It is according to the chucking and spider method used. I use a slip-on spider and my 4-jaw threads directly onto the spindle instead of using a backing plate/chuck combination.
With this setup I can work a barrel down to about 18-1/4". This is with 1-5/8" of barrel protruding from the chuck jaws. This leaves plenty of room for a 1.2" long tenon.
Using a front spider instead of a chuck, you could get down to under 17-1/4".