OK. I'll explain it in a more general way. I just assumed that most people on ASC knew what a STANAG mag was.
Anyways, it stands for Standardization Agreement, a NATO thing where members use common things (ie. calibres, comms, map symbols/blocks, etc.).
STANAG magazines are standard M4/16 magazines made for use with the standard 5.56x45mm NATO round.
As for the SR-15 and 16, they stand for "Stoner Rifle", an AR-15 (Armalite model 15) derivative made by KAC. Note that the Armalite was made by Eugene Stoner and I believe the "SR" to be some sort of homage to him however this is only speculation.
The SR-25 is actually chambered in 7.62x51mm NATO. So what I was saying is that if you (or someone else) has an SR-25 make sure you get the right magazines (since they're different than the regular M4/16 magazines).
To break it down:
SR-15/16, M4/16 (and most other armalite derivitives), SCAR-L = STANAG magazine or M4/16 magazine.
SR25, SPR, SCAR-H and such = 7.62x51mm magazine which is a totally different size.
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