From what I have been reading in this pistol forum and the interweb about the measurement of the AR pistol’s OAL is to measure from the end the barrel muzzle to end of the pistol buffer tube. My Ar pistol’s OAL come out at around 26.5”, the barrel is Daniel Defense 10.3”, the buffer tube is from Midwest Industries, and the brace is SBM4. Let me know if you have another method to measure the OAL of the AR pistol.

P.S: I am not sure if you can see by zoom in the picture but the SBM4 is sitting flush with the buffer tube.

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AGAIN?
Measuring to the end of the tube is possibly the safest way to measure with the ATF changing their opinions regularly. The ATF has never stated anything about measuring to the end of the receiver extension so it is not a factual method, just a safety measure. Safety measure, LOL . The ATF states to measure to the extreme rearward point of a weapon. The ATF also states to measure to the end of the brace. The ATF also states not to exceed 13.5'' LOP on a brace so that must factor in.

The letters below are all we have to go on from the ATF in terms of opinions. There is also The freedom shop situation where the ATF decided to measure pistols folded if they have a folding receiver extension and that complicates things because it contradicts agent Knapps opinion. The freedom shop pistols that used braces to reach 26'' were not confiscated by the ATF, only the folding pistols were taken so the ATF again stood by the opinion of measuring to the end of the brace.

Measuring to the end of the brace is the ATF's method of measuring a pistol. The question that needs answered to be 100% safe is= How is each brace measured? Do you measure extended? Do you measure collapsed? Do you measure where it is adjusted as it is being used?  The ATF contradicts itself with how to measure a folding brace so who knows what else they may change their mind about.
If you want to play it safe and you do not have a folding adapter then measure to the end of the extension. If you have a Blade that locks on in a non movable position then measuring to the end of the brace is perfectly legal. If you have an adjustable brace or friction fit brace your risking an agent moving the brace around before they measure so measuring just the tube is a ''safety measure''. The ATF does measure to the end of the brace but until they give clearer guidelines of how to measure each brace we are left guessing.

The OAL only matters if you're wanting to put a vertical foregrip of some kind on a pistol, correct?

Federally, yes.