$500 budget will buy plenty of glass. If weight is an issue.. I'm sorry. I assuming these things: budget, dpms (gettin beat around), possibility of shooting in low light; I will post.
Just bought a new scope for my 22-250 and went through all this. I called Nikon. Prostaff great light transmission and clarity, and the monarch is huge. I settled on a buck master. Excellent light transmission, durability, and clarity. If you must go with a 40mm objective I'd look for a 30mm tube to compensate for light transmission degradation.
40(obj dia) divided by 7(ideal exit pupil diameter)= less than six power. At ten power pupil diameter is 4mm and so on. Assuming that I bought a 4-12x50 buck master bdc. It's awesome. Already dropped it (from vertical to horizontal on pasture) and it hadn't budged. Online cheapest next tag $260.