Glockenshpiel- I can understand ur frustration, but CanAm sells this shotgun Im sure he's an expert or at least very knowledgeable with this arm in question.
As per these shells.. last year I got a new Mossberg 535 with the trap/hunting barrel, I had also gotten a large box of those trap loads from wally-world. The first box was a bit problematic and was for about a couple of hundred rounds, very unsetteling for a new gun, but prior to my new gun I did hear about this issue. one time I had to hold the handguard and pound the butt down on a sand bag too extract the shell. now after 1000 or so rounds its flawless.
every manufacture has some degree of differences between them, and chamber dia. is one of them. they maby the same or tighter or really loose. The dominion arms series has some compatibility issues with the parent shotgun parts it is based off of. some list the barrel's sight are cocked off a bit center when the Rem barrel is installed on a Dom receiver, a slight fitting might be needed on other things.. there are others. not too much of a problem.
Now try not to take this the wrong way, we are all friends here, just trying to get u up and rocking. In this particular forum, typically discussions here are that of the hunting and sporting disciplines, although this of a sporting nature the arm is somewhat considered a tacticool black shotgun is has a different ring here thats all. maby in the black/green shotgun forum.
so keep on rocking, or try to switch up some thing see what happens.