Will I run into any problems mounting a scope with a 50mm objective lense? It will be on a 20" HBAR flattop AR15.
I bought a scope mount block that the salesman said would allow standard scope rings to be used, but I didn't mention using a 50mm.
Should I stick to a 40mm scope?
The height at which scopes are mounted on the AR15 is dominated by the sight line, so choosing a 40mm scope won't necessarily make it rest any lower as is the case with most other guns. I would, however, consider a cantilevered mount, as most scopes have a bit too much eye relief for the AR15 platform. Getting them a bit more forward helps tremendously.
I can highly recommend the Larue SPR mounts, which put the scope into comfortable eye relief range and can accomodate a 50mm bell with flip covers. Here's a pic on my M1A using a 50mm objective. The mount is reversed from the way you'll be using it.