Sometimes shit gets stuck in the barrel. If you're lucky it won't go boom after the first gets stuck and you'll notice before it's too late. Bad luck if it happens while you're doing a mag dump.
Slap rounds are lower pressure than standard 50BMG loads. The problem here appears to be the result of a bore obstruction caused by using a sabot round in a rifle equipped with a muzzle brake.
If the manual this guy is referencing is legit, which it very much appears to be, then I suspect it could happen with a Barrett, unless the muzzle brake on the Barrett is designed to function safely with sabot rounds. You don't need to watch the whole video, really just the first half is relevant, the second half is an interview.
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This video explains this well: https://youtu.be/ardw09S7GfE
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