I bought an, out the box railed NATO. I added so much for custimizing. . Now I want to build a lower w/ what’s next lower fitted w/a blk. Does any one know if the bcm rkm rms will work w/a 300 bk. I ask that because the geissle super spring won’t work w/a blk
Any ar15 lower will work with 300blk. I caution you to do things different…like colors, or markings, etc. 300blk will chamber in a 556 rifle and that equals a bad day at the range
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I can not agree more with this advice.
Do something, anything to mark your uppers and loaded mags to distinguish between 5.56 and 300 bo.
I have as much experience as anyone but I have had a close call and it scared the 
out of me.
Thankfully I wasn’t at the range when it happened. It’s easy to become complacent and that and guns don’t mix 
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In THEORY, the .300blk should not be able to Chamber in a .556 upper due to COAL. However, in PRACTICE often the Ogive of most .300 projectiles, coupled with the, erm… “Enthusiasm” of the Buffer Spring and mass of the reciprocating assembly can easily combine to “shorten” the .300blk cartridge enough to put the Bolt into Battery…
Not only should they be Marked, but the Smart Rifleman will not even have the .300blk ammunition out or anywhere near at the same time one is even Administratively Handling the .556 rifle.
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2 Saturday’s back…
Buddy brought his .223 & 300blk.
Prior to hitting the range checking over his toys I pulled several .223 cartridges out of his dedicated 300blk loaded mags.
Turned into one of those teachable moment’s aka R. Lee Ermey impersonations, “What Is Your Major Malfunction Numb …”, well you get the idea.
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Hey, the Geissele Super 42 spring works with 300blk except w/ subsonic ammo. It’s a stiffer spring and it may not cycle correctly with lower powered ammo.
The BCM RMS is basically the Vltor A5 system, as long as you match the buffer weight and spring to the ammo you’re using you should be fine.
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