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The mechanical limit of long range shooting.

Note: This article is under development. Judge accordingly...
The mechanical limit of long range shooting is essentially the furthest distance you could shoot in a given atmosphere by dialing the elevation turret to the end of its travel. Sure, you could shoot further by dialing to this point and then using recital holdover or even further by dialing, adding all of the recital holdover, and then adding Kentucky windage but this is beyond the limitation of the optic's internal mechanism.

It is often of value, or at least entertaining to determine what the mechanical limit of a long range rifle system would be. If you are paying for and traveling to an extended range class or match, you better do your homework ahead of time and be sure that the equipment you are showing up with is up to the challenge. Older generation scopes of higher magnification with 1" or even 30mm tubes can be limiting when it comes to long range shooting.

Every factor of ballistics comes into play for this determination, so explore the variables. Usually a cartridge is already selected and the scope is mounted, but you can investigate different projectiles to see if one would serve better than the other. In extended range shooting, we are often beyond the distance where a trajectory comparison would show where a slower but higher BC bullet has less drop than a slightly lower BC but faster launching lighter bullet.

For this analysis, we will use the mechanics of one of the Sawtooth Rifles training rental rifles with a Nightforce ATACR F1 5-25x56 riflescope shooting Hornaday 6.5 Creedmoor ammunition with the 140 ELD Match bullet launching at 2780 fps. The optic's specifications of interest are:

Total internal elevation adjustment range (per Nightforce specifications) : 120 MOA or 35 Mrad
Scope base cant: 20 MOA or 5.8 Mrad

There are two ways to go about determining the mechanical limit of a scopes internal adjustment. The most accurate method would be to dial the turret from the zero condition to the end of it's travel in the up direction, while keeping track of revolutions.

We can also use math if the above method is not feasible, or when using this concept to make decisions prior to purchasing an optic or scope mount. For this analysis, we will use the math approach. By doing this, we assume that the adjustment range used to zero the rifle is insignificant, but in reality we are using some of the elevation adjustment to get zeroed.

Using the math approach, we simply divide the total elevation range in two, the usable "up" portion and the wasted "down" portion and then account for the elevation gained by mounting it on a canted base. For the Nightforce ATACR F1 above, the math goes as follows:

1/2 of total internal elevation adjustment = 60 MOA or 17.5 Mrad
elevation gained from 20 moa canted mount = 20 MOA or 5.8 Mrad
total usable "up" elevation adjustment range = 80 MOA or 23.3 Mrad

By comparing this limit to our ballistics from the "Getting to know the Shooter ballistics app" blog post, we have a good estimate on what our mechanical limitation in that atmosphere would be. In Figure 1, you can see that our mechanical limit of long range shooting would be found at approximately 1975 yards by finding where 23.3 Mrad would be in the Path column, just past the transonic distance. We can also see that using another 5 mils of reticle holdover would get us to 28.8 Mrad, and to almost 2200 yards. Although, this is beyond what we are calling the mechanical limitation.
In summary, it is very easy to compare the mechanical limitation of an optic to your ballistics to determine how far you can shoot.  This can be a decision factor when purchasing an optic or selecting scope mounts.  Perhaps an optic decision could be based off of function instead of emotion or brand recognition. 

There happens to be an optic in the stable at Sawtooth Rifles that was purchased for the science of shooting as far as possible while retaining a 100 yard zero.  This particular scope is a fixed 16x56mm from IOR with a 35mm tube.  It boasts 150 MOA of internal adjustment range, and when mounted on a custom made 60 MOA base, we get 135 MOA (39 mrad) of usable "up" travel which would get us right to the top of that solution chart in Figure 1.  When mounted on the .338 Edge, it is entertaining!
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