Lots of reloads are lead plate and not FMJ. They can probably tell from the rifling.
The barrels are probably designed and extensively tested for very specific loads. If you burst a barrel, chances are, statistically, you've used an overcharged load or some other negligence (like an obstruction) led to your failure. Warranties do not cover user negligence.
Depending on the hardness of the lead it can build up on the rifling. Glock uses polygonal rifling. So does HK, IMI and a few others. If the lead builds up it will eventually cause too much pressure and the barrel will Kaboom.