I guess I will still ask what you have in mind, 18.5" on a shotgun barrel is not the best bird gun of them all, 22-24 inches would likely be more practical for any field hunting. Screw in chokes help you maintain a pattern at varying ranges depending on constriction of choke and type of shot. How familiar are you with shotguns?
The 18 " barrel would generally be concidered a "riot gun" and not a hunting gun.
If there is a gun club about that you could learn at or practice at you may want to get more familiar with the gun and shotguns in general before going any farther; especially in light of the issues purchasing firearms in your corner of the world.
Find a few reviews and shooting tips here:http://www.americanrifleman.org/
Maybe this will be of help:http://www.americanrifleman.org/Arti...id=1842&cid=12