I want to build a CAR 15 using an A1 lower.  How short of a barrel can I put on it?

16" or 14.5" with a permenantly attached flash suppressor (like the phantom or vortex). The barrel just has to be 16" overall.

Without ATF approval for a Short Barreled Rifle (and of course the $200 tax) a rifle barrel has to be at least 16".  A tad over is always a good idea, since the regulations don't really specify whose tape measure is to be used.  The 16" length includes all permanent parts of the barrel, so the measurement goes from the barrel extension to the muzzle end, and if your muzzle device is permanently attached (there are thousands of threads here about permanent muzzle devices!), then it counts too, so a 14.5" barrel with a muzzle device that puts it over 16" is fine too.  Some people even get 11" barrels with permanent 5" muzzle devices-why, I do not know!

Quoted: 16" or 14.5" with a permenantly attached flash suppressor (like the phantom or vortex). The barrel just has to be 16" overall.

You can also have a 11" barrel with a 5" Permanently attached FS....  

Quoted: Quoted: 16" or 14.5" with a permenantly attached flash suppressor (like the phantom or vortex). The barrel just has to be 16" overall. You can also have a 11" barrel with a 5" Permanently attached FS....

or a 4'' bbl with a 12'' Permenantly attached FS ... case and point... keep it above 16'' overall and your fine.

Measure bbl length like this. Close bolt on an empty chamber. Insert wood dowell in bbl. Mark dowell at tip of muzzle. Pull from bbl and measure..

Thats the easiest way to do it.

Without ATF approval for a Short Barreled Rifle (and of course the $200 tax)

How long does that take to get the form back?

3-6 months or more.