Need help on a rifle accuracy -- actually consistency -- problem (long)

[iamkris]

Rifle experts

I have a question about accuracy / consistency problem that I have with a rifle.

This rifle started out as an FN-built Israeli Mauser chambered in 7.62 NATO that I bought in 1988 (it is the scoped rifle, fifth one from the bottom in the attached image -- wish I had a better picture of it). It's original wood was completely beat but the metal wasn't pitted and the bore was crisp and shiny.

I had the following done to it by a competent gunsmith. Barrel cut down to 22", counter-bore crowned by 1/8", bolt cut and turned down, reblued, iron sights removed and Weaver mounts installed. I personally put on a a low mount safety, installed a scope and a Bold trigger that breaks nice and crisp at around 2-1/2 lbs. I bought a California Claro Walnut stock blank and carved the shape I wanted, hand checkered it, installed a red Pachmyer recoil pad and finished it in 12 coats of TruOil. The action is fully glass bedded and the barrel is free floated. It is frankly quite beautiful and I have a lot of pride of ownership/workmanship in it. I took a pronghorn antelope with it 2 years ago and unsuccessfully carried it mule deer hunting this year.

Since refinishing, the rifle has never shot better than 2-3 inches (on a good day with a scope) with surplus NATO ammo. I didn't shoot it much prior to working on it. With Remington CoreLoct factory 150 gr ammo the best I've ever got is 2-1/2" groups. I started loading Hornady bullets and with IMR 3031 I can get 1 to 1-1/2" groups with the 150 and 165 gr SST bullets.

Here's the issue -- the gun shoots all over the place as I shoot more. The first 2-3 shots will hit one place on the target. After 10+ shots within a 1/2 hour or so, it will suddenly shift 6-8 inches...usually up and right. THe best accuracy comes when the barrel is hot (the 1" groups) and when cold shoots closer to 1-1/2" to 2" groups with the above load.

What do you think I can do to get more consistency? Obviously I don't need 10+ shots hunting (at least hopefully) but I would like more confidence in knowing where it will shoot.

The barrel is free floated today...would a pressure pad made out of thin gasket material at the foreend help? Any other suggestions?

 

[Andrew Wyatt]

I was about to suggest some forend pressure at the tip. go ahead and try it. it should help some.

 

[iamkris]

Any suggestions on what to use to apply the foreend pressure?

Tape?
Cardboard stock?
Contact paper?
Thin cork or rubber gasket material?

 

[Soap]

I like using slivers of leather to pressure bed.

 

[Al Thompson]

Do you have another scope you could use to verify the shifting? IMHO, scopes can be a leading source of lots of strange problems.

I'd shim the barrel with cardboard to check. I'd also look at the torque value on the action screws.

 

[Plain Old Bill]

Free floating is the first choice, but this is most effective when the action is bedded. If you're going to apply up-pressure to the end of the barrel, use a material that is NOT affected by moisture or oil (I use a steel washer or brass plate, bent for the curvature of the barrel.)
BUT- have you tried different ammo? Bullet weights? Sandbags? Scope? There are many, many variables to explore.
Might be a good idea to pick up a cheap Mauser and practice bedding...Acraglas by Brownell's is fairly easy to work with (get the gel) and forgiving of beginners.
Sometimes barrel temperature is the culprit. Try a string allowing the barrel to cool for 5 minutes or so in between rounds.
Good luck!

 

[iamkris]

Plain Old Bill - Thanks fo the suggestions.

The action is bedded with Brownells AcraGlass. I am a lifelong handloader and have done SOME experimentation for this rifle but not a lot. Speer 165 SP, Hornady SST in 150 and 165 gr in combinatons up and down the chart with IMR 4064 and IMR 3031. Benchrest and normal large rifle primers. Have only used RP brass. So far, the 165 gr SST with 3031 is the most consistent, giving me 1" groups at its best.

I'd suspect this is a barrel heating problem since it shifts (sometimes radically) when the barrel gets hot (say after 10+ rounds in strings of 3 in 30 minutes).

Al T - I unfortunately don't have another good quality scope to try but am in process of buying a Weaver Grand Slam to put on top of my Tikka T3 300 WSM I just bought. Will that one if the pressure strips don't work.

Good suggestions on materials. I do have some brass shimming material I could try rather than a porous flexible material.

 

[Jim Watson]

A free floated barrel radically jumping POI after it heats up. Adequate groups at both ends hot or cold. Right?
Sounds like a crooked barrel with uneven internal stresses. Maybe tip pressure will damp it out. Cheap to try.

Something else to look at that I have only read about. Is the rear guard screw bushing in place? I have seen it said that even under glass bedding a Mauser can flex if that piece is not in there.

 

[JohnKSa]

Definitely change scopes and/or check the mounts over carefully BEFORE you do anything else to the rifle.

The kind of problems you're describing are much more likely to be a scope/scope mount issue than a rifle issue, IMO.

 

[MLC]

Shots in the dark

How tight is the glass bedding and
does the bedding have uniform bearing surface on the action?
What is the OAL of your ammunition and
how close is the ogive to the lands in the barrel?

 

[TooTaxed]

There are some good suggestions above...the only thing I can add is that as you shoot and the barrel heats up, you might check to make sure the barrel is still free floated.