is it true i cant hunt with a 18.5 inch barrel

Not true

Why not? It's a legal length. Your quarry may dictate more what choke you should have for consistent, ethical kills. I'm assuming you are talking shotgun here, but is it for small game or big game?

i hunt small game in heavy cover..also rifeled slug for deer

A mod choke or barrel is fine with small game with a 18.5 barrel..

Slugs on the other hand I would shoot out of a cyclinder choke.

Rifled Slug requires an adjustable rear sight or an optic while Hunting. Barrel lenghth needs to be at least 18" or Larger to be Legal in NJ. Its all on line or in the Free Hunting Regulation book

thank you..i just started hunting again.i took my hunting course 57 years ago