I took my Remmy 1100 - 18" to the range this week to pattern it.

I used the loads I run for 3 gun / home defense - 00 buck, 000 buck and slugs.

At 50 yards, the buck shot must have been 36".  Now, I knew it would be spread - but gimme a brake!  

Slugs were point of aim - no surprise there.

Is this just missed expectations or should I be looking at another setup?

Not too unusual. Try more choke (its a screw-in, right ?). Try different load/brands. My 3-gun shotty is a 21" 1187 with a sawn off barrel - no choke at all. It spreads like crazy, which is great for static clays but sucks for long-range poppers. When extra punch is required, I switch to hotter birdshot or buck.

I use a 20" barrel, I switch between improved cylinder or modified choke for 3-gunning. You can get by with an 18" barrel if you put on the right choke for the COF. If you don't have chokes look into threading the barrel.  If you have a decent choke system the only thing limiting you with the 18" Barrel is round capacity. My mag-tube lines up even with my barrel and I can fit 8+1. I suppose you could put an 8 shot mag on an 18" but it would stick out a couple or more inches and it would take away any handling advantage a smaller barrel gives you.