Hi, and welcome.
The upper receiver is 7" long, your scope is 12.5" long and the scope's eye relief is 4.4". That and your height/length of pull puts the back of the scope properly maybe an inch forward of the back of the receiver.
My advice is to forgo standard rings and look at cantilever mounts. Reason being, the length of the scope you are talking about puts the forward section of the tube over the handguard. Handguards are not considered solid enough for scope mounting which is why manufacturers produce cantilever mounts for the AR15. They clamp on the receiver and extend forward over the handguard with a spine.
If you are wanting to shoot 800yds then the drop from an 18" barrel will be about 29MOA. Your Leupold has an adjustment range of 64MOA. Half up is 32MOA minus a couple just in case, 30MOA. You need 29 and you have 30MOA adjustment which will be close but that means you can get away with a parallel (0 MOA cant) cantilever mount. At longer ranges you will have to use the graduations on the reticle and aim above the target. This calibre can go further so limiting yourself to 800yds by the scope mount does not make sense. It would be better to get at least a 10MOA cant for when that opportunity comes along to stretch out past 800yds. The standard AR scope axis sits about 1.5" above the top of the handguard so having a 10-20MOA cant is unlikely to tilt the objective bell of a 50mm into hitting the handguard. My point is there's no sense in limiting yourself with a parallel bore axis/sight axis mount (0 MOA cant) for the same price as a canted mount.
Everyone has their favourites and they include all the brands in VASCAR's post. There is one brand not mentioned which arguably is as good or even better than most of them, SPUHR. Not discussed much on this forum because not many people use them (including me). The price will make you choke and perhaps also because they're made in Sweden so national pride in manufacturing may be an issue. $450 on Brownells [choking sound effects!]
http://spuhrwebshop.com/en/cantilever/sp-3616.html
The upper receiver is 7" long, your scope is 12.5" long and the scope's eye relief is 4.4". That and your height/length of pull puts the back of the scope properly maybe an inch forward of the back of the receiver.
My advice is to forgo standard rings and look at cantilever mounts. Reason being, the length of the scope you are talking about puts the forward section of the tube over the handguard. Handguards are not considered solid enough for scope mounting which is why manufacturers produce cantilever mounts for the AR15. They clamp on the receiver and extend forward over the handguard with a spine.
If you are wanting to shoot 800yds then the drop from an 18" barrel will be about 29MOA. Your Leupold has an adjustment range of 64MOA. Half up is 32MOA minus a couple just in case, 30MOA. You need 29 and you have 30MOA adjustment which will be close but that means you can get away with a parallel (0 MOA cant) cantilever mount. At longer ranges you will have to use the graduations on the reticle and aim above the target. This calibre can go further so limiting yourself to 800yds by the scope mount does not make sense. It would be better to get at least a 10MOA cant for when that opportunity comes along to stretch out past 800yds. The standard AR scope axis sits about 1.5" above the top of the handguard so having a 10-20MOA cant is unlikely to tilt the objective bell of a 50mm into hitting the handguard. My point is there's no sense in limiting yourself with a parallel bore axis/sight axis mount (0 MOA cant) for the same price as a canted mount.
Everyone has their favourites and they include all the brands in VASCAR's post. There is one brand not mentioned which arguably is as good or even better than most of them, SPUHR. Not discussed much on this forum because not many people use them (including me). The price will make you choke and perhaps also because they're made in Sweden so national pride in manufacturing may be an issue. $450 on Brownells [choking sound effects!]
http://spuhrwebshop.com/en/cantilever/sp-3616.html