Dedicated .22rfs like Tippman's, AM180s, and Norrel 10/22s are excellent NFA toys. But if you're looking to buy a single do-all NFA MG consider that a M16 can accept any number of .22lr uppers/conversion units. Kuehl, Ceiner, Lakeside LM7 are just a few. These kits/uppers are reasonably priced, and do not necessitate a separate NFA transfer if you have the 16.
My own preference is to run .22lr in the FS Uzi. The Vector FA kit set me back about $250. It runs like a chainsaw with Minimags. And I don't have to swap buffers or other parts out - just drop in barrel and bolt and blast away. Moreover, there's no rimfire grit getting into the FCG/lower receiver. I like it so much I bought a 2nd registered receiver just to keep as a .22. My kids love it. 'Tis the season to shoot the crap out of Frosty the Snowman. In the fall we make mush of the Great Pumpkins that grow in the manure pile. ROF is well over 1000 RPM. Can't load magazines fast enough.
Sam