View Full Version : Modifying mags to work with the 300 AAC Blackout (300BLK)

12-09-2011, 8:35 PM
Note: Sierra's new match 300BLK bullets do not require modifying the magazine.

Due to the larger ogive of .30cal bullets there can be some slight feeding problems. The rib near the front of the mag that supports a .223 bullet interferes with a .308 bullet. The 300BLK rounds tend to be forced to the centerline which causes increased tension and the rounds sometimes will not raise to be feed. The fix is to use a 12" file and file down the rib. Obviously this only works with polymer mags. Metal mags need to be machined which is too much work.


The first choice was to modify a PMAG and they worked great for a little while until I noticed the right side feed lip was crack on my three mags. I believe it was from the seating force of the rounds hitting the lip. 300BLK is much heavier than .223.

While looking for a solution I came across Lancer's Advanced Warfighter Magazine. Polymer body with steel feed lips. Same thing, file the rib only enough to feed smoothly. Testing to come.


12-10-2011, 9:31 AM
Looks good, let me know how these work for you.

12-10-2011, 6:42 PM
Are these generic .30 bullets or bullets especially designed for 300 BLK?

I keep hearing that the specially designed bullets, do not require magazine mods.

12-11-2011, 5:27 AM
The Sierra were designed for 300 BLK and are optimal in normal mags. This is also true of the new Remington UMC ammo. This is also true of the new Barnes Vortex ammo. No one who wants to use factory ammo should be modifying mags.