I have a cantilever barrel and scope for my Browning BPS and it has been rock solid steady since the day I put it on.

Now, your mileage may vary, however on my gun I can switch back and forth between the fully rifled and smooth bore barrels with no appreciable effect on the zero point of the scope.

Just make sure the end cap is tightened the same amount (I use a small paint marker dot on the cap and one on the barrel ring to get it aligned properly). The other part of the equation is to have a good scope and good solid mounts.