have a GM 7.5" barrel. I originally built it with an H2, had malfunctions, where a round gets partially stripped, and an empty stops the bolt from closing, usually with the open end of the empty case facing out of the ejection port, bolt jammed, live round partially into chamber. I went to an H1 buffer and the thing kicked quite a bit more, was into the second magazine (26 rds/mag) maybe 5 rds, started happening again. It definitely is more prevalent when warm. Today, an H3 buffer, 10 rounds in, just like above, but it's a lot warmer today than the other two days, once today I have an empty case in barrel extension, and the nose of the partially stripped live round holding it in, backwards, the damned round is backwards, the empty hole against the bolt. I think they're bouncing off my deflector, and back into the gun, is this possible? When it throws empties, it's been 12 oclock or so, today might have been more like 12:45-1 oclock. I have a standard carbine spring, I think an XP spring may make it worse, but I just don't know. I also don't how to proceed, other than attempting to capture video somehow. Should I just write off this 100 barrel and get something else? AO/Toolcraft/FN BCG, Noveske Upper, XM855, did similar stuff with Hornady 75 gr. .223 OTM. Definitely better with the Federal. Should I try XM193 . If I screwed up the ejector spring when checking headspace, I'd think it would do it all the time. This barrel extension has a pin that holds the ejector down until the bolt is almost completely out of battery. If I let it cool a bit it will run 5-6 rounds and then do it again. Maybe Slash's Heavy Buffer, or is that a waste of time? Plus that thing is $75, or 3/4 the cost of this damned barrel.

I bought a 7.5 upper from JSE surplus. Ran a ChiCom buffer and spring (standard carbine) it ran no problem at all the first time out. I think before you go adding exotic fixes like an H3, just make sure everything is in spec. Ejector, extractor, ect.

Does the bolt carrier group lock back when one round is fired with an empty mag?

Take a look at the location of the face of the bolt in relation to the rear edge of the ejection port when you manually pull all the back on the charger handle.  It should be 1/8" to 1/4" in front of the back edge of the port.  If it happens to retract further back past the port this could be part of your issue.  The quick fix is to drop a couple of quarters into the receiver extension behind the recoil spring, the real fix is if you are able to make one more turn screwing in the RE or facing/filing the leading edge of the RE for the proper depth.

Well you may have hit it on the head, I can make the (*&#$%  bolt disappear, but I gotta try. I gotta stop tossing these BCM QD endplates in the trash, or only stake them after I'm sure. First the damned Sig Brace had to go, now this crap.

Blue loctite is your friend. You can get the castle nut off if you need to, but it isn't ever going to vibrate loose.

The GM is a good barrel- I have the same one; they are Sig's factory pistol barrel, and those run all day long.
Check your tube as suggested. Start with a standard carbine buffer and spring, go to heavier buffers from there if needed. If you start with a H buffer you don't know if that's part of the problem...

My 7.5 AP barrel runs fine with a carbine buffer. OP, you are not wasting time getting yours to be reliable.  They are awesome little guns once you get them right. That GM barrel is a good one.