This barrel has 5 groove (5r) rifling. - CBC Industries 300 AAC Blackout AR-15 Barrel, 5/8x24, 1-7, Micro

Great little barrel, the description doesn't mention it but I assure you the barrel has a nice 5 groove (5R) rifling down it. I used a KAK gas tube, KAK micro pistol buffer and tube (3 oz), a milspec BCG, and Superlative Arms adjustable gas block, set to full gas (there is still a small amount of forward bleed-off on full open gas, click 18 and 19.) Breek Arms 2 port threaded brake on the muzzle. Really didn't know what to expect when I loaded up the cheap Winchester white box 200 gr OTM subsonics, but it gently cycled all 20 of them and locked open the bolt. Loaded up 20 supersonic 125 gr cartridges, backed the gas out 10 clicks to bleed off more, and had perfect function there as well. Sometimes a build just comes right together, and this was the case for me with this setup.