NSA slugs and FX hybrids fall through the magazine.
In my airgun stuff pile I found some 36gr AVS Slugs with a diameter advertised as being 0.254" They fit flush with the back of the mag just like pellets do. Don't have to force them in any way. They fall in just like the pic shows. Gonna try them at the range tomorrow. I also found some NSA 0.249" slugs that I tried powder coating awhile back. I believe that they had two light coats applied. They were tight and sat proud of the back of the magazine. Possibly a single very light coat would work fine. That'll be something to try this winter.