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Posted: 11/4/2025 9:08:11 PM EDT
| I'm going to build my first .300 blackout and I'm trying to decide if I buy that caliber specific mags, or do I just feed it from my current 5.56 mags. I'm hearing from both sides at my local club/range. Some say the caliber specific mags make a huge difference in reliability and other call BS on that one and use the same mags that they run 5.56 through. Anyone have a validated view on that? Have you experienced a difference one way or the other? TIA |
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Originally Posted By Bravo_Six: With supersonic ammo, any mag should work. The .300blk specific mags are only sometimes needed with subsonic ammo. Whether or not you need it generally depends on the load you are shooting. Both OAL and bullet profile can make a difference. What he said Running 110-125gr, any 556 mag works fine If you're running 150gr+ I'd pick up some 300bo specific mags |
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FYI: the one thing I was told early on is make sure your 5.56 mm mags and your 300 BO mags look a lot different. A 5.56 mm chamber might allow a 300 blackout round into the chamber by forcing the bullet back into the case just enough to let it go bang causing a catastrophic failure. |
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Originally Posted By Urimaginaryfrnd: FYI: the one thing I was told early on is make sure your 5.56 mm mags and your 300 BO mags look a lot different. A 5.56 mm chamber might allow a 300 blackout round into the chamber by forcing the bullet back into the case just enough to let it go bang causing a catastrophic failure. Yep, I do metal (alum or steel) mags for 556 and polymer for 300 blk -pmags for supers and lancers for subs. Never any question about what’s in the mag, by sight or feel. You can also buy mag bands. |
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Originally Posted By GUN2: Yep, I do metal (alum or steel) mags for 556 and polymer for 300 blk -pmags for supers and lancers for subs. Never any question about what’s in the mag, by sight or feel. You can also buy mag bands. Yep. GI aluminum for 5.56 and Pmags for .300, keeps everything where it belongs. I do it as much for identification as for reliability but it serves both purposes I guess. |
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Originally Posted By Urimaginaryfrnd: FYI: the one thing I was told early on is make sure your 5.56 mm mags and your 300 BO mags look a lot different. A 5.56 mm chamber might allow a 300 blackout round into the chamber by forcing the bullet back into the case just enough to let it go bang causing a catastrophic failure. Yes. The only magazines I ever put 300 Blackout in are the Lancers marketed and marked for it, and are translucent. For my 458 and 450 Bushmasters, well, I guess I just have to be smart enough not to mix them up with each other. They certainly won't chamber in a 5.56. I've put a 300 Blackout round into a 5.56 gun before. Somehow, some way, three 5.56 rounds got loaded on top of a stack of 20 some 300 blackouts in a D&H magazine, which I don't recall loading. The magazine was loaded when I took it out of my ammo storage, and I don't know when it was last touched prior to this incident. I fired three rounds from that magazine in my Colt 6933, and thankfully the first round of 300 wouldn't go into the chamber. I was nearly shitting myself, thinking I had put three rounds of Blackout through my rifle and 5.56 caliber silencer, until I inspected the brass. Since then I stripped every loaded magazine in my inventory and inspected the ammo. I also avoid keeping non 5.56 magazines loaded. There's really no need to keep them loaded. Oh, and for 5.56 loaded magazines, FDE pmags contain my 77 grain defensive ammo. All other mags are assumed to contain ball of some kind. |
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| I’ve noticed zero difference regardless of ammo. Furthermore, although not AR related, a buddy of mine just bought a Ruger American Ranch Gen2 in 300 BLK that will not reliably feed 200gr REX subsonic from 300 BLK Lancer mags, but functions flawlessly with 556 pmags. |
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I use the caliber specific 300 BLK mag marking bands solely on my blackout mags. When I grab a mag there’s no question as to whether it’s a Blackout mag or 5.56, day or night. Attached File |
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