You will need higher rings than you generally would with a standard setup because of the tube. With the forend top rail (if you use it) and the proper base you have a continuous rail like on a AR15 so you use the same height mounts as you would on a AR. For a 50mm scope you would want at least a 1.35-1.375" mount to clear the scope and NF bikini cover. If you use butler creeks which run a little thicker then you should step up to the next size mount.
If you run it without the top picatinny rail then a set of 1.125" high rings will work with most bases.
These numbers depend a lot on the exact scope base you use. I'd personally consult with AO for your exact setup, they will be able to tell you exactly what height you need based on the specs of your build.