I am not aware of a 22rf AR barrel extension... It needs to be machined directly into the barrel....So you have to start out with a large enough OD blank to accomdate for machining the barrel extension/shoulder into the barrel. Keystone Accuracy sells match grade AR15 22LR uppers and barrels using the CMMG Bolt Only. I have one in 12.5" and its extremely accurate with CCI SV. Uses standard AR15 upper and lower and Black Dog 22LR magazines. Keystone sells them as side chargers as well.