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Hi Steve and welcome. This will work no issue as well on the CMMG 300 Blkout upper. Cheers, stay safe and sounds like a great build!

To determine if your BCG (Bolt Carrier Group) will work with your CMMG 300 Blackout upper, you'll need to ensure compatibility between the BCG and the upper receiver's chamber. The BCG must be compatible with the .300 Blackout cartridge, which is generally the case if you have a standard AR-15 BCG. However, there are a few things to check: Caliber Compatibility: Ensure the BCG is designed for .300 Blackout. Most AR-15 BCGs will work with .300 Blackout, but it's good to confirm. Buffer and Spring: .300 Blackout often requires a heavier buffer and spring compared to standard 5.56mm/.223 setups. Check if you need to adjust your buffer system. Fit: Make sure the BCG fits properly in the upper receiver and cycles smoothly. If your BCG is a standard AR-15 BCG and your lower receiver and upper receiver are both AR-15 platforms, it should generally be compatible. If in doubt, consult the manufacturer's specifications or seek advice from a gunsmith.