I came up with the following minimum AR-15 barrel lengths to reach the required 26" OAL using different receiver extensions (buffer tubes). Each brace/stock was taken off and the measurements were made from the end of the receiver extension to the end of the muzzle without a device installed. Of course, different manufacturers of barrels and extensions would vary slightly.
9.9" for the KAK long pistol extension
11.6" for the Gearhead Tailhook extension
11.8" for the standard CAR extension
11.8" for the standard Pistol extension
11.8" for the KAK Shockwave extension
12.0" for the CMMG Ripstock extension
I did not have an A5 extension to measure. Also, if building an AR-10 type firearm, you can deduct about 0.6" from the barrel measurements.