BudsGunShop.com - 300 blackout conversion requirements

Hi Geoffry, technically, all you need to change from a 556 AR to shoot 300BLK is the barrel, but the easiest method is to swap out the entire upper.
Only thing that is hit-or-miss is using existing AR mags. The geometry of the 300BLK bullet-end of the cartridge is different enough that it might cause problems with some types of 300BLK ammo feeding in regular (556) AR mags. That's why they make dedicated 300BLK-specific magazines.
Whether you opt to get dedicated 300BLK mags or not, I recommend making sure mags are marked as 300BLK, either by buying dedicated mags with markings as part of mag or you marking dedicated regular AR mags to make them distinct in at least appearance. You do not want to mix mags loaded with ammo not matching the barrel: especially not loading 300BLK into a 556 upper.
This article should help you >>> badassoptic.com/best-300-blackout-upper/how-to-convert-ar15-to-300-blk/
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Stay safe & have a great day!

Best bet is to just get yourself a 300blk complete upper along with Dedicated 300blk magazines explained by Ed C. Below.

Hi Geoffry and welcome. Just the barrel would work but in more involved. Just get the appropriate 300BLK complete upper and the swap takes 10 seconds. Example 300BLK Upper : budsgunshop.com/product_info.php/products_id/718008918/m4e1-t+.300+black+complete+uppers ---------- Cheers, stay safe and enjoy!