this chamber really has me scratching my head!!! FIGURED IT OUT......

[Chuckw]

I just chambered up a shilen #2 in 6.5x47 for a light weight hunting rifle. while measuring runout I encountered something that I just cant wrap my head around! my setup is like most here. I use a spyder on the outboard and either a 4 jaw or spyder on the inboard depending on the length of the barrel. I indicate the grooves on the chamber end as close as I can get to the eventual throat, then I indicate muzzle end and shoot for less than .0005 runout front and back before I start the work. I always pre bore the chambers as well. I measure the chamber after polishing and before removing the barrel from the lathe to make sure all is ok. well this doesn't seem ok! I started at the rear of the chamber and had .003 TIR!!! moving forward it got less and less. the neck area of the chamber and throat are as close as I can get them at less than .0005 runout. HOW IS THIS EVEN POSSIBLE? should I bore the 6.5x47 chamber concentric to the throat area and punch it out to .260AI? I was wanting something with very moderate recoil due to neck issues and didn't want to go that big.
the barrel is already 20" with less than 1" of shank before the taper so I don't think I can cut it off and start over. would it be worth even seeing if the thing will shoot?
chuck

 

[Panzer22]

Re: this chamber really has me scratching my head!!!

Have you tried using a range rod to get everything lines up? You adjust the spider for the tip of the rod the chuck for the inside closest to the barrel. The video shows that the barrel is less than .0002 from concentric. The needles hardly move. (.0001 gauge)

The chuck becomes the pivot point, and if your off the base of the chamber will be out, but the deeper you go (closer to the pivot point) the better it gets.
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[Chuckw]

Re: this chamber really has me scratching my head!!!

when I true a receiver I do it in a jig and the concept is the same as what you show. I have never tried using a range rod to chamber a barrel though. explain how I could be "off" if the throat has basically no runout and the chamber was prebored? I just don't get it.....

chuck

 

[Panzer22]

Re: this chamber really has me scratching my head!!!

Where the jaws are become the pivot point, or zero, where the spider is could be off just .001, and over 20 inches, it can add up. The throat is closer to the zero of the pivot point, where the breach is the other end of the lever and will have the movement of the spider multiplied. Hard to explain but visualize a lever and the jaws as the pivot, but in a circle, rotating the movement, that's runout. Spider could have come loose slightly during the chambering process. The reamer as well, or the reamer is off center. Couple of possibilities. Weird stuff happens sometimes.

Chances are it will shoot just fine, mount it and see.

 

[Panzer22]

Re: this chamber really has me scratching my head!!!

A long as the throat area is straight, I can't think of a reason it wouldn't shoot ok. The brass cases will figure out where to go and conform to a point to make up the rest. I really think it would be ok .. It's not a BR gun, but for hunting, right? Should be fine.

 

[Chuckw]

Re: this chamber really has me scratching my head!!!

prebore should have taken away any variables from the pivot point to the end of the chamber correct? its possible the reamer could have been off center but that should have created an oversized chamber not runout correct? if it doesn't shoot what would be the course of action? I am pretty anal about accuracy even in my hunting guns. if I cant get it to shoot well under MOA I will not be satisfied.

chuck

 

[Chuckw]

well, I went out to look things over again and had decided I would just punch the thing out to .260ai after I was satisfied everything was right. what I discovered is that nothing was wrong other than my setup. when I started out, a very well respected smith schooled me on chambering and advised that I use weed eater string in the 4jaw to keep from inducing stress to the work piece. I did this for a couple years and started using heavy copper wire because I felt it would be more secure (though I never had the slightest issue with the weed eater string). a while back I ran out of copper wire and ran into a guy in town that went to the gunsmithing school in Trinidad. he suggested I use pennies in the headstock since that's what he uses and he said it worked great. we evidently, a penny has too much bearing surface and doesn't allow a barrel to pivot because I set the barrel up as stress free as I possibly could (not using pennies, or my 4jaw for that matter) and I now have well under .001 TIR at the rear of the chamber after the throat is dialed in to as close to .0000 as I can get it! it seems that some residual stress revealed itself as runout after metal was removed from the chamber. would have never believed it but I just proved it to myself anyway.

chuck

 

[Panzer22]

And that's the weird stuff happens sometimes part... glad you got it worked out.
.003 is a small amount, but huge at the same time. Everything gets magnified the further away from the clamping/pivot point you get. Even knowing its there is what seperates the men from the boys. Good on you for noticing and then solving it. Most would never even know to begin with... well done.

 

[Chuckw]

I appreciate all the help and ideas!

chuck