Going with bear creek and their gen 2 enhanced 762x39 and their recommend mags? It works like a clock.
Well worth it.
Kak is another one.
A few manufacturers make them but then it's more.propriatary of a build if you don't know what's going on.
Yeah I'm regretting selling my custom dead nuts reliable build for $650 that was awesome upper.
The Zev is nicer but I do regret...