For my RTB upper I run a mix of RTB, Black Dog sonic welded plastic & steel lip mags, 50 round drum, and the S&W mags with the stick on shims for a std. lower.
All run 100%
For the BDs I use the CMMG steel load assist tool, works fine. Cheap.
The S&W mags can usually be found hanging from a hook in most big box sporting goods stores but are often out of stock, they're either popular or made in small batches. The BDs always seem to be In Stock although the 20s occasionally go OOS.
Best bet is to buy one or two of each and see what your setup likes. Mine doesn't care, feeds round nose lead & copperwashed and various hollow points just fine.
Most of my mags are 20s, with a few S&W 10s which are shorter bodies and handy for tabletop shooting.
The BD drum is surprisingly reliable although I stick to mini mags with that.