Barrel Indexing

C

cortezit

New member
I have been reading the thimble and barrel indexing that some of the shooters are doing. I am new to the sport and want to know how one goes about indexing a barrel if the gun is set up for the thimble and barrel arrangement. Is there a step by step "boiler plate go-by" for this procedure and if so where would I find it?

Tom D.
 
Tom:

My experience has been that Barrel indexing works when the barrel is not straight or when the external surface of the barrel is not uniform...With indexing you are actually artificially balancing imperfections and unbalances in the barrel. When a barrel is straight and uniform, you can rotate it as much as you want and it will shoot the same...

I see barrel indexing as starting a formula 1 race with a patched tire, it is OK but it is not the best way to race since you are much better off starting with a new good tire and with this I mean that I believe that the efforts should be put in getting a good straight uniform barrel to start with...To me indexing is just an attempt to improve a mediocre barrel.

Best regards,

AZ
 
A different opinion. No barrel has a perfectly straight bore. You can see this easily by rotating a barrel while using the same POA. The curvature of the bore also influences the vibration patterns of each barrel. This is not to say that all barrels are the same. Some exhibit a great deal of bore curvature. Some barrels simply shoot better. Indexing is what brings out the full potential of a barrel. How much difference is hard to say until you actually go through the process with your barrel.
 
Barrel indexing

sgeorge said:
A different opinion. No barrel has a perfectly straight bore. You can see this easily by rotating a barrel while using the same POA. The curvature of the bore also influences the vibration patterns of each barrel. This is not to say that all barrels are the same. Some exhibit a great deal of bore curvature. Some barrels simply shoot better. Indexing is what brings out the full potential of a barrel. How much difference is hard to say until you actually go through the process with your barrel.
Click to expand...

I have read on a few different "gun smith" sites and manufacturing sites that there is no truly straight barrel. What I want to know is What is the process that you must go through to achieve indexing in the vertical plane??

Tom
 
I want to start by saying that what's below is base upon my experiences, and in no way reflect any type of proven method.

To obtain exactly where the "vertical plane" is you must first determine where your barrel is at before starting. To do this:
Mark your starting position of the barrel against some sort of witness mark on the action. If Stainless you can simply apply a dot with a marker at top-dead-center. If blued you can apply a piece of white adhesive tape around the barrel where it intersects the action, then place a mark on that.
Next, I would shoot a shot to see where the POI is.
Now loosen the barrel and rotate ¼ turn, then shoot another shot
Then again rotating another ¼ turn.
And again,
And finally, again, which this time should bring you back to the POI of the first shot.
If nothing else this should show you in what quadrants the "vertical plane" lie. Or, better yet, if any adjustments are needed at all.
Now you can fine-tune your barrel by choosing how you want your POA to relate to your POI (I would suggest use the highest POI) then adjust it until that POI is at the top-dead-center so that over a range of distances you are ensured your hold-over/adjustments will only involve adjusting for the vertical without affecting the horizontal.
I hope this in some small way has helped to answer your question.
Good luck, good shooting, and most of all, have fun.
Dave
 
Dave Shattuck said:
I want to start by saying that what's below is base upon my experiences, and in no way reflect any type of proven method.

To obtain exactly where the "vertical plane" is you must first determine where your barrel is at before starting. To do this:
Mark your starting position of the barrel against some sort of witness mark on the action. If Stainless you can simply apply a dot with a marker at top-dead-center. If blued you can apply a piece of white adhesive tape around the barrel where it intersects the action, then place a mark on that.
Next, I would shoot a shot to see where the POI is.
Now loosen the barrel and rotate ¼ turn, then shoot another shot
Then again rotating another ¼ turn.
And again,
And finally, again, which this time should bring you back to the POI of the first shot.
If nothing else this should show you in what quadrants the "vertical plane" lie. Or, better yet, if any adjustments are needed at all.
Now you can fine-tune your barrel by choosing how you want your POA to relate to your POI (I would suggest use the highest POI) then adjust it until that POI is at the top-dead-center so that over a range of distances you are ensured your hold-over/adjustments will only involve adjusting for the vertical without affecting the horizontal.
I hope this in some small way has helped to answer your question.
Good luck, good shooting, and most of all, have fun.
Dave
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Dave, thank you for taking the time to post an answer. This make sense to me. I may go a little further using your technique and go 1/8 turns. I am a little anal about these things when I get my teeth into them.

Tom:;)
 
All I meant about using ¼ turns was to establish where the vertical plane lie, both high and low. Yhen, after that I'd go into that quadrant and do the fine-tuning until I found what I was looking for.

Good luck.

Dave
 
Why not straighten the barrels?

I toured the Springfield Armory Museum this summer and one thing prominent in their displays of machinery is a barrel straightening machine. There was a large poster there that said straightening the barrels was absolutely essential to their accuracy. I also remember seing a picture of a fellow in the Savage rifle factory using a straightening machine not many years ago; actually, the same machine I saw @ the Armory. Even though all the Savage barrels I ever looked at were Moonscapes, most of them shoot remarkably well right out of the box. So, it beggs the question, why don't barrel makers straighten their barrels? Mebby because we don't demand that they do?

The machine is a very simple press using a hand wheel. Apparently it is possible to simply look through them at some sort of pattern on a poster to verify their straightness.

Out in the Southwest this week trying to find food that isn't Souhwestern. Can't make myself like those flavors.

Pete
 
Are you sure it does anything?

I understand that no barrel is perfectly true. Even if there was one, unlikely it would be true to the action. But so what? If the bore is canted (up, down, left, right), but parallel with the scope, what is the problem?

One hears "harmonics" and "resonance". eg maybe the barrel vibrates differently, according to the orientation of the cant - resulting in POI variation, stringing, etc.

Has anyone tested this properly? ie, indoors, same pellet lot, same cleaning protocol, dozens (hundreds?) of shots - checking for better or worse POI variance as a function of rotational orientation.
 
Why would you think

KimZ said:
I understand that no barrel is perfectly true. Even if there was one, unlikely it would be true to the action. But so what? If the bore is canted (up, down, left, right), but parallel with the scope, what is the problem?

One hears "harmonics" and "resonance". eg maybe the barrel vibrates differently, according to the orientation of the cant - resulting in POI variation, stringing, etc.

Has anyone tested this properly? ie, indoors, same pellet lot, same cleaning protocol, dozens (hundreds?) of shots - checking for better or worse POI variance as a function of rotational orientation.
Click to expand...

A barrel wouldn't fit an action accurately? Any good barrel fitter will have machined a barrel to the bore when they fit it to the action. you might be able to say it doesn't fit accurately in a production situation but not likely when a skilled machinist works with it.

That said, it's what happens ahead of the bolt face that makes most of the difference in rifle accuracy, from my experiences. I have seen a rifle with no bedding shoot great scores.

Pete
 
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Nothing is "Absolute" in terms of straightness (barrel) but a barrel that falls within certain straightness parameters can and is to be considered straight..

Ed Shilen very kindly made a ratchet barrel in .177 caliber for me some time ago...He once showed me a machine for straightening barrels (discussed below) but let's make clear that Shilen does not straighten their production barrels...When we were talking about air gauging barrels and straightness, Ed mentioned that there is a "know how" and a process for making straight barrels and that it is not just a matter of using a deep hole drill, a reamer and then to pull a button or cut the grooves; I believe nobody will argue that Ed was one of the best barrel makers ever, and proof of this is that his .22 RF Ratchet Barrel is the Gold Standard in .22 RF Benchrest shooting...

Well, after receiving my .177 airgun barrel I indexed it on my lathe (between choke and spider) for cutting a tenon and used some range rods and .0001" dial gauges for measuring it...This barrel had about 1-1.5 thousands difference in between both ends at 22 inches in length which is very good and it shoots amazingly well with a pretty good sweet spot...It will shoot almost any pellet you feed it.
Top of the line barrels are all checked for straightness and air gauged usually to .0001"...

I have two FWB 10 meter guns, a model 700 and a model 800, I measured both barrels (16 or 17 inches long) and they both had +- 2 thousands...I can rotate the barrels (index) around the action and while the POI changes a little, the precision is always the same...The same happens with Ed's barrel mounted on a Steyr LG-110 rifle, no matter how it is indexed it has the same precision. If I ever were going to align any of these premium barrels to the vertical, it would be as it has been explained by Dave and other members at the high point (12 o'clock) and only for extending the scopes's range as it is done for 1000 yard rifles; doing it for improving accuracy (precision) is in my opinion, a waste of time...

As said in my previous response, if a barrel needs to be rotated to "Bring Out The Best Of It", then we better find another barrel that doesn't need to be rotated for improving its precision; I have never seen a barrel that because of indexing shoots better than a barrel that is straight and shoots the same accuracy at any given point.

Anyhow, going back to Ed's barrel straightening machine....It was very simple tool similar to an aluminum arrow straightener with a dial...I believe that FX uses something similar to this setup and while I don't know it for a fact, I've have heard that BSA and many British airgun tuners use similar devices to straighten their barrels.

Regards,

AZ
 
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Do air rifle barrels

tend to be concentric, bore to outer surface? That is rarely the case with barrels for other rifles, that I have seen. I was thinking it may make straightening a barrel's bore a bit of a challenge if indicating the outside. For instance, I recently fitted a barrel to one of my RF rifles. While it had a visually straight bore, it was about .005" out of round on the outside surface. I turned it to be concentric for about 2" ahead of the tenon after I had the bore zeroed so that I could chuck it in a collet if I want to re-visit the chamber, for any reason, in the future.

I have heard the indexing arguement on here over the years but never tried it. Some of my friends swear by it though while others deem it a waste of time. I may try it myself sometime.

Pete
 
Pete:

When I check the barrels on my lathe, I use the range rods/dials and first check the bore and then as many intervals as possible of the outside surface also with dials...My experience over 40 years of making guns is that if the bore is pretty straight, the outside surface usually is true..If at all, a light pass on the lathe just to "clean" the surface does it.

I read about somebody putting the barrel on centers and machining the surface, but if the bore was not straight to start with I don't see how this operation could've straighten anything...

The "Straightening Tool" I described on my previous post is usually used to straighten and get both internal ends of the barrel as close as possible...I assume that if the bore is straightened and the surface is still off, then turning it on centers could help...I have never done it, to me if a barrel is not good to start with (no more than 1.5 -2 thousands off) then it does not go into any of my guns.

The topic of "Barrel Indexing" comes to discussion with every new generation of air gun shooters about every 3-4 years and the same happens with "Resizing Pellets"...Over the years that I have shot airguns, I have seen several cycles of these topics brought up by shooters and new manufacturers selling the ideas as the holly grail...Indexing helps if your barrel is mediocre as you will be shifting unbalances of said barrel to a point where things are more consistent, but if the barrel is straight and has a good bore with no loose or tight spots, then indexing is a waste of time...We have done much testing in both: Rimfire and Air shooting from a vise in a controlled indoor environment using SIUS Olympic targets to measure any improvement up to the .0005 - .001".

Is not part of the thread but I mentioned it...if you ask me about resizing, I either don't believe in it...The way I see it is that pellets are ALWAYS perfectly resized when we load them into the breech, if the head and skirt were larger the internal diameter of the barrel would resize head/skirt perfectly...If the head is smaller than the bore, then there is nothing you can do...
The one and only time when resizing is going to help is when you are shooting a very low powered rifle (5-7 joules) and the pellets are over sized and hard (like shooting 13.4 gr. Mosters out of a Feinwerkbau 300 S)...I shot them out of the tin and the velocity spread was up to 50-60 fps...I then used an old Beeman re-sizer that I acquired back in 1982 and the spread came down to about 15-18 fps if I don't remember wrong.

Best regards,

AZ
 
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I would like to thank all of the members who responded to my question. Having spent 30 years in the Navy I learned a long time ago that the best answers to a question come from men and women with experience. In other words "ask an old salt".

Thanks to the "old salts" on the board

Tom:)
 
Perfectly straight barrels are a rare thing

Perfectly straight barrels are a rare thing… I haven’t seen one yet…

Airgun barrels typically aren’t made from the same (chrome-moly 4130/4140) steel that firearm barrels are typically made from. Some inexpensive barrels are made from tubing (Drawn Over Mandrel Tubing) and some from brass. Most quality airgun barrels are made from 1117 free machining steel; the same steel used in rimfire barrels. For quality barrels the steel used in the blanks must be supplied in the correct alloy and tested for consistency.

Most of the quality airgun barrels we use today are made using the “button” rifling process where a steel barrel blank is placed in a barrel drilling machine, a special “gun drill” bores an undersized hole in a steel barrel blank which is then reamed to smooth the bore and bring it to a desired size. This process creates the bore diameter of the barrel.

A tungsten carbide button is drawn (pulled) through the reamed hole in the barrel blank to create the rifling. The tungsten carbide rifling button is ground to the size and shape of the grooves and twist rate desired in the barrel. The button is a precision tool that is compensated in size for the steel barrel alloy it’s being used in and the lubrication used in the process. The rifling button creates the grooves in the reamed hole by displacing the metal; none is removed from the bore during the process. The rifling button may include an “ironing button” or another football shaped “smoothing” button may be drawn through the bore to make/draw the bore diameter to a finished size.

fir_m04_t06_06_b.jpg

Rifling button with ironing button

During the button rifling barrel making process stresses are created and released. Many of the makers use heat treating cycles to normalize (relieve stresses) in the blanks. Anything we do to the barrel to induce or relieve stresses in the barrel such as machining will change the curvature of the barrel. Yes, you can bend the barrel to compensate for the curvature and many gun makers do exactly that; however, most barrels have some degree of curvature present.

What does indexing a barrel in the rifle’s receiver accomplish?
Indexing allows us to compensate for irregularities in the direction the barrel is shooting.

Every barrel will vibrate during the shooting process and the concept of indexing is to orient the existing curvature or a bore offset from center in a vertical plane to produce a “vertical only signature”. When the barrel is out of tune it will print groups with some degree of horizontal dispersion (horizontal stringing). It doesn’t matter if you use the 12 o’clock or 6 o’clock position… sometimes you may prefer one over the other, such as making as many clicks in your scope available as possible.

By rotating (indexing) the barrel in the receiver with the bore centered in the receiver the gun will shoot (print) groups in a circular pattern like this:
TM1000XFactoryBarrelIndex_zps76f36dd2.jpg

This 3 inch diameter pattern was shot from my TM1000 with the target at 25 yards.

Barrels or blanks used in guns with a side ported barrel will require a thimble with the transfer port fitted to the barrel to permit indexing and barrels will require preparation for thimble installation:

TM1000Thimble1_zpsd5eb68e7.jpg


This bore was offset 0.008” in the barrel blank at the receiver end requiring the barrel to be turned down to run true to the bore and a sleeve installed on the shank to fit the receiver prior to installing the thimble.

I use a dial indicator and a set of plus and minus ZZ gage pins in half thou steps to center the barrel in a run-true collet or six jaw chuck in the lathe to setup the barrel with a total indicated runout (TIR) of 0.0005” (half a thousandth) or less at the chuck and a thousandth or less TIR at the outboard side of the headstock (muzzle). If the blank or barrel can't be machined to fit the receiver with the bore centered I cut down and sleeve the blank or barrel to run with the bore centered in the action/thimble

TM1000Thimble5_zps521b2254.jpg


I use o-rings on my thimbles and improved breach seals for improved consistency and lower extreme spreads.

TM1000BreachSeal5_zps1047bd6e.jpg


TM1000BreachSeal2_zpse70514b4.jpg


The barrel or blank must have a good crown and the bore must run true with the thimble and receiver.

TM1000BarrelWork3_zps25e8eb72.jpg


The crown is cut from the inside-out with razor sharp hand honed carbide tooling. I prefer a recessed crown of the stepped or 11 degree styles.

Have Fun

Boomer
 
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I am going to try to elaborate about my perspective on why an indexed barrel will never shoot as good as a straight barrel and why I believe the efforts should be placed on getting a good straight barrel vs. indexing...

The lab has tested many guns & barrels and many lots of identical barrels with similar results...these are the results of 2 identical Feinwerkbau 10 Meter Barrels used on their 700 & 800 models for Olympic competitions...Both barrels are good shooters but the "Straight" barrel number 1 has .02" less error than the second barrel and it does not improve nor change its precision when indexing...

Please look at the group size and not the scores as the gun was nor re-sighted when indexing...Also note that the Straight barrel varies a little at the 270 degree position but this was most likely a pellet issue as re-testing at this position was done several times showing .17 groups...Nevertheless, while I have the records showing this fact I didn't find the target.

The group sizes were (inches):
Barrel 1: .17, .17, .17 & .18
Barrel 2: .21, .20, .20 & .19

Both barrels have the potential of shooting perfect scores and while .02 will be insignificant for some people, keep in mind that in the hands of an Olympic shooter it can be the difference between GOLD and seeing the podium from the bench...

The second barrel improves a little after indexing, but it never shoots as good as the STRAIGHT barrel does at any position...Both were rotated (indexed) and tested at 30 degree intervals and I am only presenting the results at 90 degree intervals to show that the improvements gained by indexing a barrel will never match the precision of a good straight barrel and that straight barrels are pretty much "immune" to indexing.

The testing was done inside a lab with controlled environment variables, the gun was on a vise/solenoid trigger. The target system is the most precise electronic available, a SIUS identical to what is used at the Olympics and at ELEY/LAPUA testing centers for their rimfire ammo...The target uses laser beams and acoustic microphones for registering the shots with an accuracy of .0005 - .001". The pellets used were RWS R-10 Select Match.

The barrels are designed for 10 Meter and were tested at 10 and 15 M (50% additional distance) and I am presenting the results of the 15 M testing to magnify the error/improvements.

Best regards,

AZ

1.Straight Barrel
2.Straight Barrel indexed 90 Degrees
3.Straight Barrel indexed 180 Degrees
4.Straight Barrel indexed 270 Degrees
5.Barrel 2
6.Barrel 2 indexed 90 degrees

Note:
7.Barrel 2 indexed 180 Degrees shown in next post (site limits to 6 attachments)
8. Barrel 2 indexed 270 degrees shown in next post (site limits to six attachments)
 
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AZ
i'm sure no one will argue your point that a straight barrel will always shoot better then one not so straight
BUT indexing did help the poor non-straight a little and even a little in benchrest helps some, correct ?
so our problem really is how does one go about getting the straight barrel ? :)

Dick

AZUARO said:
I am going to try to elaborate about my perspective on why an indexed barrel will never shoot as good as a straight barrel and why I believe the efforts should be placed on getting a good straight barrel vs. indexing...

Both barrels have the potential of shooting perfect scores
The second barrel improves a little after indexing, but it never shoots as good as the STRAIGHT barrel does at any position...

Best regards,

AZ
Click to expand...
 
Does Gravity explain the difference?

What factor could be speculated about the difference in group size?

Pete
 
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