Another Way to Check "Cartridge Fit"?

[Andy]

You can measure "headspace" using an expensive micrometer tool, dedicated to a particular round, like the 308 Winchester, or......

Why can't I just place a 38 Special cartridge over the neck of a freshly sized 308 cartridge (or a larger or smaller pistol round over something different), measure it from 308 base to 38 Special base, and compare that measurement to the length I get when I substitute a 308 round fired in my M14? The 38 Special round will be a fixed length and its mouth will sit on the same place on the shoulder of the 308 (not the SAAMI location necessarily, but the same location).

You are comparing your dies with your gun's chamber and the difference (e.g. 0.013", +/- 0.002") is what really matters, not an absolute number like 1.635" that does not consider your die's tolerances - right? Then you can adjust your dies to tighten or loosen the tolerance as you see fit, i.e. F/L, partial F/L or neck size - take it to whatever you like, e.g. 0.008".

 

[sunray]

Cartridges do not have headspace. Headspace doesn't get measured with any kind of micrometer. It gets checked with proper headspace guages. Headspace is a rifle manufacturing tolerance only.

 

[Cocked&Locked]

I see what he is getting at. He's measureing the length from a know tangent (ie the id of the 38 case) and comparing a sized one to a fired one. It will give you a repeatabale measurment. But to be useful, you have to know what dimension your comparing too. A sized case is pushed back to sami min more or less. If you could fire a case in a minimum chamber thats measured and known, then compare it to one you fired in your rifle , it would give you a working number

 

[Oldsmobiler]

Cartridges do not have headspace. Headspace doesn't get measured with any kind of micrometer. It gets checked with proper headspace guages. Headspace is a rifle manufacturing tolerance only.

Sure you can! When you have a no-go headspace guage set that gets completely swallowed you wanna know what you have for a chamber. Take one of your fired cases and invest in one of these...the principle is the same for the .308

http://www.6mmbr.com/6BRPrecMIC.html

 

[tiriaq]

I think I will run a reamer into a scrap of barrel just far enough to cut the shoulder portion of a chamber. It would sit nicely on the case to allow a basic measurement to be obtained. The base of case to end of gauge measurement would be arbitrary, but any change in length could be easily determined.
Should work just fine for measuring changes in case length when the case is fireformed to the chamber.
Measure the unfired round to establish a measurement. Fire the round, measure again.
The gauge could be applied to Go, No Go and Field gauges to establish comparison measurements.
Should be possible to determine exactly how the chamber relates to a given spec.
Sounds like a great idea, Andy!

 

[landcruiserguy]

You can also use a projectile comparator in 375 cal.

A tapered hole is better i.e. using the chamber reamer on the barrel scrap. Even better if it's the same reamer that was used on the barrel itself but a straight hole will do.

Don't forget that case shoulder length isn't exactly the same as headspace because a) the brass springs back and b) depending on action timing the action can yank the case out before all the pressure has dropped off causing the case to stretch.

 

[Andy]

I should have know that if I used the word "Headspace" that would provoke a tutorial on the word's correct usage, and the central idea would be lost (I once used the term "P17" and got educated on "US Model of 1917" - I still use P17). This could/should have been under "Reloading", but since there's a sticky that includes M14 "Headspace" Myths, I placed it here. My bad, but now it's too late......

It's really "Cartridge Fit" to which I'm referring, i.e. what you get from your dies and your gun's unique chamber, and the method could be applied to any gun you own. Absolute numbers, where on the shoulder the 38 Special sits, springback, etc. mean nothing as it's not "Headspace" being measured. Just a way to get an idea of how tight or loose your sized brass is in your chamber, to allow you to adjust your dies to suit your preference, using a simple (and free) method.

 

[tiriaq]

I think we all know what the point of your post was. Looks to be an easy, effective way of determining the relationship between rifle and ammunition.

For what its worth, if spelling and grammar count, what is a "guage"?.