I own a 12ga rem 1187 sp that I use for turkey hunting and am considering putting a slug barrel on it for a back up deer gun. I need some advise. Does anyone know if you can mount a scope on the fully rifled iron sight barrel with using see thru scope rings. Is the barrel drilled and tapped already or do I need to but the cantaliver barrel. Can anyone suggest which barrel is better.

The scope goes on the receiver not the barrel. Take it to a gunsmith and they'll drill and tap it for you.

Is it worth having that done or just pay the extra $ for the cantilever barrel and be done with it

I prefered my cantilever, seemed to hold better zero and I didn't like pulling my head up to use shoot through rings. My 2c

Having it drilled and tapped is worthless if your using it for turkey. Put the money into a rifled/cantilever barrel, get yourself at least a decent scope. I own three 1187s. One is a 12ga with the shurshot stock camo this is my turkey gun, the other two are 12ga & 20ga both with rifled/cantilever barrels both with Leupold VX-II 3-9x40 scopes

BHC told you right if you're shooting any distance, or if you just prefer a scope. Cantilever barrel is the way to go.

If you're shooting 75 yds or under though, you may fine with a smoothbore slug barrel with iron sights. This way you can shoot "rifled slug" ammo that's MUCH cheaper and highly effective as well. My Mossberg 500 with smooth slug barrel will group just fine out to 75 yds.