Indexing the barrel

Some good reading from a prior topic

Bottom line, sometimes it helps and sometimes it doesn’t. Just have to try it to see. Generally speaking, here are some factors that tend to play a role:

• thin, flimsy barrel
• long barrel
• poor anchoring of barrel in the receiver
• high power / vibration from harsh shot cycle

Every winter I am doing surgeries on my guns and start the season with re-tuning and indexing the barrels/liners. I just did couple weeks ago - and I didn't make up my mind yet .
What I learned the indexing is not shrinking the groups but rotating the pattern (of a group) around its some sort of center - how to call that ? it is not a center of POA and not center of POI but call it center of indexing.
Earlier years I set the pattern center closer to my scope optical centerline, but lately I am lurking with the idea to leave it at 12 a clock. I am not sure if better or not, but looking for a pattern that is more straight line instead of round and to make it vertical vs horizontal...
So you got the picture no worries you gotta try it diy.

Edit: sorry, I forgot the most important part of my story, here it comes making it public.
I am buying a mini lathe for at least five years and didn't got there yet. But I have couple friends working in a machine shop. Two weeks ago I picked up 4-5 of my liners (x600 and x700) and I put each in a lathe to check the runouts...
My favourite liner is the STX-A x700 and that one was making about 10" circle @ 50 meters when I was indexing it two weeks ago, but beside that issue it was shooting about 1.5MOA when I started re-tuning for 50 and 100 meters rings. That liner had the most runout of 0.014" I tried to straighten it and the best I could do was 0.004". The other liners (not my favourite calibre or twist rate) were withing 4-5 tau range initially and after straightening about 2-3 tau.
And then started snowing again, and some other commitments and so far I could not get out to my club yet since to test if the straightening can improve anything.

JohnW said:

I shoot 25 yd benchrest with an Anschutz 9015 so it is quite easy to index the barrel on this rifle.

Has anyone tested a pcp rifle/barrel to determine whether indexing the barrel does tighten a group. My barrel is bare ( no tuner or other attachments).
If so, was any improvement quantified. Even tightening the group by 1mm can make a big difference to the score.

John

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John,

I think indexing does have a place in achieving better accuracy, and may also help in achieving proper scope/barrel alignment.

After getting the barrel straight (o.d.) , and using a well centered scope, I shoot my first group with a mark near the muzzle at 12:00, then rotated 90 degrees and shoot another, and finally a third after another 90 degree rotation. Then I try to determine where the lowest group center would be and rotate to that point. My experience is that small gains, perhaps up to 1/16” at 50 yards average groups sizes can be had by doing this. That said, if I “need” a higher poi, I sometimes use the ”high” position angle. I believe the anglular shifts from a straight barrel that is rotated at a result of a non-centered bore at some point in the borer that causes it to vibrate on a different axis than verticle if aligned accordingly. I know old-world builers used to straighten by looking up the bore, but I feel its not enough unless the barrel is then turned between centers once the bore is straight.
,

Has anyone tested a pcp rifle/barrel to determine whether indexing the barrel does tighten a group.

John

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Indexing does not "tighten" groups. One indexes a barrel in an attempt to eliminate as much as possible, any windage deflection from the desired POI.

Let's define "indexing". Rotation of the barrel to find the position where the projectile consistenly leaves the barrel and strikes at either the highest, or the lowest, points closest to the POA.

In the example below where the red cross is the POA, I would select position #2 even though #3 produced the tightest group. The reason is that of the two most centric groups, #2 is the tighter and most centric, which should eliminate the greatest amount of windage drift caused by the barrel.

Headshots said:

The FX Maverick barrel I have has 2 notches one on the pellet side and one slug side. You have index it for a proper fit .

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In this statement on an FX he is actually talking about rotating the liner inside the shroud
"Let's define "indexing". Rotation of the barrel to find the position where the projectile consistently leaves the barrel and strikes at either the highest, or the lowest, points closest to the POA."

Headshots said:

You have 2 choices on how to index your liner.

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The liner can be rotated independently of the brass part.

Headshots said:

If you look at the end of liner you will see the notch that must line up . You have 2 choices on how to index your liner. Pellets side or slug side no in between.

View attachment 359105

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Ya right- I thought you were saying something different - if you index to a vertical group you should have a vertical POI group on pellet or slug - one POI group will be higher or lower than the other - at least mine worked out that way.

Headshots said:

So your supposed to leave your liner loose ?

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No, you rotate it to the preferred position and secure it there.

In the case of the conventional liner + barrel, it's locked in position by snugging up the muzzle nut (“liner lock” in FX’s terminology).

In the case of a Superlight, it's locked in position by snugging up the brass nut at the receiver end that puts a squeeze on the O-ring stack.

Headshots said:

So your supposed to leave your liner loose ? That makes 0 sense to me .

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No, what he is saying is that the "barrel assembly" consists of the liner (the part the pellet travels down), the barrel (the part that the liner slides into), the thimble (the brass part), the adapter (the brass thimble threads into this and it in turn threads into the barrel), the liner lock (which threads into the front of the barrel to hold/tension the liner).

This allows the liner (shiny metal on top) to be clocked independent of the barrel.




I "clock" my liners when I get them just so I put them back into the assembly at the same location so my POI does not change after cleaning the liner.

Thanks for the explanation. This helped. Can I see some groups that show before and after .?

Firewalker said:

No, what he is saying is that the "barrel assembly" consists of the liner (the part the pellet travels down), the barrel (the part that the liner slides into), the thimble (the brass part), the adapter (the brass thimble threads into this and it in turn threads into the barrel), the liner lock (which threads into the front of the barrel to hold/tension the liner).

This allows the liner (shiny metal on top) to be clocked independent of the barrel.

View attachment 359166


I "clock" my liners when I get them just so I put them back into the assembly at the same location so my POI does not change after cleaning the liner.

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I guess I'm used to calling what you call barrel a sleeve . That's the black part that the liner goes in , not the shroud, the sleeve.

Headshots said:

I guess I'm used to calling what you call barrel a sleeve . That's the black part that the liner goes in , not the shroud, the sleeve.

View attachment 359256

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Yupper, according to FX, the "JAM" is the 19970 Liner Lock .25 and the "SST ADAPTER" is the 19962 Barrel Attachment .25

nervoustrig said:

The liner can be rotated independently of the brass part.

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Sorry for my confusion. I hadn't pulled my barrel off in a few months and forgot how many components make up the barrel alone.

I'm new to performance airgun shooting, Joe thanks for the clarification.