Brand new Picatinny riser rail causes rifle to shoot 3 MOA to the right

[JimGnitecki]

I have been shooting reasonably tight groups (0.5 MOA on average with handloaded ammunition) with my rifle and scope setup, but knew I could do better. I had determined that the scope height on my rifle was a but too low for my cheek to get down far enough to line up my eye well with the scope, so I was looking through the very upper portion of my eyeglass lens, and my cheek and earmuffs on the stock were adversely inconsistently affecting recoil when using my Rempel bipod and rear bag. So I ordered a Picatinny riser rail that would fit between the one-piece scope mount and the Picatinny rail built into the rifle, and would raise the scope 0.5 inch.

Before I installed the new Picatinny riser rail the rifle was shooting exactly to POA at 200 yards.

I carefully installed the new Picatinny riser rail, ensuring that both the new riser rail and the scope mount were properly seated in their receiving Picatinny slots, all the screws on both the new rail and the one-piece scope mount were all tightened properly a bit at a time, and used a torque driver to get them to the same torque.

I expected the first shots from the new setup to be different in POI ELEVATION (because I added a half inch of height), but to remain zeroed for windage.

However, after the installation, after firing the first 3-shots, I found that they had impacted, at the same 200 yard range, almost exactly 6 inches (i.e. 3 MOA) to the right of POA.

I adjusted the windage knob on the scope to move POI 3 MOA to the left, and that fixed the problem. The rifle went on to shoot groups that were indeed tighter than before I installed the riser rail.

I am SURE my rifle was properly level, as I checked for that meticulously when installing the scope with the higher Picatinny riser, and again at the range, with the rifle on the Rempel bipod, just before I fired any shots at all. And once you set and lock the Rempel, the rifle CANNOT tilt, as the Rempel is locked to the rifle via a Picatiiny rail on the BOTTOM of the rifle, and the Rempel has a wide footprint (about 15 inches on the shooting bench I was using with the rear bag I was using).

So, it seems obvious that the new Picatinny rail (from TruGlo) was somehow machined such that when mounted on the rifle, it is not perfectly colinear with the axis of the rifle barrel. It is 3 MOA off of being perfect.

So, I have a couple of questions:

1. Is it typical for aftermarket Picatinny rails to be THAT far off of perfect, or did I get a bad one?

2. Is it "ok" to use the scope's internal windage adjustment to move the POA horizontally to match the actual POI? Or, does having the PHYSICAL error in the rail's (and thus the scope's) colinearity, and correcting it via scope windage adjustment, going to be a problem at some point? (e.g. when correcting for current wind conditions at longer ranges?)

Jim G

 

[CoalCrackerAl]

Did you check accuracy at different distances? If it does not shoot to the right or left. Your good to go.

 

[JimGnitecki]

Did you check accuracy at different distances? If it does not shoot to the right or left. Your good to go.

I had all my paper targets pre-mounted on one stand at 200 yards, but I used my last 6 rounds to fire at a 6" metal hanging plate target which the cliub has at 300 yards. I deliberately, aimde dand fired "off center" on that 6" gong, about 1.5" from its outer edge at 7:30pm (to make it move more visibly), and all 6 shots hit it. So, I guess it is accurate at 300 yards, but I have not yet tested at 300 yards with a precise crosshair paper target.

Jim G

 

[earplug]

Was your old scope mount perfectly machined and your scope able to be zeroed within 3 MOA of center? My point is your moving from your old zero to your new zero and your still within mechanical limits. Go shoot. Use plugs for better results.

 

[CoalCrackerAl]

I think you will be fine. If it was the new rail. The 300 yard shots would have missed the 6 inch gong.

 

[JimGnitecki]

Was your old scope mount perfectly machined and your scope able to be zeroed within 3 MOA of center? My point is your moving from your old zero to your new zero and your still within mechanical limits. Go shoot. Use plugs for better results.

With my existing scope mount, before adding the Picatinny riser, I needed to only move the windage knob on the scope a small fraction of one MOA to have perfect alignment of POA and POI. The physical addition of the new Picatinny riser necessitated a 3 MOA correction. That clearly shows that the newly added Picatinny riser introduced almost 3 MOA of error.

And again, I know that the error can be COMPENSATED for by using the scope windage knob to move the reticle internally. My question however is does this compensation introduce any negatives that might cause problems at longer ranges or with wind correction.

To put it perspective, here's an analogy:

Suppose you buy a car where one axle is not perfectly at a 90 degree angle to the long axis of the car (i.e. it's cockeyed). The manufacturer might tell you you can compensate for that either by steering a bit to the side to compensate, or with a wheel alignment that cocks the front wheels a little bit so that the car will travel in a straight line down the road.

BUT, that means the car is driving cockeyed down the road. That means that tire wear will be quite significant. It also means that the car's handling on curved roads will be pretty strange. So, in that case, "compensating" for the built-in defect does NOT really solve the problem. The right solution is to replace the axle with one that is truly perpendicular to the axis of the car.

I am asking if there might be any similar issues with using a riser rail that is not truly parallel to the barrel.

Jim G

 

[Zak Smith]

With my existing scope mount, before adding the Picatinny riser, I needed to only move the windage knob on the scope a small fraction of one MOA to have perfect alignment of POA and PO

If you are saying you mounted a new scope on a new rifle with new rings/bases, and it was already zeroed within less than 1 MOA of your POI, then that was the freak occurrence. In 30 years of shooting hundreds of rifles I've had this happen about twice.

You added a riser and the windage was off by 6" @ 200 yards. Do that math, that means that over the length of the riser it might be off by 0.005". Two strands of your hair might be thicker than that.

Whenever the mechanical parts between the scope and the rifle are changed, or a barrel is changed on an action, re-zeroing should be expected.

e one axle is not perfectly at a 90 degree angle to the long axis of the car (i.e. it's cockeyed). The manufacturer might tell you you can compensate for that either by steering a bit to the side to compensate, or with a wheel alignment that cocks the front wheels a little bit so that the car will travel in a straight line down the road.

The analogy is not apt. Scopes can be zeroed for a reason. It's how they work. If it didn't, nobody could ever zero a scope @ 100 and dial it to arbitrary range data and get consistent hits.

Also if you are being precise, which it sounds like you are trying to be, 6" @ 200 yards is not 3 MOA. It's 2.86 MOA.

 

[JimGnitecki]

Thank-you, Zak. If it is usual to need a windage correction with a new rail, I won't worry about it. By the way though, the error in the new Picatinny riser rail is even smaller than you estimated - but still not "good".

The Picatinny riser rail I bought is the TruGlo one that is 4.5" in length. that's 4.5/12 = 0.375 ft.

200 yards = 600 feet.

The error in 600 feet was 6" = 0.5 ft

So the error in the rail is 0.5 ft x (0.375 ft/600 ft) = .0003125 ft = .00375"

Now .00375" sounds small, but this is a CNC-machined rail. On a CNC machine, I suspect an error of .00375" within a 4.5" length is not considered to be good accuracy.

Jim G

 

[Show Me]

I have been shooting reasonably tight groups (0.5 MOA on average with handloaded ammunition) with my rifle and scope setup, but knew I could do better. I had determined that the scope height on my rifle was a but too low for my cheek to get down far enough to line up my eye well with the scope, so I was looking through the very upper portion of my eyeglass lens, and my cheek and earmuffs on the stock were adversely inconsistently affecting recoil when using my Rempel bipod and rear bag. So I ordered a Picatinny riser rail that would fit between the one-piece scope mount and the Picatinny rail built into the rifle, and would raise the scope 0.5 inch.

Before I installed the new Picatinny riser rail the rifle was shooting exactly to POA at 200 yards.

I carefully installed the new Picatinny riser rail, ensuring that both the new riser rail and the scope mount were properly seated in their receiving Picatinny slots, all the screws on both the new rail and the one-piece scope mount were all tightened properly a bit at a time, and used a torque driver to get them to the same torque.

I expected the first shots from the new setup to be different in POI ELEVATION (because I added a half inch of height), but to remain zeroed for windage.

However, after the installation, after firing the first 3-shots, I found that they had impacted, at the same 200 yard range, almost exactly 6 inches (i.e. 3 MOA) to the right of POA.

I adjusted the windage knob on the scope to move POI 3 MOA to the left, and that fixed the problem. The rifle went on to shoot groups that were indeed tighter than before I installed the riser rail.

I am SURE my rifle was properly level, as I checked for that meticulously when installing the scope with the higher Picatinny riser, and again at the range, with the rifle on the Rempel bipod, just before I fired any shots at all. And once you set and lock the Rempel, the rifle CANNOT tilt, as the Rempel is locked to the rifle via a Picatiiny rail on the BOTTOM of the rifle, and the Rempel has a wide footprint (about 15 inches on the shooting bench I was using with the rear bag I was using).

So, it seems obvious that the new Picatinny rail (from TruGlo) was somehow machined such that when mounted on the rifle, it is not perfectly colinear with the axis of the rifle barrel. It is 3 MOA off of being perfect.

So, I have a couple of questions:

1. Is it typical for aftermarket Picatinny rails to be THAT far off of perfect, or did I get a bad one?

2. Is it "ok" to use the scope's internal windage adjustment to move the POA horizontally to match the actual POI? Or, does having the PHYSICAL error in the rail's (and thus the scope's) colinearity, and correcting it via scope windage adjustment, going to be a problem at some point? (e.g. when correcting for current wind conditions at longer ranges?)

Jim G

Manufacturing tolerances

 

[waktasz]

Even taking the scope off the gun and changing nothing requires that you re-zero it when reinstalling. Of course you need to zero the gun again after changing the rail like that

 

[JimGnitecki]

I can understand having to change the scope windage or elevation when I change something physically. What bothers me about THIS situation is that the riser RAIL itself altered the windage, NOT any change that I made other than inserting the rail between the rifle's OEM rail and the one-piece scope mount. Neither the OEM rail nor the one-piece scope mount was altered in any way during the process of adding the rider rail.

Before the riser rail was added, the rifle was shooting to POA with the scope's windage adjsutment set to "zero" on the factory scope windage scale. After the riser rail was added, the scope settings SHOULD have required only an ELEVATION change, IF the riser rail was machined precisely enough to maintain co-linearity with the existing scope mount and OEM rail. Since it introduced a windage error, it is clearly NOT perfectly co-linear. Sure, every machined part has a tolerance, but a .00375" error in co-linearity on a 4.5" long CNC-machined part is not "good enough" for a CNC machined part. Someone either screwed up the CNC setup, or the CNC machine was badly worn when it made that particular riser rail.

Jim G

 

[Zak Smith]

You keep using the word collinear incorrectly.

 

[JimGnitecki]

You keep using the word collinear incorrectly.

Sorry. I don't know the proper terminology. What I mean to say is that the riser rail and the rifle barrel were both perfectly PARALLEL withOUT requiring any scope windage correction, before the riser was added, and should have remained perfectly parallel or pretty darn close to perfectly, if a proper CNC machining process was used to make the riser rail.

Jim G

 

[waktasz]

I'm just going to write this again

Even taking the scope off the gun and changing nothing requires that you re-zero it when reinstalling. Of course you need to zero the gun again after changing the rail like that

 

[Zak Smith]

How do you know your existing rail is straight? If the existing rail isn't straight your logic doesn't work even if your riser is perfect.

 

[JimGnitecki]

How do you know your existing rail is straight? If the existing rail isn't straight your logic doesn't work even if your riser is perfect.

I know that the OEM rail is straight, AND the one piece scope mount is straight, because with the scope windagae adjustment left at the scope's factory "zero" setting, the rifle shot to POA exactly. Only ELEVATION adjustment was necessary to shoot at 100, 200, oe 300 yards. Once I added the riser rail, the windage all of a sudden was off.

Jim G

 

[Zak Smith]

No. You only know there was a coincidence with the scope and everything between it and your barrel.

Let's say you have a set of railroad tracks whose rails stay the same distance apart but oscillate slightly left to right, and a railcar rolls down them. How does that affect the direction of the railcar in any given linear position down the tracks? How about a different length railcar?

Now repeat the same experiment but the rails don't stay exactly the same distance apart.

 

[Show Me]

Cheese with that wine

 

[JimGnitecki]

No. You only know there was a coincidence with the scope and everything between it and your barrel.

Let's say you have a set of railroad tracks whose rails stay the same distance apart but oscillate slightly left to right, and a railcar rolls down them. How does that affect the direction of the railcar in any given linear position down the tracks? How about a different length railcar?

Now repeat the same experiment but the rails don't stay exactly the same distance apart.

That is a flawed analogy, because machined picatinny rails and one-piece scope mounts (unlike individual scope rings) do not "squirm". They are always "straight", BUT if the CNC setup was not precise, or the CNC machine is worn, the Picatinny rail can be off ANGULARLY (like the one I installed).

Jim G

 

[Zak Smith]

It's less flawed than the ideas that scopes come pre-zeroed, that they should not be adjusted to zero for windage, or that you actually have any measurements of the components in this system other than the POI change.

 

[mcb]

Per you math a 3-moa shift would be a .00375 shift of one end of the 4.5 inch riser. If you do the trig directly I get a 3 MOA angle (0.05 degrees) over 4.5 inches as .003927 inch. 4.5 in x tan(.05) = .003927. Either way we are talking about a distance roughly equivalent to the thickness of decent printer paper.

But we are not talking about a single dimension we have multiple interfaces involved here. The riser itself is made up of six parts, the riser, the clamp, two bolts and two nuts. That new riser assembly interfaces with your existing OEM rail and all those parts can have some effect on how it mates, to greater and lesser degrees. Then on the top side you have a scope mount that has multiple parts that must form the clamp to the top of the riser and all the geometry that makes up the picatinny rail interface. That is a lot of tolerance build up in all these mating surfaces. Even if we assume the OEM rail is somehow perfect, and that the scope mount is also. You still have the six parts of the new riser and all the varied geometry involved that all interact with each other and the existing components.

Could you machine that riser to have every possible mating surface parallel to significantly less that .004 inch over 4.5 inches, sure but not for the price you paid for that part.

Also you have added mass to the rifle and more importantly you have moved a larger mass (your scope) to a higher less stiff position. This change in mass and stiffness could affect barrel harmonics. Your point of impact shift could be due to as much a change in barrel harmonics as mechanical misalignment of the riser.

You scope probably has 60+ MOA total windage adjustment it would not have even occurred to me to questing a 3 MOA change when changing the scope mounting hardware.

Antidotal evidence: I removed and remounted a Leupold Mark 4 CQ/T with its integrated mount and got a 5+ MOA shift in windage and I did not change anything but remove the scope and then remount it to the same rail on the same rifle in the same location. Toque was the only thing that might have changed as I did not have a toque wrench the first time I mounted it but did when I remounted it.

I think your expectations are fairly high of both TrueGlo and the situation in general.

 

[entropy]

I have been shooting reasonably tight groups (0.5 MOA on average with handloaded ammunition) with my rifle and scope setup, but knew I could do better. I had determined that the scope height on my rifle was a but too low for my cheek to get down far enough to line up my eye well with the scope, so I was looking through the very upper portion of my eyeglass lens, and my cheek and earmuffs on the stock were adversely inconsistently affecting recoil when using my Rempel bipod and rear bag. So I ordered a Picatinny riser rail that would fit between the one-piece scope mount and the Picatinny rail built into the rifle, and would raise the scope 0.5 inch.

Before I installed the new Picatinny riser rail the rifle was shooting exactly to POA at 200 yards.

I carefully installed the new Picatinny riser rail, ensuring that both the new riser rail and the scope mount were properly seated in their receiving Picatinny slots, all the screws on both the new rail and the one-piece scope mount were all tightened properly a bit at a time, and used a torque driver to get them to the same torque.

I expected the first shots from the new setup to be different in POI ELEVATION (because I added a half inch of height), but to remain zeroed for windage.

However, after the installation, after firing the first 3-shots, I found that they had impacted, at the same 200 yard range, almost exactly 6 inches (i.e. 3 MOA) to the right of POA.

I adjusted the windage knob on the scope to move POI 3 MOA to the left, and that fixed the problem. The rifle went on to shoot groups that were indeed tighter than before I installed the riser rail.

I am SURE my rifle was properly level, as I checked for that meticulously when installing the scope with the higher Picatinny riser, and again at the range, with the rifle on the Rempel bipod, just before I fired any shots at all. And once you set and lock the Rempel, the rifle CANNOT tilt, as the Rempel is locked to the rifle via a Picatiiny rail on the BOTTOM of the rifle, and the Rempel has a wide footprint (about 15 inches on the shooting bench I was using with the rear bag I was using).

So, it seems obvious that the new Picatinny rail (from TruGlo) was somehow machined such that when mounted on the rifle, it is not perfectly colinear with the axis of the rifle barrel. It is 3 MOA off of being perfect.

So, I have a couple of questions:

1. Is it typical for aftermarket Picatinny rails to be THAT far off of perfect, or did I get a bad one?

2. Is it "ok" to use the scope's internal windage adjustment to move the POA horizontally to match the actual POI? Or, does having the PHYSICAL error in the rail's (and thus the scope's) colinearity, and correcting it via scope windage adjustment, going to be a problem at some point? (e.g. when correcting for current wind conditions at longer ranges?)

Jim G

Most risers are attached with clamps held by Allen nuts. This most likely caused the horizontal shift of POI. You easily solved the problem, but the first indicator that the Allens are coming loose will be a horizontal shift.

So, I have a couple of questions:

1. Is it typical for aftermarket Picatinny rails to be THAT far off of perfect, or did I get a bad one?

2. Is it "ok" to use the scope's internal windage adjustment to move the POA horizontally to match the actual POI? Or, does having the PHYSICAL error in the rail's (and thus the scope's) colinearity, and correcting it via scope windage adjustment, going to be a problem at some point? (e.g. when correcting for current wind conditions at longer ranges?)

1. Yes.

2. Yes, it's OK to move it. You'd have to be at the outer 3 MOA of the scope's adjustment range be for it becomes an issue. If that happens: It is either very, very, windy or your target is probably farther than you should be shooting, and it's a bit windy. At those ranges, you should have a 20 MOA rail under the mount, as you will run out of vertical long before horizontal.

 

[JimGnitecki]

. . . At those ranges, you should have a 20 MOA rail under the mount, as you will run out of vertical long before horizontal.

Yes, the rifle came from the factory with a 20 MOA rail as standard.

So from what so many of you are saying, I was just plain lucky that the original installation (before the riser rail was added) was spot-on in windage? I guess I will either verify or dispute that the next time I am at the range, as I have now bought and installed (properly torqued) a different brand riser rial (Weaver). Riser rails are inexpensive enough that I figured after all this discussion I should go ahead and do the experiment, Our weather has been remarkably (even statistically) bad lately, with very low temperatures, snow or rain, and even fog. But it has to break sometime, and when it does, I'll be out there.

One advantage of the persistent patchy fog: You KNOW there is no wind to require a windage adjustment! But it makes seeing the target at 300 yards somewhat more challenging.

Jim G

 

[mcb]

Yes, the rifle came from the factory with a 20 MOA rail as standard.

So from what so many of you are saying, I was just plain lucky that the original installation (before the riser rail was added) was spot-on in windage? I guess I will either verify or dispute that the next time I am at the range, as I have now bought and installed (properly torqued) a different brand riser rial (Weaver). Riser rails are inexpensive enough that I figured after all this discussion I should go ahead and do the experiment, Our weather has been remarkably (even statistically) bad lately, with very low temperatures, snow or rain, and even fog. But it has to break sometime, and when it does, I'll be out there.

One advantage of the persistent patchy fog: You KNOW there is no wind to require a windage adjustment! But it makes seeing the target at 300 yards somewhat more challenging.

Jim G

I will be interested to hear how much better or worst the Weaver rail is. Cool experiment keep up informed!

 

[bihj]

See post #5.......

End of thread