Boltface to barrel tennon clearance

[KMart]

I have a Mod 70 Win pushfeed that has been rebarreled to 6 XC. I have 2 issues with the rifle. The first is that I have never been able to get the accuracy that I think is acceptable from the set up. The other is that the rifle starts to exibit pressure signs before I think it should. It starts to get a fairly sticky bolt around 38.5 gr of H4350 with 105 gr Berger bullets.
I have another Mod 70 pushfeed that allowed me to a little side by side comparison. I removed the fireing pin assy's from both bolts and placed the bolts into the rifles. To me it looks like the 6XC has a generous amount of slop in the bolt wheras the 25-06 does not. The clearance between the boltface and barrel face appears to be quite a bit.
Could this issue be causing the problems that I am describing?

 

[Eddie Harren]

If there is to much clearance between the boltface and the barrel, there is a possibility that you have some of the case unsupported by the chamber.

 

[fullersson]

I was taught to give .010 clearance for a hunting rifle and .005 clearance for a target rifle. If you have the proper equipment it is easy to measure. Pull the barrel off and measure the tenion then measure down to the highest part of the bolt face (with the recoil installed). The tenion should be .005-.010 shorter. Please keep in mind these clearances are just what I was taught and someone will probably tell you something completely different, but those are the clearances I use and they work very well for me. Good shooting, Brian Brown.

 

[KMart]

Thanks for the replies. This rifle has been driving me crazy.

DocEd
The unsupported case was what I was looking at. That would give me the premature sticky bolt, right?

fullersson
I do not have the ability to take the barrel off. The idea I have is to place some layers tape on the face of the bolt face until I start to feel the boltface rub on the tennon, then take the tape off and measure the thickness. I know this is crude, but it will give me a rough idea of where I stand.

Ken

 

[BoydAllen]

With the stripped bolt, is the root of your bolt handle contacting the rear bridge of your receiver when you push the bolt forward? The reason that I ask is that if your gunsmith took a light cut on the lug seats, to true them up, the bolt will sit that much farther to the rear, as will the handle. If you are pushing forward on the stripped bolt, and hitting the rear bridge, the distance from the pulled back position to the pushed forward position would increase by a like amount. IMHO the only way that you will have a support issue that causes a difference in what pressure you can use, would be if it was so great that you got a bulge in the unsupported area. Are you getting a bulge, that ends where the back of the chamber comes to on the brass? Also, how much difference is there between new brass, fired brass and FL sized brass diameters about .3 in front of the case head? A chamber that is too tight relative to the size of unfired brass, will cause a problem like you have described. A friend had a BR that was cut with an old Remington spec, reamer. Later he had the body of the chamber polished out so that it would chamber Lapua brass. Because the clearance was too small, he got a tight bolt sooner than he should have. Recutting the chamber with a reamer designed for the Lapua brass solved the problem. As long as close attention is paid to headspace, having a little more clearance on new brass is not an accuracy issue, and can yield significant benefits.

 

[KMart]

BoydAllen
There does not appear to be any contact with the bolt handle either front or rear...it appears to have plenty gap
I asked the gunsmith to lap the lugs to give me about 50% contact on each lug. Upon further review, it looks like I have 90-95% contact.
The brass is new Norma from Tubb.
New brass measurement is .468
Fired brass is .469
resized brass runs .4685
The reamer used to cut the chamber was a new PT&G that I bought for the rifle.


The measurement I got, with my crude method of using tape on the boltface, gave me between .015 and .020 clearence.

 

[fullersson]

I know the method you were using was kind of crude, but if it is accurate those clearances shouldn't give you any accuracy or pressure problems. No two barrels are the same this barrel could be a tight bore or it might have a short throat and you maybe very close or into the lands (which isn't a bad thing with the Berger VLD's, but it will make your pressure higher) it could be a lot of different things as far as pressure development goes. I wouldn't worry about the early pressure signs to much, but I would worry about the accuracy. Just for information what type of accuracy are you getting out of this barrel.

I'm not suggesting you do this but I've had barrels that showed pressure early. I didn't understand why and out of frustration I kept climbing with the powder charge slowly and the pressure went away until I got where I wanted to be. You're the only one that knows what kind of pressure you are seeing so don't tear your rifle up over something I said on a computer but I've personally seen one of my rifles do that same thing. Believe it or not I was having accuracy problems with that same rifle but when I finally got the pressure where it needed to be the accuracy came with it. Good luck, Brian.

 

[KMart]

The barrel is a 3 groove 8 twist Pacnor. And the grouping is around .5 but usually a little higher. I have had the powder up to around 40.5 gr of H4350 and the bolt just got more sticky. The rifle has never pierced a primer.

I am about 2 steps from having it rechambered to a 243 and taking it to a gunshow.

Thanks
Ken

 

[DOGCAPPER]

Buy yourself a GO guage (20.00) and a NO GO guage for the chamber(20.00)...The bolt should close on the go but not close on the no...This gives between .000 and .005 clearance....Recommended for target actions....LT

 

[BoydAllen]

Headspace gauges are for measuring from a datum line on the shoulder ( in the case of a rimless case) to the bolt face. They tell you nothing at all about the clearance between the back of the chamber and the nose of the bolt.

 

[BoydAllen]

Are those measurements at about .3 above the head?

 

[KMart]

BoydAllen
Boyd, sorry, I had a brain_art. You asked for .3...I gave you .003. The correct readings are:
new...... .4665
fired...... .4685
resized... .468

 

[BoydAllen]

They look OK to me. My take is that some barrels are "tighter" than others, and will produce different pressures because of this. Is your chamber bone dry? A little residual lubrication can give pressure signs.

 

[gamma]

It sounds to me like you bolt lugs may be minutely touching the back of barrel tenon. Take a look at your bolt lugs and see if the look to have signs of rubbing.
The 6xc is a mega calibre and should be shooting a lot better that you say it is.

 

[KMart]

Boyd, I will give the chamber drying a try. A little denatured alcohol on a swab should do the trick.

Thanks everyone for the help and advice

 

[BoydAllen]

I would wrap the swab with something disposable. Swabs are a part of the problem. They are reused and and doing so is like trying to wipe off oil with an oily rag. I use a chamber rod with an old bore brush, loosely wrapped with a long strip of paper towel. Shove it hard into the chamber and give it a twist.

 

[KMart]

I have some large patches left over from cleaning Perazzi shotgun barrels, back in my trapshooting days, that I use and throw them away.