REALLY OVERTHINKING 0 moa / 20 moa rails

[biednick]

That was my intention, sorry I was being sarcastic haha

But seriously, gravity has to bring it down and it intersects with the line of departure a second time. This concept is pretty big across the the AR cqb zero conversations with things like 36/300,
50/200 etc

It starts at the line of departure, immediately falls away, and never crosses it. The line of departure is the vector the bullet is traveling on the instant it leaves the barrel, and gravity pulls it down from there. It crosses the line of sight twice, and both rises and falls relative to the LOS.

I'm guessing that's what you meant, but don't want anyone else to read it and get confused.

 

[med358-boise]

Theoretically… the bore and a perfectly optically centered scope on a "0" MOA rail is 1.7"-1.8" different at rest over the chamber AND at your 100 yard target.
You then sight in and lower the crosshairs to match the bore axis at 100 or 200 yard "zero" where the bullet crosses over the line of sight the first time.
Gravity will then drop the bullet back below your line of sight somewhere farther down range for the second time.

However, (with more experimentation)) there is an unknown point where you can sight in to barely get the bullet up to your line of sight and only touch at one point and begins to fall from there.

That is actually a really interesting point. I just did a trial and error on one of my 308 hunting loads and a ballistic calculator. Note it only measures in 1 yard increments and .1 inches so the output you get is an elongated touch point or think of it as an elongated zero distance. For my rifle its ~79 yards. The trajectory output is -.1" at 66 zeroed from 67 to 92 and -.1 at 93.

okay, okay, my inner geek is coming out

 

[wilehunting]

I would like to point out an incorrect assumption you are working with. On a high powered rifle with a scope sighting system, zero'd at 100 yards, the bullet trajectory goes from the muzzle to the optical point of Aim at 100 yards and then drops from there. In other words the bullet trajectory touches the line of sight once.

 

[Roughrice]

You are absolutely correct!
My apologies for misleading on the 100 yard zero!
Beyond 100, depending on the velocity and BC, is what I was trying to point out about crossing the LOS twice.

SORRY ABOUT MY POST!
I have corrected my post on the LOS vs zero!

 

[wilehunting]

Mathematically you're right about the "wedge" (2 non parallel lines) however you're wrong about taller rings. The wedge angle is what's most important, and taller rings do not change the angle of the wedge. By raising the scope 1 inch higher, but keeping the angles the same, you lower the point of impact (assuming you did not adjust the scope) by just under 1 inch at your sight in distance.

This is because your scope is now aiming 1 inch higher because you raised it, but now because you adjusted the distance between 2 non parallel lines the intersect point is slightly further away, reducing some of the affect. But being that your triangle of side length 2 inches by 100 yards has now become 3 inches by 100 yards, you've pushed the intersection back a couple of inches, which is why I said just "under" 1 inch. You have technically adjusted the wedge of your reticle, but your scope remains at 0moa and have not meaningfully changed anything compared to a 20moa

It does absolutely nothing for point blank range, once you sight your scope in you can forget you have the 20moa rail, it just moved your turrets further down once sighted in so you can dial further. That's the only benefit and reason to run them. Most scopes have enough adjustment up that you can safely run a 20moa rail without thinking about it.

Got it, thank you

 

[Dirtrax]

I think what is being overlooked here is the advantage of being optically centered. This is where your scope provides the "optimal performance".
With a 20 moa canted rail on my 6mm Creedmoor I rarely if ever approach that optimum point given the capability of the cartridge. I shoot to 1300 yds regularly btw. I'm moving back to a zero cant rail. I prefer the best performance from my optics over some mechanical capability I can't use in the real world.
You all have an amazing grasp on the math, but what in reality is the actual difference between the maximum and minimum optical center point the shooter likely to use? Choose the one that keeps you in the sweet spot.
Of course this is dependent on the amount of adjustment in the optic.

 

[ajvinyard]

Not to reiterate all of the really good information on here but to land it in the simplest terms I know (and thus reiterate!?!?)
If you are not dialing the scope for long shots there is no need to use a 20 moa base but there is also no harm in it either, you will never tell the difference when hunting or target shooting.
If, for example, you are a PRS competitor, a 20 or possibly 30 moa base will be a huge benefit. You will be likely zeroed at 100 yards and towards the high end of the travel on your scope turrets. You will be dialing for part of and sometimes all of the shots in a stage taking advantage of the additional travel you have made available with the canted base. Also doing so will bring the scope to it's optical center and best optical clarity at the distances you will be shooting the most for competition.

6 to one, half dozen to the other.

 

[huntsman22]

I think what is being overlooked here is the advantage of being optically centered. This is where your scope provides the "optimal performance".

What, pray tell, IS the advantage of 'optical centering'? And *** does 'optimal performance' mean? If your scope does not 'perform optimally' while dialling, why even bother having one? Everything below your zero is totally useless, while having more above, lets you play farther.

 

[ShtrRdy]

Speaking of 'incorrect assumptions'. Where do you guys come up with this 'touches line of sight once' crap? In your 'scenario' the bullet rises thru the line of sight somewhere around 25 yards and drops thru again at 100. Prove me wrong......

This picture are the results of running the 'Shooter' ballistic calculator for my 300 Win Mag. (look at the two columns on the left) Note that the bullet starts out below line of sight when fired. This is the distance between the bore of the barrel and the centerline of the scope. The bullet reaches the line of sight at 100 yards and then starts dropping from there.

(Touch picture to expand)

 

[vern.284]

All good info...my personal thoughts and conclusions were under 500 yards no need for anything other than 0 moa rail. Over 500 it will be a benefit.

 

[huntsman22]

This picture are the results of running the 'Shooter' ballistic calculator for my 300 Win Mag. (look at the two columns on the left) Note that the bullet starts out below line of sight when fired. This is the distance between the bore of the barrel and the centerline of the scope. The bullet reaches the line of sight at 100 yards and then starts dropping from there.

(Touch picture to expand)
View attachment 625674

it works for you, I reckon. none of my scopes are mounted that high and slower cartridges, and 200 yd zero's.

 

[Koda_]

All good info...my personal thoughts and conclusions were under 500 yards no need for anything other than 0 moa rail. Over 500 it will be a benefit.

It depends on the scope not the distance. Many scopes can dial drops out to 1000yds without a 20moa rail.

 

[Koda_]

What, pray tell, IS the advantage of 'optical centering'? And *** does 'optimal performance' mean? If your scope does not 'perform optimally' while dialling, why even bother having one? Everything below your zero is totally useless, while having more above, lets you play farther.

It depends on the quality of the scope but the farther your scope dials are from the scopes axis the lower the image quality, especially in low light. If you have high quality glass its probably not an issue or noticeable.

 

[Koda_]

What real affect would putting a 20 moa rail do to the trajectory using a 200yd zero?
Would a 20 moa rail increase maximum point blank range at all?

Nothing and no.
To determine if you need a 20moa rail compare your scopes elevation range from center and estimate your drop in a ballistic calculator at the max range you want to use the scope and if you don't have enough or are closer to the scope limit than not get the 20moa rail.