Not familair with the 1300. I assume its got a screw-type piece that keeps that barrel on the magazine tube. Make sure it is tight. TIGHT!!!. Clean the barrel. Clean it again. Wait a couple hours and then reclean your barrel. Make sure your scope is tight, both tight to the mount and that the mount is tight to the barrel. Don't let you barrel rest or touch anything when your shooting. If you have a "shotgun sling" that loops around the barrel, take it off. Pull hard into your shoulder with BOTH hands. (This doesn't mean white-knuckle her so bad that you develop a flinch.) And lastly, try different slugs. These are all the trick, aside from lightening the trigger pull on my 870, that took my groups from 8" @ 100yards to 2" at 50. The clean barrel and trying different slugs are what I believe made the biggest difference.

I forget exactly where its at but google lightfeild slugs and find there page. Someone on there is a page/article about shooting slugs...and how its a slightly different game than than rifles.