I use Accurate Magazines (308) with PVA BR/Dasher kits installed for anything BR based. Works great. Some guys used the Primal Rights internals and I've not heard complaints but I have no personal experience with them yet.
I use the MDT magazines since I got them on sale a year or so ago for the best price I've found:
6BR Magazines
10 and 12 round Metal AICS magazines from 6BR, 6BRX, 6 BRA, 6 Dasher, and 22 BR. These unique purpose-built detachable AICS box magazines are designed to fit the MDT TAC21, ACC, ESS, LSS-XL, XRS, HNT26, LSS Chassis systems, and many other M24 type chassis and bottom metals.