I am using a BAT barrel extension. I will headspace it to the bolt that I buy. Headspace will be perfect. My POF lower and upper were made on a CNC machine and the tolerances appear to be better than my Colt and RRA AR-15s. I don't think there is a problem with the better lowers/uppers out there. Just make sure they are compatible with each other. I do not want a piston gun so I am not using POF parts beyond this.