.308 optics mount help

[jc000]

I'm looking for an optic for my bolt action rifle and have pretty much finalized on the NX8 2.5-20 Mil XT reticle. This is a gen purpose rifle, my only .308 bolt action, and will primarily be used for plinking out to 800 yards (with a good deal at 100-200 yards) and some limited hunting. I have selected this optic based on weight, price, (reported) durability, and my previous experience with the NX8 1-8 LPVO.

I believe my rifle has a 0 MOA pic rail. To accommodate shooting out to 800, should I consider a 20 MOA mount? And what is my lightest option? I was considering a Scalarworks mount because of the weight and I really like my existing one, but I'm wondering if a non-QD mount or even rings could shave more weight off. I really won't use the QD feature at all if I'm being honest about it.

[ccmdfd]

Shooting 165 SST's, and with a 200 yd zero, I've got 180 inches of drop at 800. For my scope that's one full turn up plus another 1.5 moa. I might have been able to do that with zero moa rail, but might not. I ended up going with a 20 moa rail.

Check out a balistics calculator and see what you get with your loads.

[SecondsCount]

I like the Scalarworks on an AR but a 1.93" high mount is going to be pretty tall on a bolt action.

I typically run a 20 MOA base with a set of low or medium rings. Seekins/Vortex and Nighforce, and I do have a set of Leupold QRW but I would only use them if quick disconnect is needed.

You may never need the 20MOA adder but you never know. A 168 grain bullet will only need about 24 MOA / 7 mils of elevation at 800. Most 30mm scopes can dial that with out an issue but if you ever find yourself at 1K yards and want to have some fun, then the extra elevation could help you.

[jc000]

I would definitely stick with the lower (1.57") Scalarworks mount. But I don't believe it's available in anything other than 0 MOA, and without a real need for QR, the LEAP probably offers little benefit. Maybe I'll look into a NF unimount.

[BobM]

I would definitely stick with the lower (1.57") Scalarworks mount. But I don't believe it's available in anything other than 0 MOA, and without a real need for QR, the LEAP probably offers little benefit. Maybe I'll look into a NF unimount.

I had that mount on an issued 700, when we bought the Nightforce scope, the vendor supplied a NF mount. It was a 20 MOA mount as I recall. I have the Seekins on a personal 700 and I prefer it’s reduced bulk.

[jc000]

I guess the real question is – what drawback, if any, is there to a 20 MOA mount or top rail vs a 0 MOA one?

[SecondsCount]

The 20MOA rail is slightly canting the scope downward toward the muzzle end of the barrel. This moves the point of impact up 20MOA, which requires you to dial down the scope elevation 20 MOA to compensate. This puts the zero closer to the bottom end of the scope adjustment, and allows for an additional 20 MOA of elevation if you need to dial up.

Let's create an example.

You purchase a new scope that has 50 MOA of adjustment and you mount on a gun with a 0 MOA base. You take it to the range to zero at 100 yards. You dial the scope to the point of impact and maybe it requires 28 MOA of adjustment from the bottom. Now you have 22 MOA of adjustment left to compensate for the elevation required to shoot the gun at longer distances.

If you are shooting at 800 yards/24 MOA from our previous example, you will run out of adjustment and be 2 MOA short. Put the same scope on a 20 MOA base and you would be able to cover the 2 MOA, and have an additional 18 MOA for longer shots.

Most of the 30mm tube based scopes today have 50+ MOA of adjustment so canting the additional 20 MOA is not necessarily needed but it can help.

I have had one instance where adding a 20 MOA base caused too much cant for the scope to barrel relationship, and the bullet impacted 2" high at 100 with the elevation turned all the way down. This was on an older target scope and I ended up moving it to another gun.

[jc000]

Thank you. Great summary.

[Clusterfrack]

Great explanation @SecondsCount. One thing to add is that your 100yd zero will be shifted down by 20moa. In most good scopes, this is not a problem. But in some cases this can create shadowing at the top of the scope when it is dialed to zero elevation.