What's the Scope Height with Factory 30 MOA Tilted Rails?

[RC_Bladeworks]

In modern digital optics with built-in laser range finders they can calculate your point of impact at different distances when you use the built in lrf (oneleaf nv500). To do so you have to put in all your ballistic info in the menu on the optic. One parameter is scope height. The problem is my fx m4 impact has 30 moa of tilt built into the rail. This means the scope is tilted, so the front of the scope is closer to the barrel than the rear. So how do I adjust for this when calculating the scope height so the optic can accurately predict the point of impact at different distances?

 

[steve123]

Try backing in the dope using actual distances via a tape measure on flat ground. You need to start with mostly correct velocity, BC, and distances. Just make sure your zero distance is sighted in perfectly because that's the baseline the rest depends on. This is what I do for FT and at close distances the difference between scope height of a 10th of an inch can be quite a bit between 10 yards and 11 yards. I measure the scope height close to start off then change the scope height a little back and forth in the app until the dope lines up as close as it can. Apps aren't perfect either so if I need to change velocity and BC a small amount to get within 1 click of all my dope then so be it.

If nothing else it's a interesting experiment to see how the change in parameters changes the dope around. Often if you can get the closest distance as well as the farthest distance to line up in the app with actual the rest will line up pretty well.

I've seen it a few times now that some friends cheap LRF's don't give correct distances. And also a guys LRF within his DO didn't range to actual.

 

[sscoyote]

Steve (good name btw ), if you had to pick one spot for that parameter for those of us who aren't worried about a .25 MOA click x2, where has that been in your experience?

 

[steve123]

Steve (good name btw ), if you had to pick one spot for that parameter for those of us who aren't worried about a .25 MOA click x2, where has that been in your experience?

Ha, yeah I remember you from the Hide years ago.

That's a tough one because they are all important and frankly it's a juggling act getting them all as aligned together as perfect as possible.

For me my zero being correct is paramount because it's foundational. Off a click or two there means figuring out the others becomes a slight hassle if backing in dope. Also if I sight in a gun on a bench I only work up dope on a bench, if off sticks and bucket then I only work up dope off from there, down prone off bipod and rear bag then I only work up dope doing that. That's only because the zero can change between them. Or at least check your zero again if you change how your gun is rested. It might be off, or it might not, but don't assume it's on.

Really close distance scope height being correct(most don't care about hitting something small at 10Y), mid distance velocity being close is a priority, out far as long as velocity is close , the BC, and/or BC drag curve law, is important.

But I could zero, measure scope height, get a velocity off my Garmin, get a BC off of the internet, plug parameters in app, dial or hold the solution, and hit a 2" spinner at 60Y. Might hit slightly low or high and then refine by juggling.

 

[Advance]

To answer this question you should first determine the maximum error that could be input into the system. Given the known 30 MOA rail and the total length of your optic, let’s pick 12” for this example, we can calculate the maximum difference in axial centerline height using simple trigonometry, sin(alpha) = opposite / hypotenuse. 30 MOA is equal to 0.5 degrees, so solving the equation for the opposite leg of the right triangle gives us 12*sin(0.5) = 0.105 in. This is the maximum error you could plug into your ballistic calculator for the scopes centerline height, given a 12” total scope length and 30 MOA rail. If you play with this number you will find this error doesn’t not have much, if any impact on the ballistic calculations. We are talking about induced POI errors that would be smaller than a single click of the scopes elevation turret. Just measure this value in a place where you can get the most accurate measurement and it will be good enough data.

 

[sscoyote]

Thanks guys-Good info!