I have built a number of "A" type rifles (those that use the SR-25 / DPMS magazine) and found a few favorites. I apologize in advance because I do not own an A-series Armalite yet to speak from first-person experience.
If you can find Israeli-made E-Landers they are black finished-polished stainless steel and duplicate Knight SR-25 factory magazines. They are minimum-dimension size to fit otherwise tight magazine wells (like my LMT).
I only own one plastic MagPul magazine because I wanted to know if it would fit a tight LMT. It does but does not drop free.
Optics Planet carries Gen2 and Gen6 Israeli Military Industry SR-25 magazines that will accommodate military-length M118LR cartridges at 2.830". You can get them blocked for 3, 5, or 10-rounds for states with round limits for deer season.
I also have Dura-Mags that work well in a Matrix and Diamondback DB-10. They are a tight fit and do not drop-free in my LMT.
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