Eye Relief and Eye box
The space between the scope and your aiming eye (for optimal sight image) is the eye relief. This implies that the bigger the eye relief, the safer your eye is from recoil. However, your eye and head will be in an uncomfortable position should the eye relief be too much.
Eyebox refers to the amount of wiggle room for placement of your aiming eye.
Perhaps you have heard people complain of “small sweet spots”; they are referring to a small eye box. An M1A scope with less than half an inch room for optimal eye placement has a small eye box. During field use, a small eye box results in much slower target acquisition, and the higher the magnification scopes, the bigger this issue.
Field of View (FOV)
How wide you can see through the scope defines your M1A scope’s field of view. It is a feature determined by the magnification power and optics construction.
Field of View is typically measured at 100 yards. it is noteworthy that the higher the scope’s magnification, the lower the FOV, and vice versa.
With the right M1A scope, while shooting from a bench rest, you can hit targets as far as 800 yards. There are those who have claimed they were able to reach targets at 1000 yards. Even though this is an admirable feat when you consider the average ballistic performance of the 7.62 x 51 ammunition, it is not an impossible one. But then, generally, people often use the .308 for long-range shooting because it is considered the world standard sniper cartridge (although it has the same similar bullet trajectory as 7.62 x 51).
The right M1A scope should help you with such long-range shots. If you are not interested in long-range shooting, the 3-9x should be appropriate, but I will advise 4-12x for shooting at long ranges.
You should also note that the higher the magnification, the less clear and dimmer the imager, not to talk about the rising cost.
Objective Lens Diameter
The objective lens is on the other end of the ocular lens.
The bigger the objective lens, the heavier the scope, and the higher the price. In some cases, you would also have to deal with higher mounts.
When you divide the objective lens diameter by magnification power, it results in exit pupil size. Therefore, if you are in need of higher magnification power, you should choose a scope with a larger objective lens and smaller objective lens if you are going for a low magnification power.
The reticle is in three main groups: the Simple (Duplex, Dot, NikoPlex, V-Plex, etc.), Mil-Dot/MOA (Crosshairs), and BDC (Bullet Drop Compensation).
Often, the choice of the reticle is left to preference. For simplicity, your choice should either be the Simple or Mil-Dot/MOA. For long-range shooters, you should go for Mil-Dot/MOA. If you desire quick holdovers, then the BDC is most appropriate.
Then again is the issue of Focal Plane. To choose from the First Focal Plane (FFP) and Second Focal plane (SFP) is quite easy. SFP reticle has the benefit of holdovers but only works on a specified magnification level. FFP, on the other hand, features holdovers and also works on all magnification levels, but is more expensive. However, if you desire tactical use of M1A scope and are serious about long-range shooting, you should choose the FFP.
Should you decide to consider illumination, well, you should check things like clarity in broad daylight, night vision compatibility, and battery life (ensure the reticle works even with a dead battery).
The standard tube sizes are 30mm and 1 inch in diameter. The bigger the tube, the more benefit you gain in durability and a wider range of internal turrets adjustment. A bigger tube also comes with a bigger objective lens. On the other hand, smaller tubes are lighter and cheaper.
Glass and Lens Coating
Good glass and lens coating contribute to image quality. The durability of your scope is also enhanced due to scratch-resistant coatings and waterproofing. Hence, look out for labels such as ED glass (Extra Low Dispersion Glass) and Fully Multi-Coated Lens – it may be costly, though.
Turrets differ with scopes; their features are determined by whether the scope is for hunting or tactical purposes.
For hunting scope turrets, the scope has a low profile and capped turrets. It prevents unintentional adjustments. For tactical scope turrets, however, it is crucial there is an allowance for finger adjustability. Also, it is advisable turrets come with zero stops and zero resettable option. For long-range shooting, parallax adjustment is good to have.
One last thing: if possible, the turret unit of clicks should match the reticle. An instance is the Mil-Dot turrets and Mil-Dot reticle.