Long-range shooting requires expansive scope reticle adjustment facilities to cope with extreme trajectory changes over great distances. Such adjustment potential can be achieved externally by means of special mounting facilities, with the finer adjustments being made internally via the scope’s turrets.
Conventional scope mount bases are usually screwed onto the rifle’s receiver which is drilled and tapped for this – a potentially problematic system as screws can work loose or shear under recoil. Likewise screw-on rails, which are superior only in that they are one-piece, offering slightly enhanced rigidity. Either way, if you are forced to use the screw-on type, avoid aluminium alloy bases (and rings) as these can fail at the worst possible time – as I learned when competing in an overseas match which I had to finish using a borrowed rifle.
The same screw-on system for mounting a scope on a typical Rem M700 SA receiver for a 6.5mm Creedmoor will not be suitable for mounting a heavy scope on an M700 LA chambered for a .338 Lapua, the recoil of which can loosen and even shear the screws or bolts. Select appropriate mounts – if necessary, go the custom or modified route.
Increasingly, nowadays, actions come with an integral rail system, which is superior. A receiver and rail machined from a single block of steel does away with mounting screws and has increased rigidity, enhancing its ability to support a heavy barrel, thereby damping barrel vibrations for optimum consistency.
To maximize the scope’s effective adjustment range, it is mounted so that its line of sight crosses the bullet’s trajectory at relatively close range, permitting zeroing at 100 or 200m with the turret dialed to maximum to provide the fullest amount of adjustment as the target ranges increase. This is possible only if the mounts permit the scope to be angled to converge with the line of bore accordingly (see diagram). Typically, mounts are rated in minutes of angle (MOA) and vary from 10 to 40MOA. Any more than this is usually achieved with a periscope lens (Charlie Tarac) attachment or adjustable mounts. Greater than 40MOA requires taller mounts to elevate the scope so that its objective lens-housing does not contact the barrel. This in turn will require a higher stock comb to elevate the eye accordingly, while maintaining consistent cheek weld.
For shooting at 1 000m, a 20MOA rail is usually sufficient, but at 1 mile and beyond you’ll require 30MOA for most cartridges with this capacity.
A scope having 80MOA adjustment, mounted on a straight 0-MOA rail, typically has only 40MOA of UP adjustment. When you angle the scope by using a 20MOA rail, you gain roughly 20MOA of UP adjustment, now giving you a total of 60MOA. Consulting your drop charts will reflect how far you would be able to shoot on the maxed turret adjustment available.
The angle of the scope tube can also be adjusted by means of ring inserts. Scope-rings are available with a range of plastic inserts affording a choice of settings to add MOA inclination on a level rail, or to increase the angle of an inclined 10, 20, 30MOA rail. The offset inserts can be rotated through 90 degrees for making lateral adjustments to compensate for mounting errors or correcting windage errors. I take pains to ensure my scope aligns with my zero with minimal need for windage adjustment. The erector-lens tube, which, in a variable scope, houses the reticle, hence moves it when you dial the turrets, will have maximum freedom of movement if it lies centrally within the main tube. The more adjustment you use to achieve a close-range zero, the more you misalign the erector tube within the main tube. In some scopes, this may reduce its vertical movement, limiting your adjustment range.
Read the full article in the February 2019 issue of Magnum.