those issues might start to disappear. Most people don't want to go heavier because it affects trajectory.

I like full bore diameter bullets although my side lock does not seem to like the conical slugs past 60 yards. I've been using 270gr Harvester Saber Tooth bullets with incredible accuracy from my 1:48 twist (3-4" groups at 100 yards with open sights) and have killed 6 deer over the last 3 years. None have as much taken a step after the shot (of course they are all neck and shoulder shots at less than 75 yards). Now I will say the entrance hole has always been bigger than the exit hole and on some occassions the bullet has not exited. If they hit the main shoulder bone, the bullet will explode - of course what's left of the bullet and bone chucks will shred the lungs.
I need to get more before next year and I'm unsure whether or not to go to a bigger size. If it's not broke, don't fix it but your experiences might vary especially if you shoot past 75 yards. I'd personally feel more comfortable with the 300 or 350 grain because they'll plow through the deer with no problems (especially the 350) but I'm unsure if my jack of all twist will be able to shoot them.

My recomendation would be to go over 300grains with whatever you choose.

http://www.harvestermuzzleloading.com/in...=1&Itemid=3

This is a picture from one of the whitetail does I killed last weekend. The bullet went in high in the shoulder (the thin blade bone), through a rib, through the lungs, through another rib, and I found it lodged up agaist the hide on the other side.