7PRC Tikka / Proof headspace question w/ Go and No Go Gauge

[SoutheastGoesWest]

Last time I put a 7 rem mag Proof barrel on a Tikka action it headspaced perfectlly. Bolt dropped on the Go gauge, didn’t drop on the No Go.

Today I torqued a new 7PRC Proof barrel onto a Tikka action and the bolt closes on both the go and no go gauges. Difference between the two gauges is .002. When I add a single piece of clear Scotch tape to the back of the NO GO gauge, it becomes .0045 longer than the Go gauge (tape accounts for .0025), and the bolt either doesn’t close, or it will close with a fair amount of force. I tried this test multiple times with the same result.

What is the consensus on whether this headspace is safe?

 

[Kurts86]

You need a field gauge to determine if it’s safe. It can close on a no go and be safe, just not quite as ideal as possible.

 

[TxLite]

Was this a proof prefit or did you have it spun up?

 

[SoutheastGoesWest]

Was this a proof prefit or did you have it spun up?

Prefit

 

[echampion8]

I’ve talked to proof before on their prefits. They said closing on a no go gauge was still safe but closing on a field gauge was not ok. The guy seemed like he was trying to say there are slight variances in gauges but that didn’t make much sense. The gun shoots fine but still bugs me in the back of my mind.


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[SoutheastGoesWest]

I’ve talked to proof before on their prefits. They said closing on a no go gauge was still safe but closing on a field gauge was not ok. The guy seemed like he was trying to say there are slight variances in gauges but that didn’t make much sense. The gun shoots fine but still bugs me in the back of my mind.


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I’m seeing different sites that claim there is usually a .004-.006 difference between a go and no go gauge and that SAAMI normally allows a .010 tolerance in a chamber. Don’t know how true this is but if it is correct, a .002 difference between the PTG gauges seems really tight.

 

[G37some]

seems like a looser than wanted fit. On my 22gt prefit from patriot valley arms my bolt closed on the go but not on the go with tape even with heavy force.

 

[Tahoe1305]

Researched this a lot a few weeks ago and had similar issue.

Different manufactures make there go and no-go variations different. Like you said .002-.004 being common. Just because it will close on no-go doesn’t make it unsafe. Agree field is the true test.

But IMO knowing some have tight gauges. Others a bit looser. If you know you are using a .002” difference (tighter end of spectrum) and confirm a piece of tape puts it to the other end of spectrum, I would feel confident.

Fwiw, exactly same situation with mine. Very tight close at .0045” from go gauge. It’s shot fine.

Another option is add 1/4 turn or so more torque to barrel. Make it closer to tikka factory takeoff stds!

 

[SDHNTR]

.002” is really tight! Get gauges .004-5 ish.

 

[762 ULTRAMAGA]

Last time I put a 7 rem mag Proof barrel on a Tikka action it headspaced perfectlly. Bolt dropped on the Go gauge, didn’t drop on the No Go.

Today I torqued a new 7PRC Proof barrel onto a Tikka action and the bolt closes on both the go and no go gauges. Difference between the two gauges is .002. When I add a single piece of clear Scotch tape to the back of the NO GO gauge, it becomes .0045 longer than the Go gauge (tape accounts for .0025), and the bolt either doesn’t close, or it will close with a fair amount of force. I tried this test multiple times with the same result.

What is the consensus on whether this headspace is safe?

Don't worry about the no go gauge, have you tried putting a piece of tape on the go gauge?
I've had headspace so excessive on a rifle once that the primer barely got struck and occasionally wouldn't fire, I measured fired cases at the shoulder and they moved .010.
You definitely don't want a chamber with more than .006 headspace, fire a primed case and look at the firing pin strike. If it looks weak or doesn't fire then you have a problem.

 

[Gonewest]

I’m seeing different sites that claim there is usually a .004-.006 difference between a go and no go gauge and that SAAMI normally allows a .010 tolerance in a chamber. Don’t know how true this is but if it is correct, a .002 difference between the PTG gauges seems really tight.

What did you end up doing? I have a similar situation.

 

[SoutheastGoesWest]

What did you end up doing? I have a similar situation.

I felt like the cause was the difference between the go and no go from Pacific Gauge and Tool being .002. From what I was seeing everywhere that seemed tighter than usual. I bought a go and no go gauge from Manson Reamers, and this worked fine. Bolt dropped on the go gauge, but didn’t drop on the no go. I believe the difference between them is maybe .004 or .005? I also ended up with a field gauge that I never used, if you are interested in buying one for the 7 prc.

 

[Gonewest]

I felt like the cause was the difference between the go and no go from Pacific Gauge and Tool being .002. From what I was seeing everywhere that seemed tighter than usual. I bought a go and no go gauge from Manson Reamers, and this worked fine. Bolt dropped on the go gauge, but didn’t drop on the no go. I believe the difference between them is maybe .004 or .005? I also ended up with a field gauge that I never used, if you are interested in buying one for the 7 prc.

Thank you for the reply and offer. I have everything figured out now.

 

[Slamfire94]

I felt like the cause was the difference between the go and no go from Pacific Gauge and Tool being .002. From what I was seeing everywhere that seemed tighter than usual. I bought a go and no go gauge from Manson Reamers, and this worked fine. Bolt dropped on the go gauge, but didn’t drop on the no go. I believe the difference between them is maybe .004 or .005? I also ended up with a field gauge that I never used, if you are interested in buying one for the 7 prc.

do you have any gauges you would lend or sell that work for a 7mm rem mag?

 

[TaperPin]

do you have any gauges you would lend or sell that work for a 7mm rem mag?

The belted magnums are somewhat special, because they headspace off the belt - more than one person has simply measured a number of cases to find the tallest belt, added a layer of scotch tape to the bottom and used that as the go, added another .002” tape layer and made that the no go.