jwai86 wrote:Sometimes I see Picatinny rails with an MOA rating. I vaguely understand that they allow for additional MOA adjustment, but I'm unclear as to how they do that, and what purposes they are relevant for.
The rail is usually level to the bore axis, a canted rail is canted upwards at the rear to point the scope down at the ground - it basically does the same thing as an old-school military leaf rear sight. The rail doesn't give you more adjustment, it moves the distances that your scope adjustment works within.
On most centrefire high-velocities a flat rail is fine out to about 1000m with decent scope adjustment.
When you start using high-mag scopes that have very limited adjustment then you can make use of canted rails, 20-minute being the most common. You can do similar by shimming the rings but risk damaging the scope. 20-minutes is a third of a degree.
On a .22LR or other low-velocity cartridges it becomes more important even at relatively close distances. To zero a subsonic .22LR at 50m requires around 8-minutes of elevation (about 5" of drop from the muzzle), at 100m it requires around 17-minutes (about 20" of drop), so you burn up a lot of your adjustment just in zeroing a low-velocity cartridge. In comparison, the 147gn in 6.5x55mm only needs about 1.25-minutes at 50m and 2.7-minutes a 100m.
When you zero a scope on a flat rail, your scope is pointing almost directly at your target. Out to about 50m you will have to dial the scope up a little to allow for the amount of drop the bullet has, probably 10-minutes or less. Some rifles have a little cant in the receiver built-in so even a flat rail will have the barrel pointing slightly upwards so you can zero close to the centre of your scope adjustment. If your scope only has 20-MoA of adjustment each side of centre you might not be able to zero a low-velocity cartridge (like CCI Quiet) out to 100m at all on a flat rail. If you put a 30-minute rail on, then you won't be able to zero it at 50m, but you will be able to zero it at 100m.
On a high-velocity rifle, if your scope has a total of 80-minutes of adjustment (40 up, and 40 down from centre), with a flat rail you can zero at 100m and dial out to about 450m (after zeroing you probably have about 35-minutes of up adjustment left). If you zero at 200m you can still only dial out to about 450m, nothing has changed except that you used up more of your adjustment to zero at longer distance.
A 30-minute rail will still let you zero at 100m, but you can probably dial out to somewhere around 700m.
If you use a 60-minute rail (your scope is pointing one-degree down at the muzzle) you can no longer zero at 100m, but you can probably zero at about 300m and dial out to about 1000m (I'd have to run it through a calculator to give you more accurate numbers).
If you use a 120-minute rail (two-degrees downward) you probably won't be able to zero closer than 800m, but can dial out to about 1500m. But if you wanted to take a 400m shot at something you would have to hold about 2.5m low.
I have no idea why anybody would choose to zero a high-velocity rifle closer than 200m (100m shots are only about 40mm to 80mm high). Choose a rail that lets you zero at 200m at the bottom of your scope elevation, then you have the full scope adjustment to use in the field.
And choose a scope that gives you plenty of adjustment, more than 100-minutes total is good. My scopes have 105MoA, which gets me out to 1100m or more with flat rails on high-velocity rifles. I could use a 50MoA rail and still zero at 200m but extend my range to around 1500m. A BDC reticle gives you a little more range when you've run out of adjustment - mine give me another 15MoA for another 100m or so.