I have bought an Undertaker for my 870 in the past and that choke did not screw into my barrel. I took it back to the shop and the saleperson slipped it into another 870 with no problems. I exchanged it for another Undertaker and that one slipped in without effort.

This year I purchased a custom choke from Precision Arms. I brought the barrel in and threaded the choke for a good fit. I let my friend borrow the choke during a patterning session. and after he tore up the patterning to shreds with it, it would not rescrew into my barrel. He pleaded with me to buy it and I gave in. I went back to the gunsmith and he told me that 870 are famous for poor tolerancies in workmanship. He examined my barrel and said it was off and rethreaded it for me free of charge. Of course I bought another one of his famous chokes. I guess you are not alone with this problem.