If he is going to hunt elk and wants to shoot powerbelts, then he might want to look at the aero tip design powerbelts. Also shoot a heavy as the rifle will take. I have shot powerbelts out of a 1:48 twist with very good accuracy. Whether his rifle will do it or not, he will have to experiment and find out.

As MLKeith already said, he might also want to look into a maxiball or Hornady Great Plains Conical. Buffalo Bullet makes a nice big conical that always shot very well out of my Hawkins and Renegades. Have him get some of each and give them a try.

If he has a .54 caliber then he could use a roundball. .54 caliber roundball is still a 233 grain projectile being pushed very fast. They have a lot of zip to them. With a round ball when hunting big game, bigger is better. I would personally use a .58 caliber or bigger even. Even a .50 caliber would work, but be sure and place your shot and get as close as possible with a roundball. Elk are tuff to penetrate.