Currently the M14 rifle in SOG:PF can accept a Redfield 3-9x scope with reticle resembling the real-life AR TEL scope. This is an adequate solution, but it's always possible to improve it in terms of historical accuracy. During the Vietnam War, US Army experimented with several "marksmanized" M14 configurations, of which two were most numerous.
The rifle on bottom is M14 equipped with M84 telescopic sight, as used before on M1 Garand rifles. This represents an early (1967) attempt to create an accurized M14, and it had obvious drawbacks, for example low power of the scope, and it's offset to the left (see this for reference photos and information). M84 scope is already in the DLC, used for M1 and M2 Carbines.
In 1968 US Army began testing of a new kind of scope - an automatic ranging sight, which sets the zero range/BDC according to the magnification level. This scope was known as AR TEL, and it used Redfield 3-9x scope as it's core. Current Redfield scope model requires few changes to properly represent AR TEL: first, the mount is a bit too high (see this photos: 1, 2, 3 - compare with in-game model), and secondly the scope itself lacks the cam, which connected the zoom dial and BDC mechanism (note how the cam itself is marked for 7.62x51 M118 round - other cartridges were tested too, including .50 BMG).
Both of these scopes can be made utilizing existing assets, so making them shouldn't be out of the realm of possibility. What may be harder to achieve is the AR TEL "automatic ranging" feature (explained here - note that figures B-4 and B-5 are described wrong: figure B-4 shows later ART II scope, while figure B-5 shows AR TEL/ART I).