[RavenVT100]

There's an issue that has revealed itself after buying an RCBS precision mic that I'd like some clarification on if possible.

The precision mic measures the deviation between minimum spec headspace and what your cartridge's headspace actually is. This allows you to knock down the neck of the case just enough to chamber a round in your rifle, without creating an excess headspace situation.

The precision mic reveals that my full length dies actually knock the headspace down to approximately -0.003" under spec, which is slightly more headspace than even a factory cartridge. I am able to raise the SB die (the X-Die, actually) so that I get around +0.001" from spec, which is only 3 thou under what the cases expand to, but in the process of doing so it removes the "camming over" action from the die.

On top of this, RCBS says don't use the precision mic for full length dies, and that the dies always have to "cam over."

What gives?

[Steve in PA]

Before going to the X-die I used the Precision Mic and FL RCBS dies for years. All the PM does is tell you what your dies are doing to your brass. If you are getting the headspace you want, I don't see where the problem is.

Where does RCBS say not to use the PM with FL dies? I'd be curious to read this since the PM came out long before the X-die did. Exactly what dies did they want you t use??

[Walkalong]

You should be able to adjust your die so your press "cams over" and still get the exact setback of the shoulder you are looking for. Check this every time you reload the brass as it can change as the brass work hardens and "springs" back further than it did when it was dead soft. :)

I can't think of any reason why you can't use the Precision Mic no matter which sizer die you use as long as the Mic goes over the case all the way to the shoulder. I would call RCBS and ask them.

[RavenVT100]

The techniques they advocate you use the precision mic for (getting the headspace to within 2 thou of where it ends up expanding to) are for use with neck sizing, apparently.

There's nothing magical about the precision mic, you can use it with anything, and I bought it primarily as a safety precaution so I could see exactly where any questionable brass had been sized to.

I will try adjusting the X-Die so that it sizes less but still cams. Thanks for the advice.

[Jim Watson]

I don't understand all that you are saying. The point of neck sizing is to get the brass to hold the bullet without disturbing the case body where it was fireformed to the headspace and diameter of the chamber. No need to gauge, then.

I would want a gauge to use on full length sizing to be sure I was getting the brass to where it would chamber in a different gun, or an automatic, but without excess cartridge headspace. I have the Stoney Point cartridge headspace gauge, but I find it simpler just to adjust the sizing die so the case goes in a Wilson fixed gauge between the steps, per instructions.

[RavenVT100]

Jim: Your response tells me that I'm most likely overanalyzing it, so I will boil down my question into something more concise:

Is it ok to back out a full length die until sized brass headspaces to where I want, even if it stops "camming over?"

[Idano]

Raven,

I am not sure you will be able to get repeatable results without the cam over. It is my understanding that the cam over is to remove the play in the in ram and pin/knuckle assembly. I suspect depending on the amount of lube, how rigid the brass is, and much it has expanded may affect at which point the some or all of the slop is absorbed. Then again the slop may be absorbed as soon as the case comes in contact with the die. I think the only way to know for sure is try it and monitor it and see what results you get.

[30Cal]

That precision case mic isn't calibrated. Take a close look at it--there's no adjustment on it for the factory to do a calibration on.

Measure your fired cases and set the die to bump the shoulders back by 0.004" on average and call it good.

Ty

[ReloaderFred]

I've been using the Precision Mic since it first came out and there's no magic to it. You size the case and measure it in the Presision Mic to see where the headspace is on that particular case. I've got them in 30-06, .308, .243 and 8x57.

I set up a sizing die in each caliber for a particular rifle, or group of rifles that have the same headspace. I set the shoulder back approximately .003" and my brass lasts much longer and no case head separations. If I have several rifles in each caliber, then the dies are set up accordingly. In 30-06, I've got three sizing dies set up at different headspace settings, since my dozen or so rifles in that caliber fall into three groups of headspace measurement.

Hope this helps.

Fred

[RavenVT100]

Fred:

Are you doing this with full length dies?

Thanks.

[ReloaderFred]

RavenVT100,

I size all my rifle brass in full length dies. I've got about 40 different rifles, so unless I want to dedicate brass to just one of them, it makes more sense for me to just go ahead and full length size the cases. I've never had a problem with doing that since I started using the Precision Mic dies. Prior to that, I was suffering case head separations in my M1 Garands, caused mainly by following the directions for die set up from the manufacturers. I was setting the shoulders back too far and causing excessive case stretching. This all stopped when I bought my first Precision Mic and was able to finally measure the actual headspace on the fired cases.

My Garands average .008" over the zero setting on the Precision Mic, and I was setting the shoulders back to .000" on the measurement. This was way too much stretching. On some of my bolt rifles, the actual headspace on the Precision Mic is .001". You can see from the difference between the chambers how this affects case life and the integrity of the brass. I do keep my 30-06 brass segregated into three groups, and since doing this, I'm now getting up to 8 loads out of brass that used to fail with the first or second loading.

I now use the Precision Mic to set up any sizing die for the calibers I have them for. It's also easier once the brass is fired in the chamber the die is set up for. The Precision Mic die really reveals the difference in chambers when you pick up range brass fired in different rifles and see how they vary in headspace. For me, they've made life simpler and I really like them.

Hope this helps.

Fred

[RavenVT100]

Fred:

That's exactly the type of info I was looking for. I'm happy to be able to draw off of your experience. Thank you.

[ReloaderFred]

RavenVT100,

You are more than welcome. There's no sense in making mistakes, or stumbling through something, when someone else has already gone through it and can help you through it or take the mystery out of it.

Fred

[redneck2]

You are more than welcome. There's no sense in making mistakes, or stumbling through something, when someone else has already gone through it and can help you through it or take the mystery out of it.
+1 on that. Some people still argue against success.

As with most things, there are variables that should be taken into account. I always knew there were variations in head space, and this points it out. thanks

[RavenVT100]

Take a close look at it--there's no adjustment on it for the factory to do a calibration on.

There is! I thought that for awhile myself, but if you look carefully, you will see that the part the screws over the half that the shell sits in actually has two halves, and they're apparently locktited together.

[30Cal]

Wow! I didn't know that. Still, I know several people that have measured headspace gages with an RCBS Prec. Case Mic and the results don't jive.

Ty

[RavenVT100]

I have heard second-hand that RCBS will calibrate a precision mic if you send it back to them, but I would check with them first.

[Walkalong]

Still, I know several people that have measured headspace gages with an RCBS Prec. Case Mic and the results don't jive.


The gage is not for measuring headspace of a chamber. It is not a headspace gauge. It is for getting a measurement of the shoulder of fired brass so you can then control how much you set the shoulder back, controlling headspace. :)