If you already own a 5.56 AR-15 and you're eyeing 300 Blackout, the real question is what you can keep and what you have to replace. The answer is cleaner than most forum threads make it: your bolt carrier group, lower, buffer, and magazines all carry over. Only the barrel and gas system change. Everything else about a 300 BLK upper is identical to your 5.56 rifle. The short an …
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