I have barrel and slide combinations in 45acp, 9mm, 40s&w, and 357sig. The only thing I don't have is the 400 corbon abd the 38 super. I am not really interested in those calibers as the ones I have are probably the best for self defense out of a really short barrel gun. All the barrels and slides interchange. You may be able to make a 9mm work in a 40 slide, but I am not sure. I guess I could put one together that way and find out quick enough. I know a 400 corbon will require a 45 slide. I have two frames and 4 barrel slide combinations and I think it is one of the best ultra compact weapons available when tuned correctly. The DEA actually approved the AMT Backup for deep concealment for their agents back in the 90's. A gun must be made 100% functional if it isn't already if it is to counted on for defense, but that can be done with the AMT Backups. The AMT Backup gets a lot of negative press, but when correctly tuned, It is GREAT for its intended purpose. I really like the fact the the trigger is stiff since that is the only safety on this pocket gun, which I also like. I also prefer the weight of an all metal gun in such a small package because it makes multiple shots easier, and multiple shots will almost certainly be needed with ANY handgun. Lastly, in an all metal option, I really like a stainless gun for pocket carry due to corrosion resistance. The restrike ability of the AMT is also an attractive feature to me in any automatic pistol. I would rather have a full size gun if I really expect trouble, but 6 sure rounds of 40s&w in your front pocket is a mighty big comfort. In 99 percent of the situations I might encounter, that will allow me as good a chance as any to fight my way clear of whatever threat I encounter.
I carry the 40s&w AMT Backup as my primary with 2 spare mags when I carry. This is easy to carry and conceal in anything I wear with a belt in the pants. The belt is needed to support the weight of the gun. I really like mine and I wish you the best with yours.
I am glad you located your barrel. If you locate any 9mm, 357 or 40 barrels that you don't need let me know. I have two frames and I like to have a NY reload sometimes. At present the only caliber I can carry two guns at a time is 45acp.