Can you measure headspace on a bolt action using a fired case and plasti-gauge?

I am sure you can, but, how? Please elaborate.

Steel gages sound like they'd be much easier to use, and, reliable.

A fired case won't work because it is firefomed to the chamber. You can measure headspace using a factory unfired case, but it will only be relative to that case. No need for plastigauge. Pull the bullet, dump the powder and snap the primer. Punch the primer out about .020. Then chamber it with force, fully closing the bolt. Extract and measure the primer protrusion from the case. That will be the headspace for that case.

short answer, no, assuming rimless case.

in theory a fired-in-that-rifle case, assuming normal pressure, will distort to that rifle's actual headspace, and then shrink once the pressure and heat are removed. therefore a fired-in-that-rifle case, assuming it's correct for the caliber of the chamber, will more or less have a close datum (technical term) to any other (within reason) case fired in that same rifle.

The headspace is set, in a bolt gun, by the distance from the bolt face to the datum point on a headspace gage. case in point, while one may (riskily) fire a 308 in a 30-06 chamber, and the case will likely be reformed into a short necked 30-06 case, and the resulting length from bolt face will occur on the case, meaning the case could contain the same datum as a 30-06, it isn't a 30-06 case - but it might actually now headspace on the datum point

I am making too much of this, answer is no, a fired case can't be used, even with plastigage, to define headspace, as the fired case is unknown

Could you compare a zero head spaced fire case to a not-so-zero head spaced fired case? sure, but that doesn't really tell you anything

put it another way - the headspace of the gun may or may not be reflected in expanded and then shrinking fired brass, in fact, likely not

it's only a little confusing, as "head space" has nothing to do with the brass, it's a measurement of the gun

Call me with the real question. Jeff is right; you can't get there from here.
You can use a steel gauge of known dimensions and plasti gauge to measure a chamber. Not brass.
For those who don't know, plasti gauge is used on engines to measure bearing clearance. You think rifle chamber headspace is made to tight tolerance; crank bearings are much more precise.

Ok… been texting with Tom. But my original thought was on rifle (98 Mauser) with known good headspace, chamber a factory round with bolt that has FP removed and plasti-gage on bolt face. Repeat process with another stripped bolt in same rifle. Compare plasti-gage of the two bolts. Will this give a comparison of the two bolts so you may see a difference or if they are the same as far as “headspace” goes.

Does anyone use the RCBS Precision Mic for comparative case headspace measurements?

I've used a few of them, with different "adapters", for many years with very repeatable measurements.
They are very helpful when case sizing, case forming, and even case sorting.

Here's a pic of one, with it's adapters.

Kevin

I use the RCBS tool to make sure fired .223 cases are sized 0.002-0.003" for correct chambering. It does give a reasonable idea of the chamber's headspace, but several cases of the same lot fired with the same load have some variation (say 0.001-0.002"), so one can't use the measurement as an absolute number.

Or electrocutions tape?

oh, Ray --- i was expecting a gem from you on headspace - i really was

quote:

Originally posted by jeffeosso:
oh, Ray --- i was expecting a gem from you on headspace - i really was

I think he really meant electric chair tape

Only in desperation have I set up a chamber using ammo ...and than from as many compainies as practical

Renting gages is easy if you dpn't want to make a puchase.

"Do it right or don't do it at all"