1) shooting SLAP with a brake is a bad thing, but does not cause the gun to blow up like this. All sorts of nasty down range, but not a dangerous over pressure. *I agree on both of these and will add that if the sabot and penatrator hit the muzzle brake, extremely bad things are going to happen. The penatrator would probably blow the brake off the barrel and keep going down range (hopefully), the sabot, being plastic will become a whole bunch of a mess going everywhere. The shooter and anyone else in the area might catch any shrapnel from this happening. So be prepared to be sued if anyone besides yourself is injured.

2) shooter says previous round was a regular round, NOT SLAP. Round impacted target. No possibility of an obstruction.
Having a short chamber will cause overpressure. I'm not sure by how much, given the sabot is plastic. Would be interesting to test. *I agree with this also and agree about the chamber. This is where the problems begin, everyone will agree that a round jammed into the chamber and rifling is also a bad thing. Mark should measure the chamber/leade as its very easy to do and then measure a SLAP-T round and let us know what he comes up with. That would let us know if there was a problem that could cause an over pressure situation. A situation that will be worsened if the round is a reload with the wrong powder.

3) correct about the rounds lacking a factory crimp, and short of a lab test, no way to know what powder. *Mark received additional SLAP-T rounds from KB, looks like there might be a couple reloads in there. He'll be able to pull them down and check the powder on them, won't be able to tell what kind but maybe if it's for use in reloading 50 BMG. He might want to send it out to a lab and see what it is, would be a benefit to him and KB.

I do wonder what Mark is gonna do. Fill a slap round full of bullseye with a pressure transducer? *Whatever he does, he needs to do it perfectly or the interwebz trolls will never leave him alone over it. It's a design that pushes the limit, the limit was exceeded tremendously and now all of that needs to be explained. Do I think it was the design that failed, no! Did it fail from something that no one could account for, yes!

One last thing, it's something that bugs me a lot and it's about KB. This guy is out there on the web shooting all kinds of guns and ammo, just like a lot of other people. He bought this ammo and other 50 Cal ammo but didn't take the time to inspect it? For what he claims he paid for it, I would've made sure it was good to go. If the seller was claiming that it was factory ammo, KB could have a case against them.

People will watch his channel and think "he did it, so can I"! Nothing could be further from the truth and he should have some kind of warning. Hell, he didn't even know that he shouldn't have had the muzzle brake on! He was relying on Mark and a Military chamber to save his ass while shooting SLAP ammo!! I do think he did the right thing by putting the whole situation out there, hopefully people will pay attention to any surplus or reloaded ammo that they buy.