RCBS precision mic/headspace questions

[xrMike]

I got one of these things recently and trying to figure out how to use it.

Instructions are pretty good but do not give me 100% confidence in my understanding of what's going on, or what to do.

1st facts: the instructions include a table showing the minimum and maximum ANSI recommended headspace for a bunch of calibers. I don't remember exactly what the values for .223 were (instructions are at home), but they were something like 1.xxx to 1.yyy inches.

2nd facts: the instructions say that a "zero value" on the mic indicates a headspace measurement that is EQUAL to the minimum ANSI standard (in this case: 1.xxx). Each line on the mic that is above the zero mark indicates +0.001 inches, and each line below the zero mark indicates "-0.001" inches of headspace. So, let's say I take 1 once-fired case, throw it in the mic, and it bottoms out on three marks above "zero", which means my chamber is .003 inches above the minimum ANSI standard of 1.xxx

3rd facts: The instructions tell you to take 5 of your once-fired cases and measure the headspace on them. Like the obsessive-compulsive I am, I take 12 cases and figure I'll get a good average that way. The results? All but 2 of my 12 cases measure right ON the zero mark on the mic. The other 2 cases measured right on the .001 inch mark. So it appears that my chamber has a headspace that is EXACTLY AT THE MINIMUM RECOMMENDED ANSI HEADSPACE FOR .223.

1st question: Is my gun not safe because I'm already at the minimum recommended headspace? Is this a good thing or a bad thing? I imagine it's good, because my chamber/throat is not yet eroded at all, so I still have lots of wear to go before it reaches the maximum recommended headspace (it's a Wylde chamber, if that means anything).

4th facts: the instructions say you want to adjust your re-sizing die so you push the shoulder back no more than .002 inches from your measured headspace value, to increase life of brass, accuracy, etc. In my case, that would put me .002 inches BELOW the minimum ANSI recommended headspace.

2nd question: Isn't that unsafe or something (being .002 under the ANSI minimum). Also, I have about 50 brass that I already re-sized and primed BEFORE I got this mic, and those were resized using the instructions that came with the die, and they measure anywhere from -.004 to -.006 under the ANSI minimum. Are those unsafe, or okay to load?


Sorry for length. I will have more questions I'm sure when I get to the next part, which is measuring distance from the lands.

[westcoastr]

i've asked the same question in a couple of forums AND to RCBS (with little helpful info from them). here are a few things i have learned.

the mic does not measure the headspace of your rifle, it measures the headspace of a fired case, or an unfired one, which I will get to in a minute. this measurement gives you an INDICATION of your rifle headspace, which would be a few 0.001 larger that the case measurement, assuming the case shrinks slightly after it is fired to allow it to be extracted.

so the fact that your cases are measuring right at or 0.001 above the min means you rifle headspace is a few 0.001's above this, which would put it at the low end, but not out of spec or "match tight".

if you mic a few new cartriges you will find that they might come out 0.004-0.005 below the minimum. for my 308 FAL, my aussie is -0.004 and my Miwalls reloads are -0.004. this is because cartridge "headspaces" are made a few 0.001's below the rifle's to allow it to feed and eject reliably. see below:

.308 Winchester Go 1.630"
.308 Winchester No Go 1.634"
.308 Winchester Field Reject 1.638"

.308 Winchester minimum cartridge headspace 1.627
.308 Winchester maximum cartridge headspace 1.633
.308 Winchester average cartridge headspace 1.630

now the question you asked is why does the stupid RCBS instructions show that -.002 is OK? well i think i figured it out as that is 1.628 (1.63-0.002), which is not below 1.627, the minimum cartridge headspace - of course they don't say that -*****holes. but more importantly how low is too low - I DON'T KNOW!!! at some point the firing pin might not set off the primer, or the case will explode or fly back and damage your bolt face. i mean my FAL fired cases are +0.004, which puts my chamber about in the middle of the range. but i am firing cartridges that are -0.005, so the brass in stretching at least 0.01 on the first shot. and that seems just fine. is 0.01 stretch the max? 0.015, 0.02?? i'd rather not find out the hard way. so no way do i resize my brass all the way back to -0.005. for 308, milsurp in particular, might be sized a little shorter than "specs" to allow for use in a dirty machine gun, or my mic might be off a few thousands

so when you sum it up, only size your brass back the minimum 0.002, or a little more if it is not loading reliably or you get better accuracy.

i'm not totally convinced that the absolute measurements are important (within reason) as much as the difference in YOUR chamber for THAT ONE GUN and the ammo you are using/reloading. as long as these are close you should be fine, you might not be able to chamber your tailored reloads in another gun of course.

this might help or just confuse you more, ignore the 308 vs 7.62 discussion

http://carnival.saysuncle.com/002453.html

i of course take no responsibility if you blow your gun up..and if the RCBS folk give you a real answer would love to hear it - i asked two people there and both were clueless (they sent some additional instructions that were even more basic the regular ones)

[rksimple]

Great post westcoastr. .01 stretch is a little excessive as it happens in the web. The thinner it gets, the closer you get to case head separation. Bumping the shoulder .002 is great advice.

[ar15barrels]

Tagging so I can write a GOOD response later.

[xrMike]

Westcoastr, thanks for your detailed reply! I'm much less confused now.

the mic does not measure the headspace of your rifle, it measures the headspace of a fired case, or an unfired one, which I will get to in a minute. this measurement gives you an INDICATION of your rifle headspace, which would be a few 0.001 larger that the case measurement, assuming the case shrinks slightly after it is fired to allow it to be extracted.

OK, makes sense, I hadn't thought of that. The value I'm measuring can't be the REAL value because if it was, the case would never eject.

if you mic a few new cartriges you will find that they might come out 0.004-0.005 below the minimum. for my 308 FAL, my aussie is -0.004 and my Miwalls reloads are -0.004.

You know, I wish I would've thought to measure some unfired, factory rounds. If I had done that, I wouldn't have been so concerned about resizing to below the ANSI minimum. Looks like I was worried about nothing, since out of 10 unfired rounds, they ALL measured between -.003 and -.005 below the ANSI minimum (the "0" mark on the mic). Obviously this is safe because I've put several hundred rounds of it through my upper, with zero problems and pretty good accuracy too.

BTW, those min/max numbers off the RCBS instructions, for .223, are:

min: 1.4636
max: 1.4736

now the question you asked is why does the stupid RCBS instructions show that -.002 is OK? well i think i figured it out as that is 1.628 (1.63-0.002), which is not below 1.627, the minimum cartridge headspace - of course they don't say that -*****holes.

I'm not sure I understand your reasoning here, but that's okay. What you said before (and later) is what gives me comfort. I do agree about the RCBS instructions though, they are NOT written clearly. For example, here they are telling you how to adjust your resizing die:

"Measure the resized cases (after resizing) in the guage to determine the headspace. To continue with our previous example, the reading is now -2 lines, or -.002 under the ANSI minimum. This is a typical reading that confirms your resizing die sets the headspace correctly to chamber in any gun with headspace diminsions within the ANSI range."

Now that is just plain contradictory information right there! Or more accurately, it is not complete, therefore it's confusing. What they don't say (like you explained above) is that when the gun is FIRED, THEN the cartridge will expand in the chamber to be within ANSI standards.

The way they word those instructions makes you think: "WTF! If there's some kind of standard or spec, how can you be BELOW it and still be okay/safe?!?!"

It's because you are only below the spec when the case is OUT of the gun, after firing. During firing, you're back within the acceptable range (or close enough not to matter).

so when you sum it up, only size your brass back the minimum 0.002, or a little more if it is not loading reliably or you get better accuracy.

i'm not totally convinced that the absolute measurements are important (within reason) as much as the difference in YOUR chamber for THAT ONE GUN and the ammo you are using/reloading. as long as these are close you should be fine, you might not be able to chamber your tailored reloads in another gun of course.

Cool -- I'm just going to take your advice and resize .002 down from once-fired and not sweat the min/max numbers.

Thanks again!

[30Cal]

Cool -- I'm just going to take your advice and resize .002 down from once-fired and not sweat the min/max numbers.

Thanks again!

Good plan.

FWIW, I've seen a couple reports where people measured actual headspace gages with the RCBS Precision Case Mic and didn't come up with the expected reading. Either way, I wouldn't use fired cases as a chamber headspace measurement.

[westcoastr]

from you quote - "I'm not sure I understand your reasoning here, but that's okay. What you said before (and later) is what gives me comfort. I do agree about the RCBS instructions though, they are NOT written clearly. For example, here they are telling you how to adjust your resizing die:"

opps... the part you don't understand in my response is because i was using 308 win measurements not the .223 in their example, sorry but i think you got my drift...i'm not sure what the 0.223 CARTRIDGE headspace values are (i gave them for 308 win) but i imagine they are a few thousands below the rifle headspace values same as 308.

someone posted a thread a while ago on reloading for semiauto rifles, if i find it i'll repost it. but basically it said to get a gauge (hornandy in his case) but the rbcs is also great and bumb the shoulder back 0.002, which many have mentioned in the past

[westcoastr]

Good plan.

FWIW, I've seen a couple reports where people measured actual headspace gages with the RCBS Precision Case Mic and didn't come up with the expected reading. Either way, I wouldn't use fired cases as a chamber headspace measurement.

i'm sure its possible, i don't have rifle headspace gauges so can't try. the RCBS mic should be accurate for absolute measuments (i.e. a zero reading on my 308 win should be exactly 1.63" or another example is x brand new ammo mic'ed exactly 1.627" and y brand 1.629") if they made it correctly. also there is the "turn the gauge until you have applied about 2 lbs of force" in the instructions - like I have any idea how much this is since god didn't include a torque wrench in my wrist...so maybe the same 2 lb force actually pushes a bit on the relatively soft brass (and is calibrated for this), while the hardened steel headspace gauges don't give..i really don't know and this would only explain if the gauges measure too long. just throwing crap to the wall to see what sticks.

But the RCBS mic (as will the hornady - which i just ordered to deal with an unrelated issue) will certainly give you relative/differential measurements (i.e. x brand new unfired ammo is 0.002 shorter than y brand new round..or after screwing my sizer die by 1/8 of a revolution, my sized brass are now -0.004" than before).

and the RCBS mic only gives you a rough indication of your rifles headspace, ASSUMING the brass shrinks at a constant rate. different brass or various powder types or charges or a million other things might skew this. I would not ever use it to determine a rifle's headspace. better to buy the real gauges and be sure.

[30Cal]

A handfull of identical fired cases will vary by a couple thousandths.