Effect of 20MOA rail on 100 yard zero

[Westend101]

Hi,

My first post here so be nice please.

I understand how a 20 MOA rail work in that in theory at 100 yards compared to a 0 MOA rail impacts will be 20 MOA higher. This requires dialing down 20 MOA on the scope which essentially frees up that 20 MOA to asstist with dialing in longer ranges.

The issue I have which I can't put my finger on to the cause is this:

I have a RPR 6.5 CM, Vortex Diamond Back Tactical, 6-24X50 scope mounted 2.25" above bore. I hand load my rounds and am firing a Hornady 140gr ELDX @ 2725FPS.

The scope has 65 MOA of vertical travel. The RPR has a 20 MOA rail. My zero on the scope at 100 yards is 20 MOA dialed up from the base, or 13 MOA from the mid point of the scope depending on how you look at it. This leaves me with 45 MOA of vertical travel left to play with.

I know that bullets don't fly straight but the vertical drop with no compensation at 100 yards is only -2.3 MOA. SO in theory a 100 yard zero with a 0 MOA rail would require dialing up 2.25 MOA.

Taking that into consideration and using that value with 20 MOA rail zero I've saved about 15.25 MOA of vertical adjustment. But why not 20?

I've measured the rings and they are all within a couple hundreds of an inch off 0.5".
I've removed the rail and while I haven't done the math to verify that it is a true 20 MOA rail. It varied by about 4/10ths of an inch from one end to the other. So there's clearly some elevation change.
I've put a bore laser on and a flat laser on the rail and can clearly see the barrel pointing higher at 30' than the rail. So the barrel doesn't appear to be offsetting the rail.

I know I'm pissing in the wind because 45 MOA is fine for my 1000 yard shooting. I'm just trying to understand why I don't get all nearer to 20 MOA of scope adjustment at 100 yards.

Any pointers? As to how to understand the mechanics of what is actually affecting the scope adjustment?

Best regards,
Pete

 

[Westend101]

Hey there,

Went to the range today to check out my zeroes for an up coming match and discovered that my zero on my rifle is actually 35MOA up from bottom. With the 2.3MOA drop at 100 yards puts my scope right in the center of travel with a 20 MOA rail leaving me only 30 MOA. So I'm just so confused as to what is up. I'm going to contact Vortex and also mount the scope on my AR to see if that is any different.

 

[_Windrider_]

Your assumption that the scope came zeroed at dead center from factory is wrong. That’s what hurting your brain.

 

[Huskydriver]

Okkkkaaay here we go...I did not look up your scope specs so going off what you posted...

65moa total internal/2= 32.5 to come up from level minus a 20 moa base to come up to level= 12.5.

12.5+32.5=issss wait for it.....






45 moa of come up travel assuming nothing lost at zero.....

 

[Westend101]

Your assumption that the scope came zeroed at dead center from factory is wrong. That’s what hurting your brain.

Hi,

Yes this was the first thing I considered. However, if this is the case then the center of the scope is so far off the center of travel that I'm losing a lot of elevation adjustment. I have no care for 35 MOA of down adjustment at 100 yards. So do you think the scope is defective?

 

[kyzim311]

What rings are you using? I second the post above, and scopes do not all come with the same amount of vertical adjustment range. Meaning you could buy one diamondback tactical with 67 and another with 71 moa of adjustment. You should check how much adjustment you can go up.

 

[Westend101]

Okkkkaaay here we go...I did not look up your scope specs so going off what you posted...

65moa total internal/2= 32.5 to come up from level minus a 20 moa base to come up to level= 12.5.

12.5+32.5=issss wait for it.....






45 moa of come up travel assuming nothing lost at zero.....

Hi,

Yes this makes sense. However, note my trip to the range today. I was wrong about where my 100 yard zero is. I thought I was dialed up 20 MOA from the bottom. I'm actually dialed up 35 MOA. Without putting this scope on another rifle I can pin this on the scope. So I will do that this week.

Best regards,

 

[Westend101]

What rings are you using? I second the post above, and scopes do not all come with the same amount of vertical adjustment range. Meaning you could buy one diamondback tactical with 67 and another with 71 moa of adjustment. You should check how much adjustment you can go up.

Hi,

From bottom to top I have 66 MOA of adjustment.

Best regards,

 

[Huskydriver]

Hi,

From bottom to top I have 66 MOA of adjustment.

Best regards,

Okay so 20 moa+66 is 86 total- 45 of useble up travel =41 moa below your zero. Have you measured how much total travel up you currently are getting ? Sounds like your getting 6 moa better than the math says you should if your only up 35 from bottom.

 

[TheGerman]

The effect will be 20MOA.

 

[kyzim311]

Okay so 20 moa+66 is 86 total- 45 of useble up travel =41 moa below your zero. Have you measured how much total travel up you currently are getting ? Sounds like your getting 6 moa better than the math says you should if your only up 35 from bottom.

It doesn't work like that. You can never get more than 66 moa out of that scope without going to an adjustable base.

 

[kyzim311]

Hi,

From bottom to top I have 66 MOA of adjustment.

Best regards,

What rings? I am geussing your rings are at fault more likely than the scope. Given your specs, I would expect around 48 moa of travel left (18 from the bottom).

 

[Huskydriver]

It doesn't work like that. You can never get more than 66 moa out of that scope without going to an adjustable base.

See post #4 for what I was trying to reiterate.

You are correct I forgot the numbers for the math I had done up above dealing with screaming twins and a defiant toddler refusing to brush his teeth lol.

 

[308pirate]

Hi,

My first post here so be nice please.

I understand how a 20 MOA rail work in that in theory at 100 yards compared to a 0 MOA rail impacts will be 20 MOA higher. This requires dialing down 20 MOA on the scope which essentially frees up that 20 MOA to asstist with dialing in longer ranges.

The issue I have which I can't put my finger on to the cause is this:

I have a RPR 6.5 CM, Vortex Diamond Back Tactical, 6-24X50 scope mounted 2.25" above bore. I hand load my rounds and am firing a Hornady 140gr ELDX @ 2725FPS.

The scope has 65 MOA of vertical travel. The RPR has a 20 MOA rail. My zero on the scope at 100 yards is 20 MOA dialed up from the base, or 13 MOA from the mid point of the scope depending on how you look at it. This leaves me with 45 MOA of vertical travel left to play with.

I know that bullets don't fly straight but the vertical drop with no compensation at 100 yards is only -2.3 MOA. SO in theory a 100 yard zero with a 0 MOA rail would require dialing up 2.25 MOA.

Taking that into consideration and using that value with 20 MOA rail zero I've saved about 15.25 MOA of vertical adjustment. But why not 20?

I've measured the rings and they are all within a couple hundreds of an inch off 0.5".
I've removed the rail and while I haven't done the math to verify that it is a true 20 MOA rail. It varied by about 4/10ths of an inch from one end to the other. So there's clearly some elevation change.
I've put a bore laser on and a flat laser on the rail and can clearly see the barrel pointing higher at 30' than the rail. So the barrel doesn't appear to be offsetting the rail.

I know I'm pissing in the wind because 45 MOA is fine for my 1000 yard shooting. I'm just trying to understand why I don't get all nearer to 20 MOA of scope adjustment at 100 yards.

Any pointers? As to how to understand the mechanics of what is actually affecting the scope adjustment?

Best regards,
Pete

Did you actually shoot the rifle?

 

[Westend101]

Hi,

Yes I did shoot the rifle. I started competing in F-Open last year. No way near an expert, the hardware to be competitive is something I'm working on this year so was shooting off my bipod. Still not a shabby shooter 189-4X is my current PB. At 100 yards I shoot sub MOA almost 0.5 MOA. So I know how to shoot the gun. I've also been shooting and reloading for 10 years and have mounted and zeroed other rifles.

Yesterday I disassembled the rifle. Took of the MOA rail and measured it with my digital calipers I use for reloading. One of my thoughts was that I had a rail stamped with 20 MOA but was actually a 0 MOA rail. I didn't write down the numbers but if I remember the rear end measured 0.58" and the front 0.54" the rail is about 8" long.

I also measured the rings from the base to the bottom of the inside of the ring. All rings were within 1/100th of an inch. The are Warne Maxima 215M. The measured between 0.50" and 0.51". Some of this error is probably me not measuring to bottom center of the ring. I was real;ly looking to see if they had an MOA adjustment built in and that I had installed them the wrong way around.

I settled the rifle on it's F-TR bipod and rear stick and boresighted the rifle on the 100 yard target. I then dialed the scope to the aim point on the target. From there I was on paper and adjusted my zero. At that point I dialed down to the bottom and noticed that my zero was 35 MOA up from the base and that my zero from last season was still good, because I had reset the turret to 0 back then and it was on 0 again.

While the rings could be part of the problem I would expect the measurements to show a larger offset between the front and rear pair.

If the rail is giving me 20 MOA and everything else is perfect. I would assume with a scope where the mechanical center is in alignment with the center zero on the scope and a world with no gravity, thus no bullet drop, I would expect to have to dial the scope down 20 MOA to achieve my zero. Seeing that I have to dial the scope UP 2.5 from the mechanical center makes no sense.

Rather than continue to hypothesize I'm going to swap scopes with my other rifle that has a 0 MOA rail. If the scope is defective then I will have to dial up over 20 MOA from mechanical center to zero my other rifle.

I'll report back. But any other suggestions are greatly appreciated.

Best regards,
Pete

 

[TheGerman]

Either your scope doesn't track worth a shit, or is your rail on backwards?

 

[VargmatII]

You are still assuming too much.
Zero your scope at 100 meters/yards and spin the turrets to see how much travel you have left. That is all you need to know and do.

Oh, and I hope you dont have a pair of vertical split rings?
Read up on those, they are known to cause issues, lots of issues.

And keep in mind that you have a low end scope that does not have the best record. So there could be issues with it or the combo of scope and rings (if they are the vertical split ones?).
But even if there is, what you have left after you zeroed your scope, is what it is.

 

[2aBaC̶a̶]

Get some burris XTR rings if you want to maximize elevation.

 

[alamo5000]

For what it's worth I also wonder about how much total elevation in the dials that I will get with certain optics. That said I try to ask questions before I buy.

Especially with optics it can be an expensive mistake to just buy "whatever". However at least now I try to get a real world feel before I spend the cash.

Forums like SH are often worth their weight in gold because odds are someone else probably bought a new wigit before you did.

That said I often wonder why it would be so much harder for the manufacturer to just calibrate the various scopes where it's applicable to a bottom 1/3 zero. It might vary a bit depending on the caliber but at least we could get more out of the box if they did.

 

[Slides]

You haven't added the offset of scope centreline to bore centreline to your calculation. Think of your triangle vertical as the sum of scope to bore height and bullet drop when finding the fine angle.

 

[sam4886]

There are much better things to worry about than theoretical moa bases. If it gets you more travel from zero than a flat base, you're golden.

Spend all this brain power on load development or something.

 

[kyzim311]

Hi,

Yes I did shoot the rifle. I started competing in F-Open last year. No way near an expert, the hardware to be competitive is something I'm working on this year so was shooting off my bipod. Still not a shabby shooter 189-4X is my current PB. At 100 yards I shoot sub MOA almost 0.5 MOA. So I know how to shoot the gun. I've also been shooting and reloading for 10 years and have mounted and zeroed other rifles.

Yesterday I disassembled the rifle. Took of the MOA rail and measured it with my digital calipers I use for reloading. One of my thoughts was that I had a rail stamped with 20 MOA but was actually a 0 MOA rail. I didn't write down the numbers but if I remember the rear end measured 0.58" and the front 0.54" the rail is about 8" long.

I also measured the rings from the base to the bottom of the inside of the ring. All rings were within 1/100th of an inch. The are Warne Maxima 215M. The measured between 0.50" and 0.51". Some of this error is probably me not measuring to bottom center of the ring. I was real;ly looking to see if they had an MOA adjustment built in and that I had installed them the wrong way around.

I settled the rifle on it's F-TR bipod and rear stick and boresighted the rifle on the 100 yard target. I then dialed the scope to the aim point on the target. From there I was on paper and adjusted my zero. At that point I dialed down to the bottom and noticed that my zero was 35 MOA up from the base and that my zero from last season was still good, because I had reset the turret to 0 back then and it was on 0 again.

While the rings could be part of the problem I would expect the measurements to show a larger offset between the front and rear pair.

If the rail is giving me 20 MOA and everything else is perfect. I would assume with a scope where the mechanical center is in alignment with the center zero on the scope and a world with no gravity, thus no bullet drop, I would expect to have to dial the scope down 20 MOA to achieve my zero. Seeing that I have to dial the scope UP 2.5 from the mechanical center makes no sense.

Rather than continue to hypothesize I'm going to swap scopes with my other rifle that has a 0 MOA rail. If the scope is defective then I will have to dial up over 20 MOA from mechanical center to zero my other rifle.

I'll report back. But any other suggestions are greatly appreciated.

Best regards,
Pete

Sounds like you have a good plan. Those are good rings, and my math says you rail is close to 20 moa, so it will be interesting to see you results on another rifle. It could also be something with the receiver of your rifle but it seems unlikely.

 

[MDof2]

You're shooting F Open. Are you running out of windage to reach 1,000 yards with the 20 base?
If you are, then you have something wrong. Let's address that. If you're not then why are you wrapping your head around this brain damage. Shoot it and go.

 

[ODCMP]

tag

 

[Westend101]

You're shooting F Open. Are you running out of windage to reach 1,000 yards with the 20 base?
If you are, then you have something wrong. Let's address that. If you're not then why are you wrapping your head around this brain damage. Shoot it and go.

Hi,

No my rifle and 6.5CM 140gr load requires 28 MOA at 1000 yards. So I'm ok but just about on the limit of the scope. Not that I am likely to shoot past 1000 at my range so most of this is academic. I just want to get to the bottom of this mystery.

 

[iceng]

Not all barrels are square to the firing pin axis. I had a RPR in the shop a few years ago (RPR series 1, 243win) and it was WILDLY to the left. Maxed out the scope on windage. Warranty, and new one was perfect.

Pro tip: scopes that are in the middle of their travel are more optically centred, so less distortion. It's a blessing. Deal with it.

 

[Steel head]

Not all barrels are square to the firing pin axis. I had a RPR in the shop a few years ago (RPR series 1, 243win) and it was WILDLY to the left. Maxed out the scope on windage. Warranty, and new one was perfect.

Pro tip: scopes that are in the middle of their travel are more optically centred, so less distortion. It's a blessing. Deal with it.

 

[Slides]

??? They just said they were near the limit of adjustment at 1000 yards, surely you want better optical performance at longer ranges where there is less light from target and more atmospheric effects?

 

[combatgrunt84]

So I have a new Ruger RPR in 6.5CM. I'm shooting a Vortex Optics Strike Eagle 4-25x50 in MOA, with Warne MED Rings, with the factory RPR 20MOA Rail. From a 100yard zero I can adjust down 41 MOA still. I went to the range today and got to hit out to 1030 without any issues, however my max up number is 60.25MOA form zero. attached is my load and dope for that load for the ranges I have so far.

 

[iceng]

You could run a 40moa rail safely, and still zero. It would "buy back" 20moa elevation. You can dial 1k, what does it matter ?