Welcome to the wacky world of AR magnified optics! Better buckle your seat belt.
There are 3 basic options for mounting scopes on an AR. Most people want the scope centerline around 1.3-1.5" above the rail so it'll be similar to the iron sights and they can use the same shoulder mount and cheek weld for either.
- Extra-high rings, with a centerline around 1.3-1.5". These don't give much forward eye relief, which a lot of people need, especially with illuminated scopes, which have long eyepieces. Stable but not very flexible on an AR.
- One-piece mounts, which come in a wide variety of heights, forward offset, and mount styles. These are simple and solid, but not very flexible. Here's the Big List of one-piece mounts:
www.maxicon.com/guns/optics/one_piece/ar15_onepiece_mounts.htm- Extended rail with rings. You want the ring centerline height and rail height to add up to the total height you want. These give lots of flexibility, but are more complex and have more connections to shift or loosen.
Also, you'll need enough height for the scope to clear the rear BUIS, which will depend on the BUIS, scope, and mount. The centerline of the rings or mount needs to be taller than the folded BUIS height plus 1/2 of the scope eyepiece diameter.
You can figure out where the scope needs to be mounted by bringing the rifle up the way you'd normally shoot it, then lining up the scope to see where the eyepiece falls relative to the charging handle, and where rings would fall relative to the rail.
Take a look at the tacked threads and the pictures of scopes, and you'll see all variety of mounts, rings, and positions.
While it's not cheap ($200), there's a lot to be said for just buying the LaRue SPR mount, which is pretty close to a perfect design. If it doesn't work out, you can re-sell it for a substantial portion of what you paid for it.
If you need to go inexpensive and you're using a 30mm scope, the CAA DVSR one-piece mount is a good starting point, and is only $25. It's not QR, but is well made, and lots of people have had good luck with it. The included 1" ring adapters don't work well, but you can buy ring reducers elsewhere that will. It's a great way to figure out what works for you for a low price.
Here's a review of it:
www.maxicon.com/guns/optics/caa/caa_dvsr.htm