This is a guest blog from reader Vana2, whose real name is Hank. Today he tells us how to index a barrel to get the pellets centered on the vertical line.

If you’d like to write a guest post for this blog, please email me at blogger@pyramydair.com.

Take it away, Hank.

In Pursuit of Sub-MOA – Barrel Indexing

By Hank Vana2

This report covers:

  • Barrel Indexing, what about it?
  • Some background for reference…
  • Do you need to be concerned about indexing?
  • Symptoms of a poorly indexed barrel
  • What to do?
  • Setting up for indexing
  • The first thing is to find where we are
  • Re-indexing the bore
  • Checking the indexing
  • Summary

FX Impact muzzle
The business-end of an FX Impact with the shroud and retaining nut removed to show the FX Smooth-Twist X barrel liner.

Barrel indexing, what about it?

As much as I enjoy casual off-hand plinking there is a lot of satisfaction in being able to place the shot exactly where I want it at longer ranges. Attention to details and optimum tuning play a large part of the accuracy we all want.

I mentioned barrel indexing as something that I would check in my guest blog “My FX Crown Story” and I’ve since received a couple of emails asking if indexing was necessary and to explain how to go about that. In this blog I’ll try to answer those questions. I recently re-indexed the liner on my Impact and took some pictures to help explain how I went about it.

Theoretically, if barrels were manufactured with their bore perfectly straight and exactly concentric to the barrel there would be no need to index the barrel if it was mounted true to the receiver and sights.

In the real world, there is no such thing as a “perfect” barrel. The chances of forming a very long, small diameter hole concentrically in a steel blank and having it remain straight through subsequent rifling, machining and the (high temperature) bluing process is challenging to say the least. To compensate for manufacturing variances the barrel needs to be indexed. It’s kinda like aligning the wheels on a car.

I’m sure that all reputable manufacturers index the barrels of their airguns so alignment should not be a concern UNLESS someone removes the barrel and doesn’t reinstall it exactly as it was. I’ve been doing a lot of testing with my FX Impact that required swapping back and forth between the “pellet liner” and the “slug liner”. With all this messing about I’ve long since lost the factory indexing and I could see that the rifle was throwing a left hook instead of a straight jab. It was obvious that the barrel was biased incorrectly and needed to be indexed.

Barrel indexing is the procedure where the bias of the bore (the direction it throws the projectile) is aligned with gravity so that the projectile flies in a plane vertical to the point of aim. I think of barrel the barrel bias as the “cant error” in the rifle.

By making a reference mark on the barrel then shooting groups as the barrel is rotated in 90 degree increments we can determine which way the barrel is biased and then adjust the bias so that it is in a vertical plane.

Some background for reference…

indexing barrel zeros
I’ve included this image because I made several references to the near and far zeros.

The take-away here is that the near zero and the far zero have the same point of aim (POA) and should (ideally) have the same point of impact (POI).

Everyone who has read about shooting theory has probably seen some variation of the picture (above) showing how the upwards pointing line of the bore causes the projectile to cross through the horizontal line of sight above it (at the near zero) then, because of gravity, arc over and cross the line of sight(at the far zero) before falling to the ground somewhere downrange. 

This simplified and exaggerated two-dimensional picture is theoretically true. But, because of all of the other things that affect projectiles, their path is more like a three dimensional curved cone of water sprayed from a garden hose. A poorly indexed barrel is one of those things that will affect the point of impact.

Do you need to be concerned about indexing?

Depends… 

If you hunt and plink at less than 35 yards you will probably not notice if the barrel is (slightly) out of alignment. If you target shoot at a fixed distance, sighting in for that specific distance aligns the sights with the path of the projectile and the fact that the projectile may not traveling in a simple vertical plane a skewed trajectory will not be noticed.

But, if you shoot at variable ranges and especially out to longer distances, the effects of a poorly indexed barrel will show up as a windage change of POI that is not related to the trajectory (gravity). This can be very confusing and frustrating. Depending on where the rifle is zeroed the POA can drift quite a bit as the distance to target changes.

Symptoms of a poorly indexed barrel

I set up most of my rifles for the optimum point blank range and do my initial sighting in at the near zero then confirming by checking the POA/POI at the far zero. Typically for my PCPs, the near zero is around 17 to 19 yards and the far zero is between 38 and 42 yards. Ideally (there is that nasty word again), the POA/POI are identical at both zero points. If I see groups that drift away from vertical in a consistent direction I check my barrel indexing.

A perfect, arcing trajectory is beautiful… drifting pellets are not! Try to setup a shooting session to be able to see the pellets. On my shooting range, with the dark log backstop and the sun behind me I can usually see pellets in flight. An alternative is to paint the inside of the skirt so it is more visible.

Poor indexing will throw the projectile off at some angle (up/down/left/right) relative to the POA with gravity further complicating matters. If you see the POI shift in a linear fashion (say, from left to right on the 9 o’clock/3 o’clock line) as the distance changes then indexing should be tested at various distances.

Sighting in at close range and checking every 10 yards out to 50 yards will give you a visual of the projectile’s flight path showing if it is drifting. A properly indexed barrel will have the POI strung a vertical plane (by distance and gravity) along the 12 o’clock to 6 o’clock line. 

Shop PCP Rifles

What to do?

As with all adjustments, if your airgun is shooting well, don’t mess with it. If you decide to make changes be sure you can restore things to where they were.

If you discover an alignment problem and can’t index the barrel then you can reduce the effects of misalignment by choosing a zero that is at half to three quarters of the range you typically shoot at. 

If your rifle allows you to rotate the barrel (or with FX products, the liner) then you can index the barrel to shoot along the 12 o’clock to 6 o’clock” line to eliminate the drift and all will be good! 

Setting up for indexing

indexing stuff
Some stuff that is needed.

For indexing, you will need the tools needed to free the barrel and a permanent marker for making reference marks. I use Isopropanol alcohol to clean the oil off where I’m marking the barrel and erase the marker ink (don’t forget to oil the metal afterwards). A bit of silicon grease applied between the barrel liner and the retaining nut reduces the tendency of the nut to turn the liner as it is being tightened. The vise-grips (with heat shrink to protect the metal) offer a positive grip for turning the barrel in its mount.

A notebook for sketches to keep track of where the holes on the target came from and a compass to analyze the pattern is helpful. And a level… because I have bubble levels on my PCP scopes to minimize cant and always level my targets.

Before starting, some prerequisites… the rifle should be well tuned, all hardware checked and tight and the barrel cleaned.

There are a couple of stages to indexing a barrel, to keep things simple and clear I prefer to use a separate (fresh) target for each stage.

A comment about safety… because I will be working at the muzzle end of the airgun I index my airguns alone. It’s safer, with no distractions I can focus on what I’m doing while I repeat the routine (disassemble, adjust, re-assemble, load, shoot and check) multiple times.

The first thing is to find where we are

index mark
The reference mark.

A reference mark is needed to keep track of the barrel rotation. Here I’ve marked the barrel sleeve and liner at the 12:00 o’clock position with a permanent marker.

Groups are shot at the 12, 3, 6 and 9 o’clock positions – as viewed looking through the scope at the target. Note to self: remember that the 3 is on the left and the 9 is on the right when facing the opposite direction towards the barrel (don’t ask why I need the reminder LOL!). 

After each rotation, the barrel and all accessories (shroud, moderator etc.) should be properly installed and tightened before shooting or the testing may not be valid.

first target
The first target.

In looking at the first target, the shots at 12 are at the sight-in point (pardon the drift – it was quite windy) which is on some arbitrary point relative to the barrel bias.

As the barrel is rotated to the 3, 6, and 9 positions the groups will rotate around the neutral point (the white cross in the picture) of the barrel bias so the pattern of holes can be in any quadrant on the target. 

To find the approximate neutral point, draw a line from the 12 o’clock group to the 6 o’clock group and one from the 9 o’clock group to the 3 o’clock group. Then, using this intersection as a reference, center draw a circle through the groups as best you can. It doesn’t matter if the lines are skewed a bit.

This is where the tires touch the road. The group that is farthest from the neutral point (outside the circle) indicates the high point of the bias. We want to index the barrel such that the high point is at the 12:00 o’clock position so that the trajectory of the projectile is in the same plane as gravity.

On this target the high point is at the 3:00 o’clock index. I also shot test groups with the barrel at the 2:30 and 3:30 positions for confirmation.

Re-indexing the bore


change bore indexing
The 3 o’clock re-indexed to the 12 o’clock position.

To re-index the barrel I like to use a fresh target and (roughly) reset the neutral point of the barrel close to the center of the target by adjusting the scope.

I then reshoot the high point of the bias (position 3 in my test) as reference point on the fresh target and proceed to rotate the barrel and shoot to walk the point of impact up until it is above the neutral point of the barrel. For clarity, I’ve just shown the first shot and the last group.

Checking the indexing

The final test is to check that there is no drift. 

after
A nice vertical line!

The picture above is of a series of shots from 10 to 50 yards done in 10 yard increments. The vertical alignment of the impacts shows that the trajectories of the projectiles are aligned with gravity.

We’re done! Now it’s just a matter of sighting in at the preferred range.

Summary

Wow, for something so simple to do it sure took a lot of words to explain! In summary, if you notice a distance-related shift in the POI it is quick and easy to run a check and see if it is indexing related. Determining the bias of the barrel and correcting it takes a bit of patience and an hour or so of time. It’s best to take your time for this kind of tuning.

Being a compulsive tinkerer I don’t need much excuse to test and tweak my airguns to get the best out of them. If nothing else it confirms that all is good and increases my confidence.

Hope this helps!

Hank