Bored and reading shooting articles- barrel indexing

[Brad Y]

Is it still possible though that Seddo's barrel is actually "bent" or "curved" but still the highest point of the bore at the muzzle is at the top or bottom? There may still be a tendancy for the projectile to travel to one side? To me a barrel could still be curved but if it is as above it may still be quite accurate? Ive seen a krieger 308 here that had a fairly pronnounced curve to it and it shot out to one side. It was still pretty accurate but the problem the shooter faced was that his windage was out a fair way and it had to be temporarily rectified by the burris zee rings with the inserts in them. Its since been rechambered and set with the curve pointing up and its much better when driven correctly. If I had a barrel 12 minutes out to one side I wouldnt be terribly impressed either.

None of this was ever meant to be gospel, just an interesting point I found while bored. There have been some very well respected people around the world discussing the merits of this. In my head it makes good sense, but the proof would be in chambering 100 barrels at many different index positions and believing what the data told you. Anyone got a few spare barrels and some lathe time they want to donate?

[Seddo]

12 minutes left is so far off centreline that the scope wind setting will be well off centre to such an extent that the vertical range of the scope will be significantly reduced. It almost demands a different scope mount for that barrel.

Peter Smith.

Not with a Sightron, i have 70moa of wind so its not an issue, it will be even less of an issue when i stick a 6-24 back on it. . the issue is remembering how far you have wound it and never forget to wind it back to zero after you have finished with that barrel. With 15moa turrets you could easily think you had 3moa of wind dialed on and go to the wrong zero.

All i can put it down to is the curvature of the bore. i will

[williada]

Brad it is very possible that the highest point of the bore at the muzzle is at the top or the bottom, however the shoulder would not be parallel with the action face given its out 12 minutes on the scope. If the barrel does the same thing each time and the joint holds up there is no reason why it would not be accurate in the short term before the joint gives up. This problem occurs when working through the headstock using a three jaw chuck as mentioned before and exacerbated if the bore is not centred in the blank in reference to its outside diameter. As Peter indicated, cutting the shoulder between centres is a way to fix a poor shoulder if you don’t have a 4 jawed chuck, cats head or spider and dial gauges and assuming no other stuffups occured. Its easy to see a crook shoulder engagement by bluing the surfaces.

However, my concern even though a barrel shoots well but the scope is not aligned with the bore would mean variable zeros at different ranges. I also prefer to shoot through the optical centre of a scope so as to avoid visual distortions. I don’t think any of this would assist a coach in a team shoot. I wonder how many people have blamed their scope for a variable zero...There are enough issues such as Magnus force at different distances to contend with to smack that super centre count. And we know shooters are pedants and will chase the last kernel of powder these days in a quest for a score.

So what is necessary to prove a theory? The minimum sample to be statistically significant is thirty. I can honestly say that I have specifically tested the index theory far beyond statistical significance and not relied on hearsay. This was done in the 1980’s testing for a barrel manufacturer and again in the 2000’s with the NRAA project Penumbra using factory ammunition. Both projects involved different bore land sizes and various twists in many barrels. Across the board, the theory was proved to my satisfaction. With the project Penumbra, I was instructed to focus on 1000 yards. So it is at the long ranges that every nuance has to be grafted. If you had a choice with a barrel bent in the lateral or vertical for a world championship, which one would you want...Its really a rhetorical question.

I too have supervised or advised and verified the smithing work of top badge or place getters at Bisley who were in the Australian team and the strike rate was 100%.

Its easier to compensation tune a barrel in the vertical plane, but I suppose for some compensation is just a theory too. If there were no theories we would still be living in the dark ages. It depends on whether you can be bothered to get that edge.

I would be happy to shoot Seddo`s gear at Moe to see what makes it tick and evaluate its super accuracy. Seddo certainly highlighted the scope problem and can be reasonably satisfied that his Sightron can cope with the extreme correction that is needed.

[Brad Y]

David, your experience and advice is most welcome here, thanks for your input. One fine day I look forward to investing in a lathe and doing alot more experimenting myself.

[Brad Y]

David- one question

"Or you could save time by taking the 12 o’clock measurement with the muzzle poking through the headstock when the barrel has been dialed in before threading and chambering. However this method can pose problems as the spider is released on the muzzle as to what is the true 12 o’clock position. You have to know what you are doing here. The former method given that not all actions are blueprinted is a surer bet and for all practical purposes will be good enough."

If a barrel was dialled in at the chamber end so that the chamber and say an inch forward of it was true, then would it be possible to determine the high point of the bore at the muzzle without the spider on the muzzle end touching the barrel? The barrel is only supported by a small amount at the action when installed so surely the lack of contact by the spider wouldnt affect it? Then once the 12 oclock point is marked, the muzzle end can be dialled in and threading take place. Once the thread is cut and the barrel indeed nip up at 12 oclock then the chamber reaming take place?

I really am thinking hard about buying Gordy's chambering DVD now.

[Matt P]

Hi Brad
I have the video if you want to borrow it, I use a variation of the "Gritters" method that works for me and have had some very good results, I like to "clock" the barrel so the muzzle is at the 12 o'clock (I'm sure the 6 o'clock would work just as well BUT you lose some elevation !!), now the bore may wander in a few directions between the chamber and the muzzle but if clocked to the 12 or 6 position the wind zero is normally off by only a small amount either way. Most of my customers with multiple barrels find their elevations may vary but their wind zeros are normally fairly close between barrels.
Regards
Matt P

[williada]

Brad you have to reverse engineer the job to understand the concepts, and you have to be able to repeat the setup so your threading, chambering and shouldering are concentric with the bore. Yes you release the spider for vertical measurements at the muzzle. The muzzle is not touched by the spider. While the measurement taken an inch forward on the barrel can be true it has to be referenced to a point aft to ensure that it is still concentrically aligned over the rear section and only the barrel is allowed to show its true colours.

Allow me to go over the steps as I might not have been clear enough before. Of foremost priority is that the throat is cut in line with the bore because if a bullet does not start concentrically more problems with accuracy are encountered. You pre-bore the chamber undersize with a boring bar without taper and give the entrance an angled lead after you have dialled the barrel in with the bore. This will then allow the chamber reamer to set up properly. The reamer will always follow that hole and in fact a pilot is not always that critical. The pilot should not bind on the rifling anyway. If you use a floating reamer holder all will be fine too. Next you face off the shank and reduce its diameter prior to threading and pull up about .1" from the shoulder.

This process forms the base for repeatable setups when dialling in again as you can take measurements from the pre-bore or the shank which should now be in alignment before you change the setup. You have to allow a full turn at the shoulder and this depends on your thread pitch. Think forward, suppose the barrel clocks up just past the 12 o’clock position, you would have to allow nearly a full turn to reach 12 o’clock. If you don’t care whether you are at 6 or 12 o’clock, then you only need to allow half a turn. If you are a miser with close shoulder thread contact you can chase the threads up. That is another skill.

The tricky bit. Because the barrel will sit in the action in its natural state, in order to take measurements of the vertical position at the muzzle you have to release the spider. The measurements should be made on a pin gauge or another range rod extending from the bore at the muzzle not the outside profile. Why, because barrels require an inch to be cut off. Even the bore can be belled here due to lapping and the outside profile can be vary due to tooling cuts backing out and linishing. Cut the crown last after lopping off an inch unless you can measure the smallest diameter of the bore and cut there.

Your aim of course is to have the barrel held firmly in the chuck so there is little movement at the chuck end when you slowly release the spider. You need long, soft copper shims that are placed on each of the jaws in the four jaw chuck to give you sufficient adjustment as you dial in the barrel otherwise short shims, copper wire or aluminium or neuroprene washers will let the barrel pivot too much at the chuck end giving added problems in muzzle measurement. This of course is fine if you are only concerned with chambering and not the additional muzzle position. The right pressure on the spider and the chuck jaws becomes an art for the initial setup. If on release of the muzzle spider the muzzle measurement swings wildly on the dial gauge you surely have a bent barrel. I can say I have yet to see a straight one and .005” variation would be great. Long barrels droop naturally and grow longitudinally anyway as they heat up on firing. I can think of nothing worse than a droop down barrel with a muzzle pointing left or right, not up or down for precision results. The poor setups are likely to take a few shots to settle as the barrel warms up or there are long pauses between shots when the barrel cools.

You will have to redial the barrel in before you chamber, thread and shoulder after taking muzzle measurements for the vertical plane. You have to dial in the breech and adjust the setup with your spider again. The spider even with a few thou of adjustment exerts considerable tension over the length of the barrel to the chuck It’s a good idea to repeatedly check breech measurements between stages.

Now you know why making a false receiver with loose threads and facing it off between jobs is better in the long run and there is reduced chance of stuffing up the chambering and threading. Use about 120 ft/pounds in torque when drawing up the barrel. Practise on old barrels to perfect your techniques. Of course there are many ways to do the same thing. Hope my explaination is clearer than before. Good luck. David.

[Brad Y]

Thanks David

Its times like these you wish you had a spare 12 grand around to get a lathe and tooling and a heap of barrels to play with!

[Brad Y]

As a follow up- I have watched the Gordy Gritters DVD a couple of times, thanks to Matt Paroz for sending it over I will send it back next week mate!

Mr Gritters indexes his barrels from the outside of the barrel. I find that a little confusing as at the end of the dvd he shows a picture of a bore off center in a barrel and says that all work on a barrel must be done from a true bore! Anyway enough of that, just thought it was rather interesting!

[williada]

Brad, check these links.

https://www.youtube.com/watch?v=7duvo7XLWIs
https://www.youtube.com/watch?v=aII2tbavKnM

The first one shows how far the bore centreline can be out at the muzzle. The measurements for this operation were taken from the bore not the outside as the bore rotates on centre in the chuck and the OD looks like it is off centre. This is a separate but similar problem to barrel curvature. We still need to know where this bore centreline will point when your barrel is screwed in. So it is imperative to take bore measurements not outside diameter measurements.

The barrel is screwed in based on the breech operation shown in the second video because you have to start a bullet straight.

The second one is one by Gordy Gritters taking measurements from the bore before he cuts the chamber and it does not show where the muzzle will be pointing when it is released from the spider as it would sit in your action.

However you must take measurements again on the muzzle from the bore when you release the spider to find the highest part of the bore as the barrel would sit in your action. This is necessary if your barrel curves and OD is still concentric with the bore as in most cases or in the case of the bore not being concentric with the OD as depicted in the first video. As I said its tricky to keep a barrel from moving away from the established breech alignment when you release the spider to take muzzle measurements.

If you have never operated a lathe its easy for others to skip steps they take for granted.

Firstly, the bore needs to point up for compensation considerations whether the bore is offset or the barrel is curved (two different scenarios). I have had barrels where the offset is greatest at the breech and because they were chambered at the breech it gives the appearance the rest of the barrel is really bad. In the case of an offset bore from outside diameter, the belly created by the bore offline acts like a keel on a yacht. So the belly is effectively a counterweight. You want that counterweight swinging in the vertical plane. If it is at an angle the groups tend to arc more as you increase or decrease velocity. It is easier to tune vertical with powder charge if the bias is in the vertical. The old adage used to be, more vertical more powder. Secondly, the scope with a 12 o’clock orientation uses less adjustment.

I would like to add that fluted barrels pose a greater risk to accuracy if they are not cut concentrically with the bore and this would be my thinking based on the first video. Most barrels are bent and some bores are not concentric. I am reminded of an Australian Team to South Africa where some with fluted barrels failed. It was found that on the crook barrels not all flutes were cut to the same depth. The would obviously walk as the barrel heated up and even if they were in the correct orientation little would save them. Like a lot of teams gone by, the final gear preparation is done at the last minute and bugs so obvious to some are not picked up. So I applaud Peter and DaveMc and others for their meticulous preparation of a couple of years to knock off that world title. The corporate knowledge has to be remembered so mistakes are not repeated when it counts and it has to be shared so new blood can increase the talent of future teams.