Noob questions about scope hieght from centerline of barrel and sighting in

[DMS1]

Ok - I have an xcr, and I am not new to guns by any means, but I am having a brain fart here. The gun in its current configuration are shown below. I am changing to a collapsible stock on a new adapter from GRSC (see xcrforum for GRSC straight stock adapter info), which will put the top of the stock right around the height of the top of the rail, and putting a 1 pc high rise scope mount on it. The scope mount is the NC star MARQ with QD lever, I want to try a cheap one and make sure I like it before I buy a PEPR or RRA mount. Within the next week I should be able to post pics with everything on the gun.

Since the center line of the scope will be about 3" over the centerline of the bore (1.5" from bbl cl to top of rail and 1.5" from top of rail to scope cl), my questions are:

1) at close distances, say an initial sight in at 25 yards, how does the height affect the the trajectory or the set aim point? If I am shooting 6.8 SPC, right on at 25 yards = about 3 inches high at 100 yards. Where does the 3" come in to play, would I have to adjust higher or lower for the scope height? I will shoot the gun at 100 and 200 yards also, but want to understand the theoretical disadvantages of mounting a scope so high.

2) what difference does it make at longer ranges 200-500 yards? I see guys with tall mounts on guns that are regularly shot out to 500 yards, but have always mounted my bolt guns with scopes as low as possible because I was told this was best practice

I just want to understand if mounting the scope so high means the gun will shoot very different poi to poa from very close to very far differences

Thanks!

 

[BigUglyMan]

At super close range your shots will hit low. This shouldn't be a big issue for you as it sounds like you want to shoot the XCR out at 100 and beyond. Just initially sight it in at 25 yards and get a zero, then move to 100 yards and use a big piece of paper or cardboard as a backer for your target. If you're zeroed at 25 yards you should still be on a 8.5x11 piece of paper at 100. I like to use a bigger target board like a piece of cardboard from a moving box or appliance box and put my targets on that, securing them to the target frames with spring clamps. If you put a big sheet of paper under the target on that board you will be able to spot your hits should they end up way off the paper. Plus you can replace the paper for cheap. I like to use paper that movers use to wrap your stuff in when they box it up. A roll of that stuff should last you forever and it's lightweight so you can cut it out and put it in your data book without turning said book into a bulging monster.

 

[460v]

Nice rig!

Use this and key in your data including scope height. play with the numbers if you want to see the effects

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

 

[USP]

At super close range your shots will hit low. This shouldn't be a big issue for you as it sounds like you want to shoot the XCR out at 100 and beyond. Just initially sight it in at 25 yards and get a zero, then move to 100 yards and use a big piece of paper or cardboard as a backer for your target. If you're zeroed at 25 yards you should still be on a 8.5x11 piece of paper at 100. I like to use a bigger target board like a piece of cardboard from a moving box or appliance box and put my targets on that, securing them to the target frames with spring clamps. If you put a big sheet of paper under the target on that board you will be able to spot your hits should they end up way off the paper. Plus you can replace the paper for cheap. I like to use paper that movers use to wrap your stuff in when they box it up. A roll of that stuff should last you forever and it's lightweight so you can cut it out and put it in your data book without turning said book into a bulging monster.

Great idea, but what if you blast the spring clamps and its windy out there?

 

[Grizzlypeg]

I've played with these figures in calculating the trajectory for my Ar-15, and what I see is that having the scope mounted so high, results in less required clicks to compensate for the first 200 yards, half as much as where the scope is mounted closer to the barrel. But beyond 200 yards it starts catching up and becomes closer to what it would be otherwise. So in other words, it produces what appears to be a flatter shooting rifle from 100-200 yards, but that appearance quickly disappears as you shoot out further. The calculated results coincided exactly with what we observed out in the field. With 3" above centre of the bore, the adjustment for drop from 100-200 yards is half what it would need for a 1.5" above the bore.