Rifle Scopes Scope mount alignment question....

[Wannashootit]

I'm trying to adapt/fit a Weaver side mount to a Mosin Nagant action.
As I consider alignment, it seems that it is not critical to have the center of scope (center of rings) directly aligned over the center of barrel.
We have an AK with the original side-mount POSP, and it's waaayy offset.
As long as the mount/scope is perfectly parallel to the bore both vertically and horizontally, am I correct in assuming horizontal offset can be easily compensated for with windage- just as the vertical distance over the bore is always compensated with elevation?

 

[Wannashootit]

Re: Scope mount alignment question....

Nobody?

 

[exhogflyer]

Re: Scope mount alignment question....

You are correct in this...

 

[Wannashootit]

Re: Scope mount alignment question....

Thanks for confirming...
I'd hate to f it up.

 

[Lindy]

Re: Scope mount alignment question....

Just bear in mind that when you zero at, say, 100 yards, you introduce an offset angle between the scope and the bore.

Let's say that offset is 0.5 inches.

That means that your lateral error from that offset is:

0.0 inches at 100 yards
0.5 inches at 200 yards
1.0 inches at 300 yards
1.5 inches at 400 yards
2.0 inches at 500 yards

and so on.

Depending on the size of your targets and the distance you're shooting at, that may or may not require compensation for.

 

[dieselten]

Re: Scope mount alignment question....

The best way to accomodate for the off set windage is just to zero with the off set being constant at all ranges. If scope is .5" left of center line of bore then zero it so rifle hits .5" right and it should be close to this at all ranges.

 

[Lindy]

Re: Scope mount alignment question....

Yes, that will work. There's even a name for that - it's called a parallel-bore windage zero.

There are also uses for a parallel-bore elevation zero, too.

 

[Wannashootit]

Re: Scope mount alignment question....

Thanks, guys.
I'm going to be very close- within a few mm's.

On a bit of a tangent, but still on the topic of scope alignment- if one uses a MOA rail with built-in elevation, doesn't that change the usual method of compensating for elevation at different distances?

IOW, because the scope itself would now be canted, and not parallel to the bore vertically- does that mean that changes in distance can't be compensated for simply by using bullet drop? Seems that you would need to sight-in at each distance you would be shooting to get the correct MOA adjustment which is affected by the angle of the scope's mounting rail? OR...am I just overthinking this?

 

[TiroFijo]

Re: Scope mount alignment question....

You can zero at 200 yds:
100 yds = 0.25" off
200 yds = 0
300 yds = 0.25 off (opposite direction)
400 yds = 0.5" off "

A rail with built-in angle does not change anything once the scope is zeroed. You have to use a program where you introduce the scope height, zero range, bullet type and muzzle velocity, and it will give you the vertical trajectory of the bullet at any range in relation to this base zero.

 

[NoExpert]

Re: Scope mount alignment question....

A down angle base is used to get more elevation travel out of the erector tube. If everything is perfectly parallel then you just used about half of your elevation in the scope. Using a down angle base makes the scope aim down so you need to adjust elevation to bring it close to parallel again. This will give you more elevation to use for making longer range shots.

They have no effect on alignment other than that.

 

[Wannashootit]

Re: Scope mount alignment question....

Sometimes it's hard to articulate in words exactly what I'm thinking...and I'm probably overcomplicating the issue anyway...
But- since the "lines of sight" are no longer parallel with a down-angle base, does the mathematical "method" of compensating for bullet drop change?

IOW, let's assume I'm zeroed at 200 yards. I have 9" of bullet drop at 300 yards. Would I go 36 clicks on a 1/4 moa scope as usual, or is it necessary to sight-in at this distance because the scope is not parallel and the "4 clicks to one moa" changes?

 

[TiroFijo]

Re: Scope mount alignment question....

36 clicks on a 1/4 moa scope as usual

The scope does not know what the base angle is once it is zeroed (other than optimizing internal adjustment as NoExpert pointed out). Remember you aim through the internal erector tube, that always have the same angle relative to the bore when zeroed at a certain distance.

 

[Wannashootit]

Re: Scope mount alignment question....

Thanks!
I just put an EGW rail on our new .308 that we expect to take long range soon. 100-200 yards is no longer a challenge and we're ready to move up.

I know I'll have to calculate and see how much adjustment I can get out of each of our rifles' scopes...

Yet another newb question...
One of our rifles is a Rem. 700, .223. I handload for it, but for sake of discussion, a Hornady 55 gr. V-Max shows 46" of bullet drop at 500 yards. The Bushnell 5x15 on it has a max adjustment of 50". So, it looks like 500 yards is the max for this setup as-is, and I would need to install a MOA rail to exceed that distance.

Can you still shoot at 100 yards after installing a down-rail?

 

[BobinNC]

Re: Scope mount alignment question....

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">Thanks!
I just put an EGW rail on our new .308 that we expect to take long range soon. 100-200 yards is no longer a challenge and we're ready to move up.

I know I'll have to calculate and see how much adjustment I can get out of each of our rifles' scopes...

Yet another newb question...
One of our rifles is a Rem. 700, .223. I handload for it, but for sake of discussion, a Hornady 55 gr. V-Max shows 46" of bullet drop at 500 yards. The Bushnell 5x15 on it has a max adjustment of 50". So, it looks like 500 yards is the max for this setup as-is, and I would need to install a MOA rail to exceed that distance.

Can you still shoot at 100 yards after installing a down-rail? </div></div>

Assuming your figure of 46" of drop is correct for the 55 gr Vmax, running a JBM table and forcing a 46" drop, would require an initial muzzle velocity of 3430 FPS.

Working from those numbers, and calculating from a 100 yd zero, the elevation adjustment required from your scope would only be 8.8 MOA.

If your scope does indeed have 50" of adjustment, that's 25" up and 25" down. 25" of adjustment is equivalent to 23.9 MOA, and you only need around 8.8 MOA to reach 500 yds from a 100 yd zero. 23.9 MOA of adjustment will get you to around 770 yds or so.

Sounds like you have plenty of adjustment for a 500 yds target, without a canted base.

Bob

 

[TiroFijo]

Re: Scope mount alignment question....

Normally you can, unless your scope has an extremely small adjustment range, or your canted base has a lot of angle.

1.0 MOA is 1.047" at 100 yds, but is an angular measure, so it is 2*1.047 = 2.094" at 200 yds, 3.141" at 300, and so on...

So if you have a load that drops 46" at 500 yds, in relation to you 100 yds baseline zero, you have to adjust:

46/(5*1.047) = 8.8 MOA

the ballistic programs will give you the come ups in MOA or inches

and BobinNC beat me to it!

 

[Wannashootit]

Re: Scope mount alignment question....

Ahhh...thanks, Oh Wise Ones...
I was forgetting to convert the total bullet drop, to minutes of angle...DOOOOHHH....