Tuning FX Impact barrel indexing question

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Corvid_hunter

Corvid_hunter

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Dec 21, 2019
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Ontario, Canada
I have a new MKII and did a little bit of indexing on my barrel (12-3-6-9) and managed to get my windage from 8 MOA at the 12 o'clock position to 3 MOA at the 9 o'clock position. I did this in my garage at 9 yards and will wait for some nice weather and try to fine tune it at 25 or 30 yards. I was wondering what most people end up with for windage correction after doing this type of tuning. I marked the threads on the piece that tightens the liner into the sleeve where the threads start and where to tighten it to so I get that part consistent every time. I know my scope is perfectly level as I used a Spur scope mounting tool that squares the bottom of the scope to the rail so those variables should be eliminated.
 
3 MoA is good...of course you can always try to get it closer to zero if you want. If your POI swung to both sides of the reticle during your 4-point indexing experiment, that means you can hit 0.

Just to be sure, you did center the scope's reticle beforehand, right? 

Lastly, please note that orienting the scope by way of physical features on the scope and gun by no means ensures it is aligned properly. There are more than half a dozen reasons why that approach may yield a canted scope. We discussed the topic here a while back: https://www.airgunnation.com/topic/check-out-this-scope-leveler
 
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3 MoA is good...of course you can always try to get it closer to zero if you want. If your POI swung to both sides of the reticle during your 4-point indexing experiment, that means you can hit 0.

Just to be sure, you did center the scope's reticle beforehand, right? 

Lastly, please note that orienting the scope by way of physical features on the scope and gun by no means ensures it is aligned properly. There are more than half a dozen reasons why that approach may yield a canted scope. We discussed the topic here a while back: https://www.airgunnation.com/topic/check-out-this-scope-leveler
Click to expand...

I used a level on the side of my Impact when I installed the bubble level on the scope so that was true. The grid lines on the target I used were plumb as per the level I used on the target. The reticle lined up perfectly with the grid lines on the target and the bubble level on my scope said it was level so I'm guessing I'm pretty close with regard to everything being lined up. Now I just need to work on the indexing when the weather cooperates.
 
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It's probably close but fundamentally there is no surface on the gun or the scope that can reliably be used to orient the scope correctly. Using a mirror as described in the link will ensure it's installed properly.

A level only comes into play _after_ the scope has been mounted. That is to ensure the shooter does not induce gun cant. Scope cant is a distinctly different source of error, though often times the two get conflated together. 

The reticle also should be mechanically centered first. If you were to start with the reticle (erector tube) already dialed to one side or the other, the result will not be optimal.
https://www.gatewaytoairguns.org/GTA/index.php?topic=116903
 
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I used a machinist's square to check to see if the rail is square with the body of the gun. It is (FX should be ashamed of themselves if it wasn't). I used a Spuhr mounting tool to square the scope (Athlon Midas Tac) with the rail. I leveled the rifle vertically and installed the MK Machining billet scope level. The target I used was plumb and the reticle lined up perfectly with the target while the scope level said the gun had no cant. It's probably close to being set up correctly unless the scope is garbage.
 
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At least check the reflection in the mirror to confirm the reticle simultaneously bisects the objective bell and muzzle.

Why am I beating this dead horse? Because installing a scope based on a flat feature of the gun's action requires that:

1. The barrel is perfectly straight, and
2. the bore is centered within the barrel, and
3. the scope rail is precisely parallel to the receiver's mortise, and
4. the barrel-to-receiver interface has no lateral bias (a typical O-ringed barrel tenon secured with a grub screw through the top of the receiver will have some bias), and
5. the scope mounts are precisely centered above the rail, and
6. the reticle is clocked perfectly to the scope body/turrets, and
7. the selected flat feature is perfectly perpendicular to all of the preceding items.

If this rather demanding combination of things were frequently true, we would seldom need more than a few clicks of windage adjustment to get a gun zeroed, starting with a scope which has been mechanically centered. That is rarely the case in my experience.
 
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At least check the reflection in the mirror to confirm the reticle simultaneously bisects the objective bell and muzzle.

Why am I beating this dead horse? Because installing a scope based on a flat feature of the gun's action requires that:

1. The barrel is perfectly straight, and
2. the bore is centered within the barrel, and
3. the scope rail is precisely parallel to the receiver's mortise, and
4. the barrel-to-receiver interface has no lateral bias (a typical O-ringed barrel tenon secured with a grub screw through the top of the receiver will have some bias), and
5. the scope mounts are precisely centered above the rail, and
6. the reticle is clocked perfectly to the scope body/turrets, and
7. the selected flat feature is perfectly perpendicular to all of the preceding items.

If this rather demanding combination of things were frequently true, we would seldom need more than a few clicks of windage adjustment to get a gun zeroed, starting with a scope which has been mechanically centered. That is rarely the case in my experience.
Click to expand...

I've eliminated every variable except for the barrel. That is where the indexing portion of the testing will come onto play. For what it's worth, I had the same scope on a Red Wolf HP and it was 4 MOA out in the same direction. 
 
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I've eliminated every variable except for the barrel.
Click to expand...

You have found a barrel orientation that has a mostly vertical bias. (Presumably...I don’t think you ever answered whether you had mechanically centered the scope first).

You have not eliminated scope cant from the equation. This potential deficiency likely does not affect your effort to index the barrel at such a short distance, but it very well may affect your results when you go to shoot at a distance. 
 
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I just purchased a bubble level that attaches via the picatinny rail. Got it at Trenier. Seems like a more logical way to align a scope.

This bubble level would allow the gun to be accurately leveled and then you can use a plumb line or some true straight edge to level the scope's reticle then you're done. The plumb line is the best as it will always orients a true vertical line. I like to level my reticle at around 20 yards or so on a calm day. works great

Before hand, optically center the reticle using the mirror technique or count clicks on the turrets. Then level the system out as stated above.

Use adjustable rings to keep the scope optically centered at your zero distance, then do your liner index. It's so important to start with a solid and true baseline.

Finally I think that it is best to Index at distance say, 50 yards. 9 yards is way too close. At 50 you can really fine tune with ever so slight adjustments of rotation to the liner.

Good luck
 
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