I've found that the PMAG is the only magazine other than USGI to fully activate the bolt catch when empty. I've used all mags to include USGI, PMAG, Lancer L5, Lancer AWM, TangoDown ARC, Troy Battlemag, and USGI 20 round. Only USGI 30 round and PMAG press the bolt catch up as high as it can go and give me consistent lockback on my Bren. All other magazines function fine and generally lock back, but sometimes allow the bolt to close when the magazine is removed. Doesn't always happen, and I've inspected all parts to ensure proper function. With the lower removed, you can see the bolt catch activation yourself. When the gun is assembled and the bolt is drawn briskly to the rear, it is clear that some magazines lack either spring strength or have a follower design where the dummy cartridge is depressed by the bolt lugs and causes incomplete depression of the bolt catch arm. YMMV but I only use USGI or PMAG in my Bren. Given that the STANAG Bren lower was likely designed around the use of USGI magazines, it doesn't bother me much and I don't expect aftermarket AR-15 magazines to function in all guns aside from the AR-15.