I have had issues with cantilevers, specifically a change in the point of impact. All my slug guns are drilled and tapped. If you press up or down on the eye end off a cantilever mounted scope, it will move. I believe with the expansion or contraction of metal due to temperature changes with a cantilever allows or contributes to a change in the point of impact because it has only one point of contact.
I also have my barrels pinned to the receiver for added accuracy. I believe the new Remington super slug comes with a pinned barrel.
Hastings Laser accurate slug comes in a 3 1/2" 20ga version and Hastings partnered with H&R to produce the Hastings field slug gun with a 3 1/2" chamber. This custom gun is currently the only production gun that this slug can be fired from. I read an acticle that it kicks like a 3 1/2" 12ga turkey load.