RE: slug vs. buckshot
Hi Peyman,
I noticed from your first post that you have a "rifled" barrel on your shotgun. And from the last post I'd gather that you have adjustable sights. If both of those are true you'd sell yourself short if you don't try either a "saboted" slug....or for "rifled barrels only" slugs. Either way with a good adjustable sight you should receive sufficient accuracy and trajectory to zero your gun for 100 yards. Zeroed as such it should shoot flat and accurate enough for shots on deer sized targets to at least 100 yards....and maybe 125 yards.
You might as well take advantage of your equipment when you can?!? Let us know how it goes!
Best of luck....Dave.