Thread: Zero MOA rail/base - why?

Assuming we have a droop-free centrefire barrelled action that shoots 820m/s. when shooting horizontally it takes 0.12s to hit 100m. At t = 0.12 s, d = (0.5) * (9.8 m/s2) * (0.12 s)2 = 0.07 m. I.e. 7cm, or around 3 MOA, drop. On average, scopes are mounted about 5cm above the bore, measuring from the centre of the scope to the centre of the bore. At 100m, that higher difference is just under 2moa. Basically if you have a droop free barrel and perfect scope, you still need about 4~5 MOA to zero the gun at 100. Lets call this effect the "inherent slope".

The inherent slope is more pronounced on 22LR zeroing at 50m, because it takes 0.15s to reach target and drops 0.112m, i.e. 8 MOA. add the scope to barrel angle which is about 3.5 MOA for 50m you need about 11 MOA to zero the gun.

The inherent slope can be smaller if the scope is mounted lower, or if the barrel slightly points upwards (never happens by design), but it will never be negative - because you never mount the scope under the barrel and gravity always pulls downwards.

Guns where barrel is not screwed into the action, instead, is sled into the action and held by screws, such as CZ455/457 and Ruger 10/22, have the potential of barrel droop. This further increases the inherent slope.

This is why I think MOA rails is not just for dialling snobs, it is also useful to anyone who is shooting rimfire and also to those running long and/or slim scopes (that tend to have limited adjustments). sloped rail entirely removes the possibility of running out of adjustments before zeroing the gun.