I am amazed this is still going but I will add my experiences. I have 2 BCM lowers, a blem and non blem, a custom JD Machine Tech lower with EGA and a custom formerly 80% lower. All of them work with all generations of PMAGs including M3. I don't get the love affair of USGI mags here as I've had the majority of failures with them. Old, beat up military issued mags are the biggest culprit but I have old, beat up PMAGs as well that still function 100%. Lancer AWMs and PMAGs are still my primary mags while I relegate all my USGI mags to reserves or backup. And I don't trust USGI mags unless I personally retrofit them with Magpul followers. I don't like any of the USGI followers, especially tan!
The only exception is 20 round mags. They are the only USGI mags I like as they are the most reliable and by design the follower has to tilt for reliable function.

As for USGI mags fitting into everything, I actually had multiple mags not drop free from my EGA lower while they drop free from my non-blem BCM lower or my buddies Colt LE 6920 lower. All of these "out of spec" mags were made by Center Industries and they are all newish condition, not issued to and beat up from LCpl Schmuckatelli.