Over the years I have taken around 35 whitetail deer with slugs. In Manitoba we have a shotgun/muzzleloader season. You can use a muzzleloader or shotgun with slugs. For several years we could take up to 3 deer in this season using also a second and third tag. No poaching or bragging involved here. The season starts the last week of September, closes for several weeks and then re-opens typically until Christmas eve.
I use Mossberg shotguns with rifled barrels. I have a Nikon Slugmaster 3-9X scope on my "posting" gun. I have two others with rifle sights, one has fibre optic sights. The old Remington vs Mossberg debate is a great one and I won both, but I bought my first Mossberg when I was 16 and it's what I'm used to. I don't load my own slugs but I've done fairly extensive testing with various slugs at various ranges (out to 300 yards). I've recently started hunting the high power season again and it wasn't for the weight of my shotguns...I'd be using one during the season.
So here's a Coles notes of my 20+ years of shooting deer of various sizes with slugs in temperatures ranging from +15 to -30 C.
1. Cantilever barrels suck. There's movement involved because groupings with the one's I've tried are terrible. At 100 yards I'm sure they're fine but anything further I'd forget about using one.
2. Without a rifled barrel you're lucky to make any kind of respectable shots past 50 yards. A smooth bore shotgun is useful to shoot an angry bear that just tumbled out of the bush in front of you. I do carry a Remington with a 14 inch barrel in bear country for this.
3. A properly outfitted shotgun with a slug barrel and scope is freaking heavy. I've been reminded of this the last couple of years hunting the high power season with my synthetic stocked rifle.
4. Federal slugs have served me the best over the years accuracy wise. Currently I shoot 3 inch Federal Premium Vital Shok Trophy Copper Sabot Slugs. These are rated for 2000 fps and in my Mossbergs I get on average 1950 fps (chrony'd). Accuracy wise these things are amazing. At 200 yards they group consistently within 10 inches (some groups are better). At 100 yards they get 2 inches pretty consistently. The drop from 100 to 200 yards is significant and you need to adjust for that. I have a Nikon Slugmaster on my posting gun and while the predefined markings are no where close....I have a cheater card stuck to my stock that tells me which to use at which range. Using this setup and a rangefinder, I have actually made a 280 yard shot on a big doe. I would not really recommend trying to make a shot that far but in my defense it was the end of the last day of the season and I knew my elevation mark to use - windage wise there was probably some luck involved. Before these slugs I used a hollow point copper slug from Federal. Accuracy was good but not quite as good as the Vital Shoks. I did prefer the way the hollow points opened up and I still use these slugs in my smooth bore bear shotty.
5. A proper slug makes for an incredibly clean kill. The exit hole on a whitetail deer using the two slugs mentioned above is incredible. I've had exit holes the size of my fist and never smaller then the base of a beer glass. I have recovered two slugs from whitetail deer and only because they hit darn near every bone in the animals body. Slugs penetrate a lot which leads me to believe they'd be fine on a moose too. All but one deer I've shot dropped in it's tracks - and some with less than ideal shot placement. The one that ran, went about 40 yards. She ran in front of me and I could see blood pouring out of her - I've never seen a deer that tough before. You do not want to ever take a shot with a slug where you might hit the animal in the rear end - the wastage would be horrible.
Side note....last year I shot a buck from a tree stand with my .270 which was my first high power deer taken in several years. I was shocked when it ran off, thinking I somehow missed a 30 yard shot. Turns out it was 25 yards away with an hole in the heart - but I had not seen a deer not simply drop in it's tracks for many years (hunting with slugs).
6. Rifled sights on a Mossberg need locktite on the screws. I've lost a rear sight in the bush. The fibre optic sights that come (or came) with, are too much light. Maybe it's my old eyes but the blurr around them if you're facing into the sun around dusk or dawn makes them almost unusable. My scoped shotgun was the fibre optic setup - I took them off.
7. Speaking of locktite...the weak point accuracy wise on the Mossbergs (and I'm guessing the Remington) I believe is how the barrel attaches to the receiver. I use a set of slip pliers to tighten the barrel nut to make sure it's as snug as possible. There is no way that you're removing this nut by hand on my slug guns. I know that some custom Remington's have a pin that goes through the receiver to hold the barrel in place. From what I've experienced, you can achieve similar results by really tightening the barrel nut.
8. Recoil pad. Recoil pad. Recoil pad. 3 inch slugs kick like a mule. The Federal 3 inch vital shock kicks like nothing I've ever shot before (and I've shot some pretty big stuff). A lead sled is a must when sighting in.
9. Shooting good slugs ain't cheap! I believe the last time I bought the Vital Shok slugs they amounted to being around $6/round. Once you have things dialed in...dedicate that setup to slugs. This is why I won more than one shotgun as I shoot geese too.
This are one old man's experiences...