Optic height is expressed as the measurement between the top of the mounting surface and the exact centerline of the scope’s glass.
Common “AR height” 1.5-inch height AR-15 scope mounts will place your eye 1.5 inches above the top of your rifle’s Picatinny rail as you look through the scope. For reference, standard AR-15 iron sight height is just underneath that, at 1.41 inches above the rail.
One-piece scope mounts for AR-15s are so popular that it’s pretty hard to go wrong with optic height. Recently, extra tall 1.93-inch AR scope mounts have been gaining popularity.
These were originally designed so the scope’s field of view would clear bulky accessories like lasers mounted above the rifle’s handguards.
Extra tall AR-15 mounts may be more comfortable for taller shooters or shooters with bigger heads. They may also provide a minor advantage to sight acquisition speed when standing up.
However, when shooting from the prone position your cheek weld may not be as stable and repeatable with an extra tall AR mount.
The big word of caution on one-piece scope mounts is for shooters using rifles other than AR-15s. An AK or a bolt action rifle with the stock positioned significantly lower than the Picatinny mounting surface demands a mount with a much lower central height.
Installing a 1.5-inch AR height mount on a standard Remington 700 will force you to abandon your cheek weld entirely.
You will have to hold your head completely above the stock to look through the scope, and this is far from ideal. For these applications, you need to restrict yourself to looking at mount options with height measurements of 1-inch or even less. Fortunately, the proprietary Ultimak and side rail AK mounts are intentionally designed to place the scope’s centerline as low as possible.
Forward offset refers to the distance between the position of the mount’s rings compared to the position of its Picatinny rail interface area, where it physically attaches to the rifle.
Some rifles like the FN SCAR feature long sections of uninterrupted Picatinny rail, allowing the mount to be placed anywhere along the top of the rifle and making forward offset irrelevant.
However, for rifles like the AR-15 featuring handguards that are separate from the receiver, it is not desirable to place the scope mount on the handguards or use the scope mount as a “bridge” between the receiver and handguard.
A better choice is using a scope mount that only attaches to the rifle’s receiver, with enough forward offset to place the scope at the correct position relative to your eye looking through the scope.
In general, if you are using a long eye relief scope (approaching 4 inches of eye relief), you prefer a shorter length of pull style rifle stock, or you shoot with your head in a forward position in the “nose to charging handle” style, look for a “Recon scope mount” or “SPR scope mount” type with significant forward offset.
If your optic has a short eye relief (2 inches or less), you prefer a full length stock, and you shoot with your head further away from the charging handle of the rifle, then a forward offset mount won’t be needed.
When in doubt about the proper forward offset, AR-15 shooters should err on the side of too much offset, rather than not enough. If your mount doesn’t have enough forward offset when attached to the front edge of the AR-15’s receiver rail, you’ll need to extend your collapsible stock or otherwise find a way to re-position your eye further back.
It is much easier to move the scope mount a couple of slots further to the rear on the receiver due to too much forward offset.