The law specifies the lengths below which a barrel cannot be altered without changing status.
What if the barrel was a factory legal barrel below 18" to start with - like a 10/22, AR-7, or 94 Trapper with 16 odd inch barrels? Can one of these be shortened further? The barrel is not being altered, modified or shortened so that it becomes less than 18".
The law does not address this situation, and as far as I know, there is no official interpretation or ruling. IF, and IF is the operational word, an already under 16" barrel could be shortened, then the only restriction would be the 26" overall length.