You need to see what it looks like with a borescope for multiple reasons.
#1) A LOT of smiths that build prs rifles cut the chamber like it is a benchrest gun. I have had some that will shoot that good (benchrest good), but they are not "field guns" where you can shoot out the barrel and never clean.
#2) 6mm cartridges that are that overbore and sometimes even dashers and bra with tight chambers have a lot of changes and problems with fouling. I have been through several 243 Ackley barrels and a bunch of Dasher barrels. I have been given some bra and dasher barrels that people said were "burnt out" at under 2000 rounds. To a man, those barrels were cleaned religiously and there was "definitely no fouling or carbon ring." No bore scope was ever used. Fouling is HARD. When I looked in the borescope I found a shitload of fouling. I used thorroclean and iosso to clean them to bare metal and they went back to shooting acceptably. They were not screamers and they fouled faster from the abuse, but they would shoot average or above for what they were. I let my kids shoot them at one day matches and was very grateful for the free half barrel. When I first bought a borescope, I was new to prs and running a 243 Ackley in a sendero contour. At 1700 or so rounds it stopped shooting. I had cleaned every 200 rounds religiously including JB in the throat. The first 2 barrels had made it 2500 rounds before going over 1 moa. I pulled and replaced the barrel. After I looked with the borescope, I realized it was excessively fouled. I cleaned it to bare metal, and it went back to shooting around 1 moa. I'm still using it on a spare hunting gun because the countour is heavy for hunting and light for prs.
The moral of the story is, 6 CM is a lot of fouling and pressure down a small hole. The fouling is probably a lot harder to remove than anyone imagined. It was probably chambered fairly tight by gap. In my experience, these need to be treated one of 2 ways. Clean every 200 rounds religiously (with borescope) and monitor velocity increase across round count, or get it dirty with 200+ rounds, reduce the load to handle it and run it to 1000-1500 rounds. Then pull the barrel and throw it in the garage. If you want to shoot without cleaning, you are going to need more throat clearance. Raw precision will suffer and I have no idea how much more clearance you will need to never clean, as I have never experimented with it.
I will say that the barrels I have had that will put 10 shots under 1/2 moa while shooting prone from a bipod have all had crazy tight freebore diameters and fairly generous neck clearance. The more you open it up or push out the round count between cleaning, the more the groups trend toward 1-1.1 moa even with top end components and high quality barrel blanks.
The general consensus that I have seen is to chamber for precision with the need to clean more, especially in overbore 6mm. If top end precision is not a concern, or if you are not going to clean, you can open up the freebore some. It seems like pretty sloppy chambers that are perfectly concentric will do 1-1.2 moa with good ammo. Unless you are cleaning (at least some) or throwing barrels away early, you are not going to see any better accuracy than that with a tight freebore barrel, and you are going to have problems with rifle functioning like you are experiencing.
The first thing I would do is buy a teslong borescope, so you can see what is going on.
eat the shipping back to GAP or at a minimum have the original seller split it with you. Nothing worse than trying to save a few bucks and then still having issues with no more chance of OEM support. Sounds like a short chamber combined with a little carbon build up.
