Free: Contests & Raffles.
Should be end of the muzzle to bolt face. You can use a cleaning rod or wooden dowel... mark it at the muzzle, then measure it. To make sure, it wouldn't hurt to check with the BATF.
Barrel length is from the bolt face when the bolt is closed, to the end of the muzzle. Less than 16" on a rifle, and you fall under the classification of "Short-barreled Rifle". Illegal in WA State. The only way a barrel shorter than 16" is legal is IF the length to the end of a muzzle brake or flash hider extends the length to 16" or more, AND the muzzle brake/flash hider is welded, pinned, or otherwise permanently attached.
Slight thread hijack... How about revolvers? I assume you don't include the cylinder length in that measurement, right? Measure from the gap to the crown?