Upon attempting to sight my 935 Cant. Slug barrel in, I found that there must be a major problem with it. I tried Winchester BRI, Federal Premium Hydroshok, and Federal Premum Barnes expanders. None would group and I started noticing the holes in the paper were keyholed, buddy shot same slugs as I was into the same target and no keyhole!!! Ended up getting some remington Buckhammers and sighted them in and they flew fine(as expected) but I just coulnt believe that sabots were tumbling out of a new rifled barrel!!!! Any one ever heard of a problem like this???

Assuming you have verified that the barrel is in fact rifled, I'd say it doesn't like those other brands. Can you see rifling?

I always suspect the mounts and the scope itself, but you say the 'hammers shot good. If you have time and the inclination to mess with it, tear it all down, degrease screws and holes, remount with lok-tite, and try again.

Otherwise, shoot the 'hammers.

The scope and mounts have nothing to do with the bullet tumbling. That barrel shoud not have a problem with 3" 1oz slugs. I would not want to shoot those 600gr buckhammers if I didn't have to.

Those bullets might be too fast and light for the rifling in your barrel? My H&R slug gun hates the faster Barnes bullets. Can't even keep them on the paper at 100 yards. I've decided I don't think my gun likes bullets that go over 1700-1800 fps.

Paul

I know theMossberg 835 barrels have a 1-36 twist, I am fairly sure your 935 barrel would be the same.
I would be surprised if the twist rate alonewas the source of your problem, considering you used three different types of ammunition.