I bought an AMT new many years ago, it was chambered in .40. I did some experimenting and bought a used .357 Sig barrel, dropped it in, it worked like a charm. I also bought a 9mm barrel, had to do a minor alteration on it, but it works fine even with the .40 mag. I recently purchased a .45 slide and mag but haven't had the chance to shoot it yet. All total I have about $325. in everything (that would have been a good down stroke on a Sig)
I don't know about the other calibers, but I have fired at least a thousand rounds throught the .40, never a problem. I heard stories about feed problems, so I took a Dremmel tool and polished the feed ramp on all the barrels, I never experienced any kind of feed problem even with the 9mm.
I think they are a pretty good little guns, just too heavy for carry.