There are a gazillion 18" rifle gas guns & most all of them, if set up right, run absolutely just fine.
Get the gas port right, the BCG & the buffer & spring all set up right & there's not a problem with any decent ammo.
If you get a barrel from a source that hasn't a clue on the gas port size & makes it too small, you could run into issue.
If the port is around .098" - .103", it should be fine; I used them with ports down to as small as .093" with no problem.
No need to dink with a piston system unless you just want to.
MM