DISCREPANCIES BETWEEN MAXIMUM CHAMBER HEADSPACE AND FIRED CASE MEASUREMENTS

[Edsel]

TIKKA T3X, 6.5mm Creedmoor

Bolt closes with medium resistance ("sticky bolt") to finger pressure on Forster Chamber Headspace Gauge 1.541...

...which is +0.002" above the upper ledge of the Wilson Case Headspace Gauge…

...when using the Wilson Case Headspace Gauge and Micrometer for measurement.

Does not close on Forster Chamber Headspace Gauge 1.542, which is +0.003" above the upper ledge of the Wilson Case Headspace Gauge.

Brass subject to full - length sizing headspaced to +0.002 above the upper ledge of the Wilson Case Headspace Gauge likewise demonstrates medium resistance ("sticky bolt") to finger pressure on bolt closure.

A difference in +0.001" makes the difference between bolt closure and otherwise.

FIRED CASE MEASUREMENTS

Brass 1: +0.007" to +0.009" above the upper ledge of the Wilson Case Headspace Gauge
Brass 2: +0.008" to +0.009" above the upper ledge of the Wilson Case Headspace Gauge

To reiterate - bolt does not close on Forster Chamber Headspace Gauge 1.542, which is +0.003" above the upper ledge of the Wilson Case Headspace Gauge.

ACCURACY INTERNATIONAL AT, 6.5mm Creedmoor

Bolt closes with medium resistance to pinky pressure on Forster Chamber Headspace Gauge 1.544, which is +0.005" above the upper ledge of the Wilson Case Headspace Gauge. Does not close on Chamber Headspace Gauge 1.545.

Brass headspaced to +0.005" above the upper ledge of the Wilson Case Headspace Gauge exhibits the same resistance.

FIRED CASE MEASUREMENTS

Brass 1: +0.012" to +0.014" above the upper ledge of the Wilson Case Headspace Gauge
Brass 2: +0.010" to +0.013" above the upper ledge of the Wilson Case Headspace Gauge

To reiterate - bolt does not close on Forster Chamber Headspace Gauge 1.545, which is +0.006" above the upper ledge of the Wilson Case Headspace Gauge.

ACCURACY INTERNATIONAL AT, .308 Winchester

Closes with medium resistance to pinky pressure on Forster Chamber Gauge 1.635, which is +0.002" above the upper ledge of the Wilson Case Headspace Gauge. Does not close on Chamber Headspace Gauge 1.636.

Brass headspaced to +0.002" above the upper ledge of the Wilson Case Headspace Gauge exhibits the same resistance.

FIRED CASE MEASUREMENTS

Brass 1: +0.002" to +0.003" above the upper ledge of the Wilson Case Headspace Gauge
Brass 2: +0.001" to +0.003" above the upper ledge of the Wilson Case Headspace Gauge

To reiterate - bolt does not close on Forster Chamber Headspace Gauge 1.636, which is +0.003" above the upper ledge of the Wilson Case Headspace Gauge.

This rifle has fired case headspace dimensions closer to what I would have expected.

--------------------------------------------------

SUMMARY OF ISSUE

Fired case headspace measurements appear to exceed those of Chamber Gauge sizes that preclude bolt closure, and by a huge margin at that.

--------------------------------------------------

QUESTION(S)

1. What is causing this discrepancy? Chamber Headspace Gauges, FLSized Cases, and Fired Cases were all measured in one Case Headspace Gauge. The Fired Case measurements do not agree with the Chamber Headspace Gauge and FLSize Case measurements. For instance, the bolt closes with resistance on Chamber Gauge 1.541 and FLSized Cases headspaced identically, and not on Chamber Gauge 1.542 (+0.001”) - but Fired Cases (which chamber properly) have headspace values as high as +0.007”.

This does not appear to be an issue of wide case bodies hanging up on the Headspace Gauge - the brass slides in and out freely.

a. Is there a significant variation in chamber shoulder angles between manufacturers?

b. By that token, where does a Chamber Headspace Gauge index off?

2. I feel I might be overlooking something - any suggestions?

 

[ShtrRdy]

I would guess the first step is to prove out the dimensions of your measuring tools.

Is there a problem with the rifle you're trying to solve?

 

[JBoomhauer]

A. There can be.
B. About here

 

[Edsel]

I would guess the first step is to prove out the dimensions of your measuring tools.

The tools measure fine, cross - referenced them several times.

The Chamber Headspace Gauges do follow increments of 0.001", as does the L.E. Wilson Depth Micrometer.

The Wilson Case Headspace Gauge doesn't need to be particularly accurate, as long as it is the only gauge used throughout measurements.

Is there a problem with the rifle you're trying to solve?

No particular problem, the rifles shoot within expectations of their price range - but I'm just hoping to avoid committing a systematic error in measurement I'm not even aware of.

 

[Edsel]

A. There can be.
B. About here


View attachment 8777508

The variation in shoulder angles - can it give me indexing variations as high as 0.007"?

Not saying that this is the issue, but if it is - seems a bit much.

 

[whatsupdoc]

Saami chamber dimensions for 6.5 Creed are
Min 1.541
Max 1.551

 

[45-90]

Wilson cartridge gauges are a bit off...that's common.
Check them with a surface plate and a drop dial indicator by inserting your headspace gauge, and taking a measurement.
Now you know how far-off your cartridge gauge is.

The headspace gauges are ground accurately to a datum line as per SAAMI print, and are not usually a problem.

Or make your own cartridge gauge with the reamer that chambered your rifle to set your rifles headspace.

 

[Edsel]

Original post reworded for clarity re: "SAAMI maximum" - should have been written as "Upper ledge of Wilson Case Headspace Gauge."

 

[ma smith]

Maybe you need proper gunsmithing tools to measure headspace. iirc...manson used to sell a legit headspace guage measureing/setup like $150+, maybe op can call them It might be special/made to order...afaik, nothing on the consumer/retail space like LE wilson or Hordady etc are for saami-spec measurements, as they are sold as "headspace comparators"...not "measurement" tools...they are good for some things but maybe not what OP is trying to do.