IMHO it would be extremely foolish spending ~$150 on an 18.5" barrel when you can buy a Maverick Security or Stevens 350 Security for ~$220.
just put the money towards a second gun. alternately you could purchase the $300 Mossberg 2-barrel combo set (with 18.5" barrel), and sell whatever you do not need. Mossbergs/Mavericks are so cheap that it really is not worth buying barrels separately for them.