I've been using PMAGs and HEXMAGs in my RDB-C/Hunter with no problems. Although I'm limited to 10/30 and 10 round mags in my state.

 

I second the vote of confidence for PMags. They seem to work well in my RDB-17 and RDB-S.

 

The only magazine I have had problems with have been the thin walled, cheap alumium ACS magizines. Everything else (even a P60) have worked fine.

 

Third here, I admit I'm one of the few that actually prefers the super-short 10-round PMags but across half a dozen I've had no magazine issues at all except for the literal first mag I put in ever at the range I didn't slap it in hard enough so it didn't actually latch.

The issue with mag retention is generally a user error I think, the RDB is... somewhat stiff compared to others I guess? KT seems to have a habit (KSG/KS7 being another example) of building stuff that has resistance just before you're done with something, but finish going that last quarter inch or it won't work.

As for feeding it's turning out to actually being over-gassed a lot; under-gassed it won't lock the bolt back if single-fed, but some folks just cranked the gas up... and that makes things operate incorrectly too and just shred some ammo unsuccessfully. RDB's sensitive to using the right settings for the right ammo, so find an ammo you like when adjusted properly, mark down that adjustment, and stick to it.

 

See, individual experiences are all over the place.
You won't know till you try a mag in your own, or potentially your own rifle.
I would test fit some mags before you buy it.

 

Yeah, this.

With KT almost every one is unique and distinct in their own way, no getting around that.