question of barrel length on firearms?

With the exception of revolvers, yes. Revolvers do not include the length of the cylinder. All other firearms include the chamber and are legally measured from the bolt face when closed.

The ATF measures barrel length from the bolt face with the gun in battery. That measurement would include chamber which is typically part of the barrel on firearms with a bolt. In many firearms, the bolt locks into the barrel, so the actual barrel length can be less than the overall length of the part called the barrel.

I'm fairly sure its from the bolt face to the end of the barrel, or at least that's the measurement that determines if a rifle is a SBR or not.

With the action closed and clocked (delete the 'L' from clocked; silly censors), measure from the boltface to the muzzle. If you have a permanent, fixed muzzle attachment like a flash hider, measure all the way out to the end of the attachment. If the muzzle attachment is removable, measure to the muzzle without the attachment in place.

i only use bolt action rifles and shotguns, bu 24 inches is generally the best allround lenght.