I have a Mossberg 695 bolt action, 22" rifled with ports, 12 ga that will put 5 Fed Prem Barnes Expanders ito 1.5" at 100. I have taken deer at 180 yards with it. I am shooting the 2 3/4 shells at 1400 fps. I have tried the 1900 fps models and only thing it did was clear my sinuses and pull me out of my boots. Definitely not for the faint of heart.
If you use a gun like the 870 or similar with interchangable barrels, have your smith drill and tap the rcvr and barrel so you can put one screw in and make them tighter. Will help groups quite a bit.
Just about any smoothbore will group well at 100 with the forester type slugs (ie Win, KO, Brenneke etc), but the drop after that will get you. That is where the sabot slugs shine with less drop. You can work drop tables very easily also.
For muzzleloaders try http://www.prbullet.com
for very fast and accurate bullets along with breech plug mods that will really make your muzzleloader shoot well. I have their encore mod plug for the 25 ACP case with small rifle primers. Really tightened by groups up. They really have some low cost mods that make normal T/C and Knight guns long range shooters. Way under a custom gun. also you might find that the new Nikon BDC reticle for a muzzleloader is perfectly matched for a shotgun shooting sabot type bullets and that is geared for 250 yards.
I am ordering the 20 ga barrel for my Encore, I think that is flat and has enough knockdown to go 200 yards.