Glock drums are cheap, PPSh drums aren't too bad, Thompson drums are somewhat affordable.
The upper is really designed around rifle cal stuff but I figured it would be cool to have the option of pistol cal stuff, especially with the CMMG delayed blowback bolts and barrels on the market.
Glock drums make less sense outside of the grip IMHO. They would stick out too much for my taste. PPSh drums would be great, shame I missed out on the cheap ammo but it's not too expensive. That would be cool. Thompson drums are too expensive for me. So PPSh or Soumi? I wonder what the RPM would be with the CMMG. Being an m11 and "A1" owner, the CMMG 45 might be interesting actually. Anyway, realistically people will probably buy whatever adaptors you offer once they've bought the upper.