I'm going to be the bad guy here...
First, scopes with a 50mm bell have a very difficult time fitting an AR. They have to sit so high, you'll need an expensive stock (or stock add on components) to compensate. Otherwise you'll never get a proper cheek weld.
Second, you bought the AP4... which is absolutely fine, and a very decent rifle... but you have a Looooong range scope for it? That power range is way over what that rifle should be wearing. A 3-9x would have sufficed. Maybe a 4-12x at max.
Last... that Sightmark is going to get beat to shitt on a .308. You could've found a Bushnell, Burris or Vortex for around $100-$125, that would outlast the Sightmark.
Personally, I'd start over from scratch.