Magpul PMAG Magazine M3 Black .223 Rem / 5.56 NATO / .300BLK 20rd

223 magazine

This Magpul PMAG 20 Round .223 / 5.56 Magazine, M3, fits AR15 style rifles, and has a black finish. Mfr part number MAG560-BLK.

PMAG® 20 GEN M3 Magazine Specs

• Self-lubricating anti-tilt follower for increased reliability
• Over-insertion stop on the spine helps ensure that the magazine will not over-insert on compatible weapons
• USGI-spec stainless steel spring for corrosion resistance & field compatibility
• Aggressive non slip texture at the front & rear for superior grip
• Paint pen dot matrix to increase durability of mag identification
• Strong slim floorplate design that retains positive mag handling & easy disassembly

about 223 magazine

When I first started building AR’s, I ordered barrels without thought of how they were marked.
5.56mm or .223 Rem.
Neither did I pay any attention to what ammo I fed them, 5.56mm or .223 Rem.
I know I burned at least a thousand or so indiscriminate (5.56/.223) rounds through barrels with both markings.

I never had any signs of over pressure or lack of 3F (fire, function, feed).

Just how critical is it that chambers and ammo match on this particular round?

Has anyone out there had an incident due to firing a 5.56mm in a .223 chamber?

…and yes, I’ve been told, .223 in a 5.56 is ok. 5.56 in a .223 is a no-no.
…and why.

The technical part is that the freebore of the SAAMI 223 is smaller diameter, shorter and the throat angle is steeper. Optimized for shorter bullets and less bullet “wiggle” before it sees the rifling. Because the bullet encounters the rifling sooner, the pressure will generally be higher. The effect of the steeper rifling in the throat is less clear according to a study about bullet engraving that I read a number of years ago.
Also of note is that most of the more recent cartridges that I have seen use a shallower throat angle, often 1 1/2 degrees, like that of the 6.8 SPC.

I am not a 223 shooter, so I have no practical experience for the other questions.