How is it possible to determine calibers that a stripped lower will support? To a 100% AR-15 rookie, it is hard to tell.
Here's an interesting article that shows a number of the available options. I don't know if it's an all inclusive list or not, but there are a lot of options.
Welcome to the forums, Rlmoorex!
Basically, any cartridge under 2.25" overall length with a rim diameter of 0.440" or less.
Magazine length determines the overall length and rim diameter is limited by the bolt face. If too much material is removed from the bolt face, bolt lug strength is compromised.
Currently available choices range from 22 rimfire to .50 Beowolf plus 410 ga.