Help with RCBS .308 Precision Mic

[BamBam-31]

Okay, I've used this a couple times now, and I think I'm doing it right. Getting bass-ackwards readings, however.

So I'm using virgin .308 Winchester brass and resizing with a Redding Type S FL Competition die (w/ 332 bushing). Shooting it out of a Rem 700 w/ Shilen barrel. Fired brass measures out at +.001. Fine. Instructions say I wanna back the shoulder down no more than .002 off that. Fine.

So I back out my die, resize, and read....+.004. Hmm. Crank down the die til it's touching the shellholder, and now I'm reading +.003. So my completely resized die is reading a shoulder longer than the fired brass?

Am I reading my gauge backwards?

Btw, same thing happened when I tried it at my friend's house with Magtech brass (and the same Redding die). Resized brass read higher than fired brass. Is that just how the Redding die works?

I called RCBS and the tech sounded kinda confused by it as well. He said it may be that the brass is still too new to get accurate readings, so I had to fire it a few times first. Not sure what difference that would make, but I'll try it anyways.

Can anyone shed a little more light?

[yakmon]

assuming you're using a full length sizing die, you shouldn't back the die out. you should screw it in untill it touches the shell plate plus an additional 1/4 turn.

[Navyguy0023]

Just as a sanity check.... Left of zero is negative, right is positive when reading scale on the mic. Also your using the top with the larger hole for reading headspace, not the smaller one for Ogive. Pretty sure your good here.

That said. My 308 Remington 700's both measure +.001 on fired brass. If I setup my lee die accoring to instructions (screw in die till it touches shell holder + 1/3 to 1/4 turn down) it will size brass to -.004 -ish.

I think you need to lower your die another 1/3 to 1/4 turn. You can go past touching the shell holder and still bump that shoulder a little more.

Hope this helps, I used the crap out of the RCBS mics both in 308 and 223.

[BamBam-31]

Yes, using the top with the larger hole (RCBS #4 stamped on top), not the ogive top.

I'm positioning the mic with the case head and mic bottom down, case neck and mic top up, and I'm reading it as a + being towards the #5 and a - being towards the #45. Is that where my wires are crossing?

[Navyguy0023]

Yes, using the top with the larger hole (RCBS #4 stamped on top), not the ogive top.

I'm positioning the mic with the case head and mic bottom down, case neck and mic top up, and I'm reading it as a + being towards the #5 and a - being towards the #45. Is that where my wires are crossing?

If you start with both Zero's being lined up then yes + is towards the 5 and - is towards the 45.

I think your reading it correct. Just think you need to screw your sizing die down a little more!

[BamBam-31]

Just screwed down the sizing die a hair over 1/8 past touching. Got it down to zero. Whew! Should I go down to -.001 (fired brass measured at +.001) or leave it there at zero?

Thanks for the help, btw.

[Navyguy0023]

For my 700's I go to -.002. I think you should be good at -.001, size about 10 and try chamering them. I tried zero and had about 1 in 5 that were tight bolt closures.

[BamBam-31]

Hmm, I'll try -.001 for my next batch, see how that goes.

[JamesY]

If your fired brass measures +.001, then you should be good at sizing them to .000 or not even FL sizing them at all (just neck size) since it's a bolt gun. For semis, its a good idea to size .002 below the fired case measurement. At least that's what I've been told and the way I do it.

[westcoastr]

the mic measures off an imaginary line halfway up the shoulder as the datum point (and the bottom of the head).

Weird stuff can happen when resizing, like if your die isn't down far enough, the neck gets pushed down and the shoulder pops out a few .001's if there is no die there to keep it in place. ....Or the if the expander ball doesn't have enough lube and grabs the neck on its way out and stretches your case back some. I've experience both situations and my cases got "longer" after resizing.

Also remember, the RCBS mic is a relative measurement. If you want to make assumptions about actual cartridge headspace you need to drop a head space gauge to see what "0" really is. most are off a .001-.002.

[westcoastr]

btw: i just neck size for my bolt guns.