Steven model 520-620

I just discovered these shotguns and have been buying a few for my collection. I recently picked up and extra barrel and went to mount the slide when the screw on the end that threads into the lug on the barrel, just didn't want to seat. Figured the threads were a little bugger and needed to be chased with a bottoming tap. Since I am an amateur gunsmith, figured this was a perfect excuse to buy a set of firearm taps, so I did, just not here yet. As I have been pondering the problem, it seem that this screw is bigger than the 8-40 tap supplied in the kit. When I use a pair of calipers the reading on the ID is something around .203, bigger than a #8 screw. So the big question is does anyone know what size this screw is, is it a #10 or #12-40. I found taps at MSC, ouch, they are expensive.

thanks all,