Pistol Caliber Carbines make a lot of sense, since pistol ammo is cheap, magazines tend to be common types, and you can shoot them at indoor ranges which do not allow rifles, or require very expensive frangible ammo (that you buy from them, of course). Pistol-Carbine conversions make less sense. The only reason I can think of that you would buy one is if you had an extra version of that same pistol that you didn’t use for some reason, and wanted a long barreled version of the same gun. I bought a used Chi-Com 1911 for $150 once that needed more work than it was worth to make it more than a pop-and-drop. Did a little work and sold it to a buddy for cost, but that would have been a good candidate for a Mec-Tec upper.