Clymer 9mm gauge question

[Cali Mike]

Please excuse my terminology. I'm a machinist, not a gunsmith.


So I'm building a 9mm firearm and I'm trying to check headspace. I purchased some Clymer go-nogo gauges. When I drop a gauge into the bore, it fits really sloppy.

Saami spec says the 9mm is .381 diameter on one end and .390 diameter on the other. So it has a very slight taper..
But these gauges measure .370 throughout without a taper. wtf? This precision gauge wobbles in my bore. I even put it in 2 other 9mm bores with the same result.

Am I missing something? (1st time using this type of gauge. But it doesn't seem right)

[klewan]

Isn't head space a length measurement? It's .754" plus a couple of thou; do you have that? The diameter of the gauge isn't important, just the length.

[tujungatoes]

^this is correct. Headspace on a straight wall pistol cartridge is a measurement from the breech face to the where the case mouth stops in the chamber.

[ar15barrels]

Rimless cases headspace on the END of the case so the diameter of the gauge does not matter as long as it is large enough to stop in the chamber you are measuring.
The reason those gauges are made undersized is so that you can use them with tighter "match" diameter reamers as well as more common SAAMI spec reamers.

[Cali Mike]

Thanks for the responses. I'm aware I'm checking the length, but it's so wobbly it feels as if it's beginning to go into the barrel. I'll shoot a quick video when I get to the shop tomorrow.

[Cali Mike]

[ar15barrels]

If it really bothers you, make a new set of gauges that fit nicer.

[kcstott]

Did you ever think that your barrel might be out of spec? as in way to much throat??

I can guaranty that those gages are fine.

the gages are relieved on the sides to prevent any undesired influence to the feel of the gage. MEANING it needs to be either used with the bolt, slide, breach face. what have you, to insure that the gage is being held square or set it up in the lathe and use a center to support the arse end of it like you would a tap and use a indicator to measure your depth to the breach face.

[klewan]

If it really bothers you, make a new set of gauges that fit nicer.

All he really needs to do is find a piece of brass that measures .754" and use that. Then he can put a couple of pieces of masking tape on the head and see how that fits.


If you can measure the datum line accurately, you don't need a gauge. Of course you need to know what and where the datum line is, among other things....

[kcstott]

All he really needs to do is find a piece of brass that measures .754" and use that. Then he can put a couple of pieces of masking tape on the head and see how that fits.


If you can measure the datum line accurately, you don't need a gauge. Of course you need to know what and where the datum line is, among other things....

That would be the only time i would say using tape would would work.

[klewan]

That would be the only time i would say using tape would would work.

Thank you.

[Cali Mike]

I checked the gauges in 2 built firearms, and that barrel was the project I'm currently working on. Same sloppy result in all.

If you guys say that's normal, then I'll proceed with the project. I'm slowly realizing that precision as I know it is very different in gun world. I guess I expected more from a "precision" ground gauge. I appreciate the help

[kcstott]

I checked the gauges in 2 built firearms, and that barrel was the project I'm currently working on. Same sloppy result in all.

If you guys say that's normal, then I'll proceed with the project. I'm slowly realizing that precision as I know it is very different in gun world. I guess I expected more from a "precision" ground gauge. I appreciate the help

You still don't get it.

The only thing precision on that gage is the over all length and it is ground to within .0002" or better Also the rim is over size in thickness on a no go gage to check extractor tension. The part that you don't get is where i mentioned undue influence on the feel.
they clearance the gages so that the only place that it will come in contact is the gaging surfaces. This is so you do not get a false "no go" or "go" feel when closing the action. If the gage was sized like a case with a taper and the same od as a case then the gage would not work because then you would have the potential for a excessively head spaced chamber but you would never know because your "precision" gage would bottom out on the taper not the shoulder.

You do realize there are five sets of dimensions typically between gages, chambers, and cartridges?? the chamber is machined to the go dimension or .001" to .002 larger in some cases. the gages come in three sizes "Go" No Go" and "Field" then you have the size of the cartridge itself which is undersized from the "go" dimension. This is to insure that the cartridge will chamber and that no factory ammo is made to close to the "Go" spec

You do understand tolerances as they are applied to hole fits correct??
Same thing applies here. The cartridge can be -.xxx" but never +.xxx"
The chamber can be +.xxx" but never -.xxx" and the gage well the gage is your basic dimension and has no tolerance. It is ground on size and is your datum reference.

[Cali Mike]

I put it in a v-block on the granite. Measured with a height gauge. I checked the depth of bore from the face. I then put the 9mm gauge in and it's going into the bore by .007.

I just received PTG gauges. Problem solved.


You keep talking down to me like that and I'll pull the drawers out of your Gerstner. (Hoping you get the joke).

[Cali Mike]

Also I fully understand why I don't want it snug. Just figured it wouldn't be so sloppy.