A scope mount for any AR', big block or small block format, requires at a minimum two things:
1. The right height above the comb for a proper cheek weld and ...
2. Sufficient forward offset to provide adequate eye relief for the optic used.
There is a third, but we can get to that later.
Let's use the flat top upper receiver as our point of reference here. The axis of the iron sights on an M-4 have a height above the flat top upper receiver of 1.4". It's not surprising, therefore, that most scope mounts have an axial height in the 1.4" - 1.5" range. The good news here is a 1.5" high mount will allow for a 56mm objective scope - plus lens cover.
Here's a shot of a small block gasser in 5.56, with a LaRue mount 1.5" high:
Here's a big block gas gun in 6.5 CM with a Spuhr SP-4016 mount, 1.5" high:
Another SR-25 pattern rifle in .260 Rem, with a LaRue LT845 mount, 1.535" high:
Keep in mind that a big block gas gun has about 1/8" more drop to the comb of the stock than does a small block gasser. In other words, the SR-25/AR-10 will have 1/8" more distance between the top of the receiver and the top of the lower receiver extension. You pull the scope and mount off your AR-15 and drop it on your SR-25/AR-10 and you will have to elevate your Mk 1 Mod 0 eyeball ever so slightly higher for a correct field of view.
Eye relief on most scopes used today is approximately 3.5", plus or minus. We have shooters of all shapes and sizes to accommodate. We accomplish this by working with the length of pull of the stock and where on the rifle the scope is placed. Normal scopes on AR-15 format rifles always require some forward offset. Most shooters benefit from some degree of forward offset on the big block gas guns.
An exception to this 3.5" eye relief rule of thumb is the Leupold TS-30 2.5-8x36, developed in a hurry for the military's SPR Mk 12 Mod 1. A scope was needed with a shorter eye relief (3" at max power) so that conventional ARMS rings mounted to the upper receiver could be employed. Note that the W&E turrets still had to be jammed up against the forward ring, and the forward ring had to be as far forward as possible. (You would not want this scope on a lightweight, hard kicking hunting rifle.)
The third thing I mentioned? Clip-on night vision devices have an axis 1.53" above their mounting surface. They work optimally with scopes mounted coaxially, or at the same height. While this may not be ideal for shooting prone with a big block gun, it's worth the trade-off. How many prone shots do you get in the field, anyway? This is not a gas gun, but the photo gives you the idea of the arrangement. It's an AI with a 1.5" high scope mount:
OK, there's a fourth thing that comes up, sometimes. Some shooters like a hard cheek weld. Or shoot prone most of the time. Or have super thin cheek bones? These shooters will benefit from a lower scope mount, like the Spuhr SP-3026 or SP-4026, 1.26" high:
I actually prefer this height on SR-25/AR-10 pattern rifles, if there is room for the objective housing and lens cap. I even use this height on my own AR-15 pattern rifles, since I have trouble shooting prone with a 1.5" high scope height:
