An SBR is based on the overall length and the barrel length. From the ATF: https://www.atf.gov/firearms/firearms-guide-identification-firearms-section-5 The AR-15 link you posted is technically a pistol regardless of what marketing words they are using. What is somewhat confusing is the designation of the AR-15 lower. This is how the manufacturer designates the lower upon completion of fabrication. Keep in mind the following is "technically" what you are supposed to do, not what may happen in real life. Designated as a Rifle lower: May only be made into a rifle Designated as a Pistol lower: May be made into a rifle or pistol Designated as Other* lower: May be made into a rifle, not prohibited from being made into pistol The chances of someone stopping you while out shooting, and validating that the lower is being used for intended purpose as above is way less than zero unless you are planning a Broome styled raid on the local bank. Then it's just extra charges after the fact. *Other in this case is not the same as the ATF AOW designation. As previously stated, in that photo, due to the addition of the vertical handgrip, you would need to purchase that as an SBR, and thus $200 stamp.