http://www.thehighroad.org/archive/i.../t-481389.html
AR-15 (List 1)
.204 Ruger
.20 Tactical
.20 Practical
5.45X39mm
.222 Remington
.222 Remington Magnum
.223 Remington
5.56x45mm
6mm PPC
6.5 Grendel (and various wildcats)
6.8mm Remington SPC
7.62X25mm Tokarev
7.62x39mm
.458 socom
.50 Beowulf
AR-10 (List 2)
.243 Winchester
.260 Remington
7mm-08 Rem
.308 Win
.338 Federal
6.5 Creedmoor
List #1 are cartridges that will work with a standard AR-15 or M16 lower. There are several other cartridges, as well, including several WSSM chamberings and wildcats based on the .223 case. List #2 requires a longer, .308-size upper and lower receiver. As you can see, I added in a few additional cartridges for you that are currently popular in this platform.
One consideration is that inexpensive 'plinking' ammo is not readily available in many of these other cartridges. Several of them are also 'barrel-burners,' and will erode the throat of a barrel very quickly - especially in full auto shooting. The hotter a barrel gets, the faster it erodes. A full auto rifle in one of these cartridges would be a very expensive proposition.
I'm sure that several people will be along shortly to add their favorite cartridge that I didn't mention. There are several, and I didn't try to list them all.
I'm no expert on NFA technicalities, but I believe that you have to list all of the different chamberings of your NFA weapons on the appropriate ATF form, and update it when they change.