Headspacing on Ruger 77 Mk II?

[DonOK]

Just for curiosity I checked the headspacing on my Mk II 270 WSM. It measured .008. This is a stock gun with stainless sporter barrel. It has probably not seen much over 100 rounds lifetime. Does this seem acceptable, and how much does this affect accuracy?

TIA
Don

 

[Jim K]

Huh? .008 what? How did you check the headspace? If you didn't use a proper gauge, I don't see how you can determine whether the rifle has a problem or not. The only gauge needed for a used rifle is a FIELD Reject gauge, and you have to take the firing mechanism out of the bolt before checking.

I think I can pretty well say that, barring any tampering, a Ruger rifle with 100 rounds through it has no headspace problem. If you had said 10,000 rounds, it would be different.

In any case, headspace normally does not affect accuracy, so what is the concern? Is the rifle inaccurate? Are you seeing stretched cases or any other signs of excess headspace?

Jim

 

[nalioth]

Thread title had me thinking this was gonna be about a .22 semi auto pistol.

Edited:

. . and so the (editing) butterfly flaps it's wings, causing ripples of change to sweep across the thread. .

Original title was "Headspacing on Ruger Mk II" [so now my comment makes sense ]

 

[DonOK]

The spacing is .008 inches. I checked it with plastigage first and then with pieces of shim stock with known thickness. The plastigage measured .007 to .008 inches, and the shim that was .008 inches was a tight fit but the bolt closed completely.

I checked the rifle for curiosity only. I have understood that headspacing has an effect on accuracy and posted for the opinions of this forum. The rifle does not have terrible accuracy issues, and I am no bench shooter.

 

[rbernie]

Just as an FYI - unless you used a SAAMI gauge, you really didn't check headspace as a formal measurement.

I suspect that you used a brass cartridge as your chamber insert. If so, you have introduced the variability of the brass dimensions into the measurement and what you see is not a true measurement. After all, the brass itself is not formed to SAAMI chamber spec but is deliberately undersized from the chamber max dimensions.

I've posted a copy of the SAAMI 308 spec, for you to look at. The spec includes both brass and chamber dimensions, and will help you grok the issues at play.

Measuring headspace is usually done with pre-sized chamber inserts, sized in 'go', 'eh', and 'no-go' increments.

 

[DonOK]

I appreciate the info.
How much does headspacing really effect accuracy?

 

[Jim Watson]

Anything short of benchrest shooting, hardly at all.

Looking at rbernie's diagram, you can see that the headSPACE between cartridge head and bolt face can run from a -.004" press fit to +.013" open space. So if you gauged that space over a typical cartridge, as I suspect, your .008" is within the normal range. I doubt .300 WSM is all that different from .308 WCF.

There is no user adjustment to headspace (in other than a Savage rifle) except by knowledgeable adjustment of loading dies to use fireformed brass without setting the shoulder back any farther than necessary for easy chambering.

A guy here freaked out when his AR showed .007" excess headspace above spec, not over a round. He whined until the company replaced the barrel. Dumb, the gun shot well under MOA and his main use for it was 3-gun and tackytickle matches with the brass lost in the weeds and no need to worry about it stretching too much for the next reload.

We are frequently warned to "check the headspace" on any used or surplus rifle. But the followup is not well described. Gunsmith correction of excess chamber headspace will cost more than most surplus guns are worth, and a lot of sporters.

 

[DonOK]

Looking at rbernie's diagram, you can see that the headSPACE between cartridge head and bolt face can run from a -.004" press fit to +.013" open space.

Jim,

How did you calculate these numbers?

Thanks,
Don

 

[rcmodel]

Chamber drawing shows headspace datum line on shoulder as:
1.630" Min chamber.
1.640 Max chamber.

Or an allowance of .010" in chamber length.

Cartridge drawing shows datum line on shoulder as:
1.634" max, -.007" min., or 1.627" Min.

1.640" max chamber minus 1.627" min cartridge = .013" allowable.

It is hoped that the min & max manufacturing tolerances always fall somewhere in between, and they very likely do.

But if they don't, and you get a Min cartridge in a Max chamber you have an allowance of .013" where nothing bad will happen.

rc

 

[DonOK]

Got it. Thanks for the explanation.