...where did you get a smoothbore, cantilever scope mount barrel for an 11-87? I tried to find one for my 870, but couldn't, although the B-Square I'm using works great.
For .45K: You're getting great groups, although I don't know why: are you really using a rifled choke tube with rifled slugs? The slugs want to spin at their own rate, and the rifled choke wants to spin them at another. Somehow, all the planets must be coming into alignment for you!
I'm using an 870 Express with smoothbore barrel and Leupold 2-7x32 atop a B-Square mount. With Remington 12 ga. 2 3/4" rifled slugs, I can cover my 50 yard groups with the palm of my hand, whatever that works out to be in inches. All I know is it kills deer, no tracking required. It took a few "settling rounds" after I assembled it to get the groups down to that size, and it also seems that it shoots better when not completely clean (but not too dirty, either).