Recently I read an article where some had mixed up .300 AAC Blackout (300 BLK) rounds in their non-dedicated rifles with disastrous results. My first suggestion would be to dedicate a lower receiver and magazines to the round, but I realize that may not be practical for some and does negate the benefit of being able to change out uppers on the AR to create a different setup. At a minimum dedicate magazines to your 300 BLK rounds so the mistake never happens to you. For those unfamiliar with the round it does not require a dedicated magazine and so you can load your .223/5.56 AR magazine. With the low cost of magazines currently simply purchase extra magazines and mark them as 300 BLK. The issue comes when someone inadvertently tries to shoot the a 300 BLK in a .223/5.56 or vice versa. Because the 300 BLK is 7.65 x 35 and the 5.56 is 5.56 x 45 the shorter and larger diameter 300 BLK will fit in a .223/5.56 chamber as depicted here.

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As you can imagine it would be a disaster to try and fire a 300 BLK round in a .223/5.56 and you could suffer a horrible injury when the bullet has no ability to travel outside the chamber. Spend a few bucks on dedicated magazines and mark them for the round so you never make the mistake while shooting your 300 BLK. If you are worried about appearance then color fill some part of the 300 BLK upper and color fill some part of the dedicated magazines. With a few minutes of preparation you can be sure this mistake is one you never make.

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