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Optics, Mounts, Rails and Sights If it aims your firearm, post about it here.

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  #1  
Old 10-11-2017, 11:30 AM
sfvshooter sfvshooter is offline
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Default 2 exact scopes on 2 different rifles and weird(?) outcome

I have 2 NXS scopes. One is on a MPA 6.5 CM and the other is on a Barrett 98B 300 win mag. Both zero'd at 100 yds. Both on 20 MOA rail.

With the MPA, I have about 45 MOA of elevation. With the Barrett, it's tops out at 37 MOA. I have yet to swap the scopes to see if I get same results, but it seems odd there's so much difference between the two. At 100 yds, there shouldn't be that much drop between the 2 cartridges which would result in 8 MOA difference, or should it?

I've had the scope on the Barrett looked at by NF twice and they've assured me it's fine.

It's probably something basic I'm not seeing, but I'm puzzled.
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Old 10-11-2017, 1:41 PM
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Imagine two identical right triangles. You change the length of one side of one triangle. Now they are different. Simple as that. You have different geometry because you have: different guns, different height over the bore axis, different caliber, different velocity. Makes perfect sense. If you swap the scopes, you'll need to re-zero them both.
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Old 10-11-2017, 3:02 PM
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I don't think that it is a big deal. 8 MOA is not a huge shift from one rifle to another. There are differences from rifle to rifle and barrel to barrel. I shoot with a rifle that changes cartridges and swaps barrels and bolts. All barrels have some curvature and the run out for the barrel is generally indexed to 12 o'clock when the barrel is fitted. That way, you'll get some vertical dispersion among the different barrels, but there will be very low horizontal dispersion. So, this known offset in vertical dispersion is programmed into your starting points on the ballistic calculator.
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Old 10-11-2017, 4:01 PM
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Quote:
Originally Posted by sfvshooter View Post
I've had the scope on the Barrett looked at by NF twice and they've assured me it's fine.

It's probably something basic I'm not seeing, but I'm puzzled.
Basically, you are getting your money's worth on the warranty program...and helping the shippers increase their profits.
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Old 10-12-2017, 6:13 PM
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You are confusing the zero as mounted on those respective guns, not mechanical zero. There is a difference. That's why to zero a scope, you have to fire a few shots; you can't just turn the dials to the middle and call it good.

Like octavist said, there's different geometry between those 2 guns. The angle of the receiver, height over boreline, the alignment of the rings, any of those factors could contribute to the 0.133 degree (8 moa) difference.
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Old 10-12-2017, 10:42 PM
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Thanks for the feedback. I assumed it was something obvious.

I'm running out of elevation around 1200 yds with the 300 win mag so I'll have to try a 20 or 30 MOA mount to see if I can go out to 1400-1600 yds.
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Old 10-13-2017, 5:23 AM
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What type of rings or one piece mounts are you running?
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Old 10-13-2017, 7:27 AM
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I had a 20moa difference between two very similar guns.

http://www.calguns.net/calgunforum/s....php?t=1309069
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Old 10-13-2017, 9:18 AM
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Quote:
Originally Posted by HK Dave View Post
What type of rings or one piece mounts are you running?
Barrett zero-gap rings. I have an American Defense 20 MOA mount so I can try that, though it might be tight with a 56mm objective lens. It's pretty tight with existing rings so not sure if the 20 MOA mount will work.

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Old 10-13-2017, 3:26 PM
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LOL been racking my brain trying to figure this one out for you...

But then I just realized you are talking moa. My brain only thinks in mil. What a dummy I am.

Yah 8 moa isn't that big a deal. Different tolerances could probably cause that kind of a difference. It's more than I'd like or hope for but I could probably live with that.

I originally thought it was 8 mils of difference and my mouth dropped open... I'd be pissed if I was losing 8 mils worth of elevation.

I'd just keep on truckin at 8moa or add more elevation through a 1 piece mount.
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Old 10-13-2017, 4:45 PM
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Two rifles and two scopes doesn't provide enough data for me form a solid theory on why there is 8 moa difference. Would you be willing to buy another rifle to give us a third data point?
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