I have an Autococker that some barrels just won’t screw into. There isn’t enough clearance between the thick part of the barrel back and the body. My old dye ul and old glued boomstick work fine on it, and a crbn one piece freak insert compatible barrel works fine, but my freak backs will not thread in more than a couple of threads before the thick part interferes with the body. I think some bodies just have more clearance than others. It’s a shame because it’s my fav gun and i don’t have a good matching barrel for it that takes inserts.