I live and hunt in Ohio where we can only use shotguns loaded with slugs for deer.
Now we can use handguns (certain ones) and of course the best of all, MUZZLELOADERS!
When I was into shotguns only seasons, all we had were smoothies and foster slugs.
A bunch of us serious about making a true 100 yard deer shotgun did a lot of experimenting and a TON of shooting.
Here is what we learned from actual tests and range work.
#1) make sure the barrel is TIGHT on the action. That means your pump or auto loader must be tightened as solid as you can get it.
The Remingtons had the deep cross notches in the mag cap so it was no trick to use a large blade screw driver to get the sucker tight.
The Mossbergs didn't have the deep notches so we drilled a hole in the side of the cap and used a hard steel rod, like a drill bit to force the cap tight.
You'd be surprised how much this helps as if the barrel seems tight it isn't and moves with recoil spoiling accuracy.
#2) Good sights. Forget those junk side saddle mounts. Too much slop. We had our 870's drilled and good Weaver mounts, rings and good scopes installed. This in the days before they came predrilled.
#3) shoot several brands of slugs and see which your gun performs best with.
Foster slugs are cheap compared to sabot slugs.
#4) CLEAN the barrel. Slugs lead the barrel quickly. Mossbergs are the worse, they have very rough insides. Smooth them by sticking a cleaning rod in a good electric drill and use a 'bore mop' with 400 grit paste and polish until mirror bright, then polish some more!
After 10 rounds use a wire brush and don't spare the elbow grease cleaning the lead out!
I used to take a cordless drill to the range and use the rod, brush in the drill and clean the lead out.
Lead leaves 'lump' deposites and spoils accuracy.
#5) LUBE THE SLUGS> How? Get some white lithium spray grease and squirt it into the mouth of the slugs. You can see the grease fill the "rifling" in the "rifled" slugs. This cuts leading down a good bit.
Does all this help?
If I had a smoothie tuned the way I described and it wouldn't one ragged hole at 50 yards it found a new home right quick.
My best 100 yard group?
Once, just once, I got an even 1" center to center 100 yard group. The gun was an 870 Remington Police Riot gun, 20" barrel, no choke, polished, Winchester foster slugs, greased, drilled and tapped with Weaver mounts/rings and 2.5 Weaver post crosshair scope.
Average groups ran 3" at 100 yards.
Longest kill???
175 yards, paced off and witnessed. Lucky shot? Probably so.
Some guys I hunted with actually had the 870 barrels permanently pinned in place so it would never come off and never move under recoil.
If I still used a shotgun I'd never WASTE my money on sabot slugs and rifled barrels.
I've seen guys with $800.00 rifled shotguns including expensive optics that pronounced the 5" groups 'good'. I'd say that SUCKED.
I've chronographed about EVERY brand of foster slug on the market.
Brenneke is o.k. but is 200 fps SLOWER than Winchesters.
What is the best foster slug?
The one YOUR gun shoots most accurately.
Stay away from double barrel shotguns for slugs. Most won't keep both barrels on a pie plate at 25 yards with slugs.
Single shots can be deadly if you have one in modified or imp. cyl. NO CHOKE is best.
Trick up your pump or auto loader and do what I suggest and your in for a surpise.
Another last thought.
Once sighted in..DO NOT remove the barrel. You WILL change your point of impact.
Clean the barrel ON THE GUN using the rod in the drill with a brass brush.
Listen I've shot THOUSANDS of slugs getting this thing working, my buddies shot even MORE than I did, buying slugs by the case.
It works.