I am looking for input on a problem with an ar pistol chambered in 300 blk. The upper assembly was a pre-built AR-Stoner brand with an 8.5 inch barrel 1:8. The ammo used was Seller & Billet 200 gr subsonic, FMJBT.

The failure was that the bullet from the second round fired out of the gun became jammed in the barrel directly passed the chamber. The bullet was jammed so tight that it actually blew the lead core out the front of the bullet, while pressing the copper jacket against the inside of the barrel. Because of the location of the copper jacket remnant, the subsequent rounds would not load all the way into the chamber. It was very lucky that the failure did not happen a little further down the barrel.

The copper jacket was retrieved from the barrel by using tightly-packed wadding inserted from the muzzle and a ram rod to hammer it backwards and out of the barrel. A visual inspection shows no obvious damage to the inside of the barrel and examining the outside does not show any signs of bulging.

Has anyone here had experience with a failure of this nature, or know of any endemic problems with AR-Stoner and S&B? General thoughts and advice?

The plan forward right now is to buy some different types of ammo and test them, perform an inspection checklist before each round, (one round at a time, ensure barrel is clear, ensure bolt closes, etc), and build a simple fixture so it can be fired remotely for the first few rounds.

Others will chime in I’m sure.
First welcome to Arfcom.
I would say this kind of failure is catastrophic and I would not use the barrel again !
Ballistic advantage has some wonderful blackout barrels. also
I would double check the bullets making sure that somehow SB difnt cram AK bullets  in which are slightly larger than .308 diameter.

My first suspicion would be the ammo.

If you don't have a borescope, this is a really good opportunity to get one. Inspect the barrel, make ABSOLUTELY certain that thre is nothing left in it, and that the finish of the bore looks good, especially around the area where the jam happened.

Next, buy, beg, borrow or steal a box of good quality factory (US factory!) ammo. Load up one round, and test fire. If it wre me, I would not have the gun anywhere near my face when
doing this. It will probably be fine. In which case, inspect the bore then load up five rounds and fire them, paying attention. If any one of them sounds or feels different stop and inspect
the bore.

My guess is you will run through the whole box with no issues. What you do with the original ammo is up to you, but at the VERY LEAST I would be carefully measuring it before
trying to use it again.

Thank you both for the input. If I just buy a new barrel, obviously that would be the easiest and safest route, but would either of you suspect the bcg for damage? Midway has a decent return policy, that I qualify for but they discontinued the 8.5" upper, and the barrel. So right now I can only get store credit, so I am also exploring other options.

Probably no damage to the bolt,  however if there was it would be on the locking lugs.
If retailer will give you credit  towards a better upper, thats what I would do.
As Phillip recommend mic out your ammo

was this the first time you shot the rifle? If it was running perfectly fine before I suspect a squib load in which case the rifle and barrel would be perfectly safe to fire again (as long as the barrel obstruction was completely removed).

Yes it was the first time I shot it. The failure happened on the second round.

I still think its a squib load but just to be sure I would slug the barrel

I think so as well.  But the core leaving the jacket has me puzzled.

Sellier & Bellot have a rep as being loaded hotter than American brand ammo for given calibers.  I doubt that had anything directly to do with your issue.  Sounds like the problem was the bullet itself, possibly some internal flaw.  Lead core may have been loose from the jacket and the friction and squeeze into the lands just caused the jacket to stick and the loose lead core punched on out.  Possibly, when the bullet was made, the lead core wasn't full sized and the jacket never bonded or sealed to it, I dunno, just guessing here.  

Can't say I've ever seen this exact failure before, but I've seen quite a few stuck bullets from squib loads.  Good recommendations to have the barrel carefully checked, measured, or slugged.  

Depending on how quickly or easily the core blew out, it might not have severely over pressured the barrel or chamber, so it might check out ok and be fine.  I would certainly have it checked.