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On my LMT I had to sand down p-mags to drop free.
What lower are you using?
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I don’t know about DPMS but LMT have a tight mag well here is a linky https://www.m4carbine.net/showthread.php?t=30029
I sanded my P-mags as rod_s post shows in his pics it only took just a little bit for them to start dropping free.
I don’t know DPMS might be tight some where else in the mag well.
My DPMS lower drops PMags just fine. I would suggest the same thing as Thomas M-4.
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