Have you tried inserting the barrel without the gas pistons installed? Perhaps that would help you locate what part of the barrel assembly is the problem.
My M4 barrels all fit snug the last millimeter or two inserted into the receiver but not a “something metal is in the way” kinda snug. While inserting the barrel tang into the receiver I’m usually slightly twisting, jiggling, and firmly pushing the barrel towards the receiver; it sounds like you are doing the same.
I just recently picked up my M4 Entry model that had been awaiting ATF Form 4 approval for one year and one week. I purchased it new from a high-volume Benelli dealer so I’m guessing it was manufactured about 16 months ago +/- . This newer M4 Entry is significantly tighter in fit and finish than my other M4’s (also purchased new). None of my M4’s are loose but this M4 Entry doesn’t go together nearly as easily. Maybe a couple of range trips with slugs will help this shotgun adjust and get broken in.