The Bartz mags were next but I ended up striking gold with Sig Sauer poly mags.
I tried E-Lander metal mags with varying length springs and 30 RAR single stack (which worked but held the round too high and caused barrel extension scoring on the brass near the neck). Tried mutilple other 556 mags on hand.
I missed out on a killer clearance deal from Sig they were running 2 packs for 25.00 but they're all gone so had to spend 18.00 a piece.