There are two different 12 gauge chokes that Trulock (Colonial) make. The regular chokes for thicker barrels (870's and the like) and thin walls for sxs and o/u's with thinner barrels. The regular chokes measure .795 on the unthreaded section and the tw's .773. Both are threaded 44 tpi double lead. If you drop one of the thin walls into a regular barrel the threads will match up but it will fit like a saddle on a sow. I'm sure some regular size chokes will solve your problem.