Are you wanting to put this on an AR pistol or put it on an AR rifle? Your first post is confusing. The serial numbers on a lower classify the gun as either a rifle or a pistol, you cannot just up and convert one to the other, it would be an SBR. If you put this on a rifle and it makes it shorter than minimum legal length, you just committed a felony. The buffer tube has nothing to do with it
(When buying a lower to build an AR pistol, your 4473 needs to be filled out as pistol, not rifle. Lots of guys unknowingly running around with illegal SBRs )