Revelations on Mounting a Scope with Leupold Rings and Bases

[Peppysan]

Ok so I am just about to switch out a scope from my Browning A-Bolt, partially because "I thought" it was not holding zero. I bought the rifle used, it came with the Leupold two piece bases, the Standard" single front pivot and rear windage adj. base that the rings go on. This year, and in previous years I have shot game with it and the point of impact was not where I was actually aiming. I always wrote it off as "Buck Fever", wind, yadda yadda, then the more I thought about it, there was also RANGE to Target, usually more than 100yrds. (This year a Mulie at 230yrds)

As I delved into the purpose of the windage adjustment I have realized that IF the rear base is off center it has pivoted the scope on the front base making the Line of Sight (LOS) now track out left or right of the actual Bore of the rifle.

Then when I take it to the range and sight in the scope at 100yrds it groups perfect at 100yrds. Which is because I have now moved the windage crosshair to intersect that point out at 100yrds. BUT in fact, (I believe) that the bullet trajectory, being straight and the LOS is actually twisted off axis, any target past 100yrds has the impact spot left or right of what the crosshairs would have you believe. This would be less obvious on targets under 100yrds.

Now getting geeky about it I have made a model of the effects of the difference that could make. It has astonished me that by as little as .005" (5 thousandths) of an inch off center at the rear mount, which amounts to only .0573 of a DEGREE (@5.00" Radius) will move the LOS to be as much as 3.62" out at 100yrds and this will increase, (just like MOA) to be double that at 200yds 7.22" and 10.8" at 300yrds. and so on. This explains why I hit this year's mule deer, (at 230yrds) about 4-5" left of my aim point.

So if anyone would like to prove otherwise I welcome that. But if this is indeed the case how the X#x$ do you perfectly align the scope to the centerline of the barrel with these Leupold Mounts??

I know that there are scope kits with the alignment bars that taper to a point, but the Leupold front ring does twist in the base. So should that be slightly turned off of the center line, you COULD move the rear, (side to side) to align with the point.

I suppose that the scope may bind trying to align to the two rings but as per my sample measurement you don't want to be off by much more than .001" inch which I don't think you would feel/see that. How could you even measure that?

Any input would be great, maybe I am over thinking it but I believe the math would verify my measurements. Actual shots in the field have convinced me.
Some of you guys doing Long Distance shooting may be aware of this, maybe you don't use Leupold Bases & Rings HaHa.

I created this graphic (Not to scale of course) to visually explain my findings.



(Exaggerated to show effect & make it fit the page)

 

[shackleford]

I use a 3' piece of wood dowel clamped in ring to turn in front ring. I stop when its directly over the muzzle. then loosen rings. slide the rear ring on dowel then alternating between both sides .tighten down windage screws. But only shooting 100 yards as well....

 

[tiriaq]

Are you suggesting it would be a good idea to center the scope's windage, use the rear clamping screws to get the crosshairs centered on the target, and then use the scope's internal adjustments refine the final zero?

 

[270 totheend]

You are overthinking it and mixing up your reference points for misalignment. If you loosen one windage screw and tighten the other for a .005” movement you would be 3.6” off at 100yds. If your point of aim was off 3.6” at 100 yd you would miss by 3.6” at 100 yards. When you zero your scope at 100yds if your scope is .005” left of center on your rifle your group would be .005” left at 200 yards. Think about a Win 94 side mount 1” off center. It would be 1” left at 200 yds. Your 4-5 inch off target at 230 yds was probably a 5 mph wind deflection somewhere between you and the deer.

 

[Peppysan]

You are overthinking it and mixing up your reference points for misalignment. If you loosen one windage screw and tighten the other for a .005” movement you would be 3.6” off at 100yds. If your point of aim was off 3.6” at 100 yd you would miss by 3.6” at 100 yards. When you zero your scope at 100yds if your scope is .005” left of center on your rifle your group would be .005” left at 200 yards. Think about a Win 94 side mount 1” off center. It would be 1” left at 200 yds. Your 4-5 inch off target at 230 yds was probably a 5 mph wind deflection somewhere between you and the deer.

Correct you are on an offset mounting system. Under the presumption that that system in mounted perfectly parallel to the rifles bore, then yes at any distance the bullet (theoretically) will be 1" to the side, IF YOU DIDN'T ADJUST the windage in the scope to hit a bulls eye at 100yrds. I believe that once you do that the "Line of Sight" of the scope will now be crossing the true path of the bullet. Hence at ranges greater than, (or less than) "Zero" the impact will increasingly move to the side.

Easy to imagine, visualize two strings* (lines) out to 200 yards perfectly parallel but 1" apart. Now at the far end of one string move it sideways until it crosses (intersects) the other string at 100 yards. I think you will find that at the far end the end of your string will now be 1" off on the other side of the line.
That is a very simple way to verify the situation. The effect in this case is not that great, as the "Pivot Point" is 100 yards away at the center of the string. In the case of the Leupold mounts the "Pivot Point" is only maybe 5 inches from the starting end. Now visualize how far off you would be out, if you rotated that short end 1" to the side of the pivot which is 5" away. I can tell you, it will be 719in (59.9ft) to the side at 200 yards.

Your thought that I would only be off .005" would be correct IF the scope was moved perfectly sideways, maintaining a parallel reference to the bore. But it does not adjust that way with these mounts. The forward ring PIVOTS in the front mount to maintain a common centerline of the two rings, (which prevents stress on your scope tube) which then changes the point of aim of the optics. Now we are not parallel to the bore.

As far as the shot on the mulie it was a dead calm day. Which is why I am trying to figure this out. It's a "Thing" with this rifle/mount I think, because it's also not the first time I didn't hit exactly where I aimed, (always on the longer shots). I initially though it was the scope, a decent 1" tube Leupold and I was going to change out to a 30mm tube scope so I needed rings and discovered how the bases work. That's where I had the "Ah HA" moment. Since I had this brain fart I haven't been able to really test this at the range.

Now the task at hand is still how do you precisely, and I mean perfectly align these rings to the rifle's bore.
Maybe I should just get a rail . . . .

*If you don't have that much string use two rulers or any two sticks, any lenght.

 

[Peppysan]

I use a 3' piece of wood dowel clamped in ring to turn in front ring. I stop when its directly over the muzzle. then loosen rings. slide the rear ring on dowel then alternating between both sides .tighten down windage screws. But only shooting 100 yards as well....

I think you may have a viable method, I may use some aluminum bar stock.

 

[dan belisle]

I use a 3' piece of wood dowel clamped in ring to turn in front ring. I stop when its directly over the muzzle. then loosen rings. slide the rear ring on dowel then alternating between both sides .tighten down windage screws. But only shooting 100 yards as well....

Good idea. I do the same, though my dowel is only a foot long. Also, go easy on extended front rings when twisting them in (even when lubricated), they break easy. - dan

 

[propliner]

If the scope body is angled form the bore, the erector tube inside the scope will eliminate the offset angle by an equal amount the opposite way and erase the error.

 

[Bad Bob]

Way too much over thinking. 1) Mount scope as close to optical center as possible given whatever mounts are being used. 2) Zero scope at what ever range desired. 3) Go shoot.

 

[Peppysan]

Good Info

If the scope body is angled form the bore, the erector tube inside the scope will eliminate the offset angle by an equal amount the opposite way and erase the error.

I was hoping someone would be familiar with the way a scope works, I have no idea how the internals work in relation to cranking the dials and aligning the cross hairs.
I would guess that there must be some UTube posts explaining the mechanics of the optics. Going to get on that.

Thanks

 

[Bad Bob]

That is why the windage and elevation turrerts exist (the little dials on the side of the scope). Turn them up or down, left or right, which ever way you want the bullet to go and shoot. If you need to know more that is why Goggle exists.

 

[propliner]

I was hoping someone would be familiar with the way a scope works, I have no idea how the internals work in relation to cranking the dials and aligning the cross hairs.
I would guess that there must be some UTube posts explaining the mechanics of the optics. Going to get on that.

Thanks

In actuality it is a great system for correcting inherent windage issues in a rifle but is unnecessary for rifles drilled with properly aligned mounting holes or rails.

As Tiriaq said, when using one of these mounts, make sure the windage is centered in the scope (the same number of clicks from each side), then mount the scope into the rings. If the windage is off, adjust the rear screws in the mount accordingly to center crosshairs on the target. Then fine tune using the windage knobs on the scope.

The Leupold mount is meant for fixing windage issues in the rifle and when used properly is a great tool to correct it.

If the mount is used improperly ie the mount's windage screws start off out of alignment and the scope is crooked, the scope can internally make up for the windage error. This of course this is not ideal but since the scope misalignment and erector tube correction are virtually the same distance from the target, the correction error is extremely negligible and it should work fine set up like this. But you won't be looking through the ideal optical center of the scope's objective lens and there will be more unnecessary spring pressure on the erector tube mechanism.

 

[Duramax6.6]

In actuality it is a great system for correcting inherent windage issues in a rifle but is unnecessary for rifles drilled with properly aligned mounting holes or rails.

As Tiriaq said, when using one of these mounts, make sure the windage is centered in the scope (the same number of clicks from each side), then mount the scope into the rings. If the windage is off, adjust the rear screws in the mount accordingly to center crosshairs on the target. Then fine tune using the windage knobs on the scope.

The Leupold mount is meant for fixing windage issues in the rifle and when used properly is a great tool to correct it.

If the mount is used improperly ie the mount's windage screws start off out of alignment and the scope is crooked, the scope can internally make up for the windage error. This of course this is not ideal but since the scope misalignment and erector tube correction are virtually the same distance from the target, the correction error is extremely negligible and it should work fine set up like this. But you won't be looking through the ideal optical center of the scope's objective lens and there will be more unnecessary spring pressure on the erector tube mechanism.

This is exactly how to utilize this mounting system. By making the big windage adjustments externally and only needing a few clicks to perfectly zero the impact, you will always be looking through the clearest image the scope is capable of because you're looking through the center of the lenses. Also the less deflected the erector tube assembly is the better the image and light transmission. After 40 years I've never had the windage screws shoot loose or work loose on me of these mounts including a 378 Weatherby.

The picatinny and Weaver slotted bases are great if you like swapping scopes back and forth because the slot spacing should all be the same. But if your mounting holes on the receiver are off by a tiny bit your scope may not have the internal adjustment to compensate. Plus on some scopes when you have one of the turrets maxed out, the adjustments using the other turret may not track straight in the direction intended but almost diagonally. Modern scopes have way more internal adjustment than the scopes that were around that inspired Mr. Redfield to first design and patent his STD and Jr mounting system.

 

[Peppysan]

Good Information

Thank you Propliner and Duramax6.6 for some informative and educated responses, (Unlike Old BAD BOB) this is the kind of information I was trying to pull from the resourses of this Forum.
I am usually quite hesitant to post a question, or a result, and heaven forbid an opinion on forums generally because of the usual smart a z z replies I have come to witness hanging around here for the past 9 years.
At least I did gain some knowledge, and a trick or two going forward on dialing in these rings. Thanks also to Shacklford & 270 Totheend.

Until next time I am really really curious or stumped. I may give it another go.

 

[Wendell]

 

[Peppysan]

Found a way

Thanks Wendell for that post, (UTube) has some bearing on what I am trying to learn more about. It also lead me to come across another video that has just the hardware to verify alignment between a set of rings, this I had never seen before. (see link below for UTube) I am going to machine me a set.

Anyway on to my update.

I put a lot of thought into how I was going to swap scopes and install the new scope as perfect to the bore centerline as possible. I have in my tool box an older Bushnell Optical Bore Sight, (link below) this is the unit that has an arbor that centers in the bore and positions an "Optical Window" with a Grid in front of the scope.

So first thing I did was mount the Bore Sight to the rifle, (Browning A-Bolt) BEFORE I removed the original scope, (Leupold 3-9x40 w/1" tube).
This scope was "Sighted In" to this rifle now for years, so I am pretty confident it's kinda close. Anyway the sight picture in the Bore Sight placed the crosshair low and to the left of the Center of the optic grid. Not much but the equivalent of about 1-MOA each way.
Next step was to verify, and adjust on the new scope, (Sig Sierra3) that the range of travel of the crosshairs, (both windage and elevation) were precisely in the center for each, with respect to the range available.

I installed the new, (now 30mm) front ring into the "Dovetail" hole and rotated it to eyeball close to the center of the bore. I then placed the rear ring on the pad, with both windage screws backed out. Placed the new scope into the saddle and put the caps on with the screws, but not at all tight. I then somewhat tightened the saddle down on the front ring to grip the scope enough that it was held in alignment with the ring. The rear ring was still floating and will now follow the scope tube position.
Looking though the scope now I can see the grid in the Bore Sight, focusing on the windage element, (vertical line).
It WAS NOT near close to the "0" vertical grid line. Using a small 5/8" wrench, I could get it under the scope on the neck of the front ring and turn the ring until I was able to bring the Scope windage to match the "0" line on the grid of the Bore Sight.
Now very carefully I turned in the rear mounts "Windage" screws from each side, all the time observing through the scope that it did not swing the scope left or right. And I mean it was pretty evident that if I torqued on one side too much the windage was shifting off center. But really if you take it slow and easy from side to side it doesn't take all that long to get it right.
Followed up with torqueing the caps as per spec. and constantly checking back visually if the scope deviated from the center, which it did not.

Next step Range Day. Went to my closest available range which is only 100yrds, but this is where you want to start.
Fired a couple rounds and I was high, to be expected as the Bore Sight indicated the Horizontal to be there, (I didn't change it from the centered original position).
Brought the elevation up, to bring the hits down and to my delight I was grouping 1-MOA (1 inch) to the right of center. So 4 clicks left in windage and we are good. Interesting enough when I got home I again mounted the Bore Sight and it is indicating about 3/4-MOA again both to the left and low on the grid, a very small bit of variation, maybe the original scope installation was just that little bit off.

Now how perfect are the optics aligned with the bore? I would say pretty dam close, even those four clicks to correct may be well within the tolerance of the windage centering travel of the scope, or the precision of the Bore Sight Grid, or the base screw hole placement or the ammo characteristics. I was diligent to use the same hand loads that had been previously fired in this rifle to hopefully eliminate that factor. I can usually get groups of 1/2MOA at 100yrds with this rifle/ammo.

Ok wait you skeptics, stand back from your keyboard . . . .

I have TWO rifles and another new Scope and those tricky Leupold rings.

Repeated the same steps and process to replace a Vortex with another Sig Sierra3, this time on a Weatherby.
Same day, same range, same 100 yards. ZERO CORRECTION to the windage, I had to bring down the impact in elevation but that was it. The follow up with the Bore Sight was perfectly in line with the original results of the Vortex scope alignment on the grid.
I believe I have a method, which was the whole point of this thread. I mean the real proof will be shooting at distance, 200, 300, plus. Hopefully on a calm day and no serious deviation in the windage impact, which would indicate, (to me anyway) that the back of the scope, or in essence the center line of the rings and optic are perfectly in line/parallel with the bore. I will update this thread with those results.

Very Clever Ring Alignment Tool
Just an observation with a tool like this is, it may confirm that the two rings are aligned with each other but NOT necessarily inline with the bore.
These would be far more accurate than those "machined to a point" alignment rods you can buy.

Link to the Bushnell Bore Sight

https://www.amazon.ca/Bushnell-Professional-Bore-Sight-Case/dp/B0002LWWBW

=======================

 

[OkayShooter]

I use these on a mauser that have the holes drilled off bore. I make rough adjustment with the base, then fine tune with the scope.

I used a wrench to turn in the ring.

 

[USPfan]

All the more reason to get a European rail mounted scope.