I have a new Remington 870 rifled cantilever barrel and put a Leupold 3-9-40 on it. What I have noticed is a lot of flex in the cantilever. If you put some pressure on the scope and squeeze it slightly toward the receiver you can see that there is flex. Is this normal?

I've only shot Hornady SST slugs out of it and sighted it in at 50 yrds. They were grouping really well. But when I moved the target to 100 yrds, I was hitting about 18" high, consistently. I moved back to 50 yrds and was hitting zero again. What could be the cause of this? I was shooting from different type benches and my shooting bags may not have been in the same resting point on the gunon each bench. Could this have that much effect? And if this did make that much of a change in impact, what effects will it have when I shoot from a rest in a hunting situation? Any suggestions and help would be appreciated.