Center fire barrel indexing

[Tozguy]

First let me ask about the underlying principle. What does indexing compensate for? Why does a barrel shoot better in one position than another? Is indexing related to minute variations in barrel and/or bore dimensions, in barrel steel homogeneity, in internal stress of the barrel, or to some other characteristic?
Have there been any successful attempts at predicting the best barrel index before threading the tenon?

Traditionally, indexing a barrel for a center fire rifle is a rather tedious procedure. Gene Beggs has developed an approach for indexing a barrel that does not require shimming and adjustment of headspace every time. However barrel indexing still seems to be a relatively demanding procedure. Has the collective experience to date shown whether indexing benefits a factory barrel more than a custom match barrel? Does it benefit a light contour barrel more or less than heavy contour barrel?

 

[LHSmith]

Indexing can be for cosmetic reasons..i.e. barrel stamp, front/ rear sight holes, fluting ...........or to orient the imperfect barrel so that the muzzle points upward vs. sideways or downward.

 

[Bob3700]

For long range shooting it is beneficial to index the bbl so that the high or low spot is oriented to 6 or 12 O'clock. That way your sights and bore are more closely aligned.

Why use up 5 MOA of windage or elevation just zeroing the gun.

It really isn't that hard to do when you are chambering a new bbl. Just takes a few more minutes to set up.

Bob

 

[Tozguy]

Bob,

Any details about how you set up to find out the 'top' of a barrel before threading would be much appreciated.

 

[Bob3700]

TOZ,

It is simple really. I chamber thru the headstock using a four jaw and a spider. To get the chamber end of the bbl centered, you center the end of the breech and then up an inch or so beyond the throat. This segment of the bbl is where you are interested in the lowest run-out possible. See Gordy Gritters video.

The muzzle will be adjusted with the spider to get the chamber/breech end centered. The muzzle will NOT be centered after you do this. I put a Deltronics pin in the end of the muzzle and find out where the high point is. Mark it with a sharpie. You now have the bbl indexed. You just need to thread the tenon and fit your action. You want the bbl high or low point to index with 12 O'Clock on the action. You will have to adjust the bbl shoulder and breech length to get the bbl to index.

Windage corrections will now be minimal. You can switch iron sights or scopes with minimal adjustments, probably only elevation depending on the cartridge.

Bob

 

[TRECustom]

Bob, I never even thought of indexing except for cosmetic reasons. This makes a lot of sense. I measured .0075" tir in the middle of a new non-fluted 32" custom blank between centers. That barrel ended up being a hummer, but it got me to asking....How much runout is too much. If the breech end has any runout, I take a light cut with it on the live center. I called barrel makers, and got replies that ranged from.0075" (from the maker of that barrel) down to Dan Lilja who asked me why I would accept any runout.

Would you offer an opinion on maximum runout in the middle of a more common 28" blank? I have a bad feeling that I'm missing something else I should have thought of besides indexing the runout.

Good thread Tozguy!

Thanks, Tom

 

[Tozguy]

Bob,
Thanks for the explanation. Makes a lot of sense. Intuitively I would fit the barrel to the action in the muzzle up position but you write that it can be either up or down. Do you have a preference?

Tom,
Your question ''How much runout is too much?'' has been an issue for me too.
Those bore straightness gauges that are supposed to freefall through a bore seem like a rather coarse standard for run out. How much run out will a 6'' long rod that is .231'' in diameter show in a .236'' bore? It doesn't seem like a very sensitive tool but I have never used one. Anyone's comments on these would be welcome.

Mike

 

[xs hedspace]

Seems like I remember a test in Prec. Shooting, where the low impact index point gave the tightest groups, on a 22 RF br barrel. Always wondered if this was due to the barrel bore being aligned with the natural drop of the bullet as it went down the tube...less forces from the bullet, giving less vibration in the barrel as it was fired. My engineering brain theory...