First off, since you didn't see his group, I'd be hesitant to tell the man I didn't believe him. A muzzleloader can easily best a centerfire with just a little time tweaking, and many times even out of a box, a muzzleloader makes centerfires look sick. My Super91 can out shoot my 308 easily, just not to 300 yards. My 30-30 isn't even in the same class as my White muzzleloaders ... my worst can out shoot it by several inches.
If you don't believe his representation, ask him to bring his gun down to the range sometime as you have never seen a 3/8" group and would love to see how he does it.
Aloha..
