I have no idea what’s going on. I was just trying to do a quick mock up with the receiver, rail and barrel but the rail will only screw on about 3/4 the way and seize up. When I screw the rail on without the barrel in the barrel extension it goes on all the way with little to no effort. The receiver is an old Genesis with a Kac EC barrel and kac urx II rail. Anyone have any suggestions?

ETA: another thought, does the barrel extension on the receiver "flex" when you screw on the rail or barrel nut? Is the barrel in the extension preventing it from flexing to allow the rail to screw on? Only reason I ask is because the barrel has a little wiggle in the extension without the rail but is rock solid with the rail partially screwed on.

Does the barrel have a heavy profile down at the chamber end, possibly too wide for the inner diameter of the rail somewhere?

It’s a heavy profile but there’s plenty of room for the rail to go over the barrel. A factory kac EC and EMC uses this barrel/ rail combo.

i think the question is:
with the barrel unattached, not on the upper receiver, does the barrel nut (or rail if the barrel nut is integral to the rail) slide onto the barrel all the way down to the barrel extension flange, and spin freely on the BE flange?

Yessir