The KP-15 receiver has been thoroughly tested with .223/5.56mm, .300 Blackout, 7.62x39mm, and 9mm blowback uppers.
We have not done any substantial testing with .450 Bushmaster, .458 SOCOM, .50 Beowulf, .350 Legend, etc.
Will it hold up for the normal firing cycle of most users who zero with one box of ammo and shoot 3-5 rounds a year hunting with it? Probably.
Will it hold up to mag dumping 10,000 rounds of .50 Beowulf? No idea. We're not going to spend the tens of thousands of dollars testing this edge case use.
Our stance on big bore uppers is use at your own risk.
If you damage a KP-15 lower using a big bore upper, our $65.00 replacement fee for a new lower applies.
If you do use a big bore upper on a KP-15 lower, use the heaviest buffer possible that still allows the rifle to cycle and lock back on the last shot.