I have a couple 1100's...one from the early 60's and one from the late 70's.
Neither will group a slug through the smooth barrel to save their life. I bought a cantilever rifled barrel and it's game on....shooting 2" groups at 100yds w/ it.
Maybe you can keep trying slugs...but your best bet may be to look for a barrel now. Problem is that a good slug barrel for the 1100 costs more than an H&R slug gun...that will outshoot the 1100 in my opinion.