measuring headspace on once fired brass

[SPAK]

Hey guys,

Just wondering if you measure your headspace using once fired brass or not. I've read that some folks use their once fired measurments on moderate loads and use that number as a baseline to bump the shoulders back to...and I've read that it might take a few firings to get the true headspace measurement due to the "spring back" of the brass.

Is it better to use a max load on the first firing to get an accurate measurment? or just fire twice and take that measurement?

I'm trying to set up my body die to just bump the shoulders back .001" from the chamber's actual headspace measurement.

Thanks for the help.

 

[MitchAlsup]

Re: measuring headspace on once fired brass

What I try to do is to find three cases that measure within 0.001 of each other on the sholder. One case will let the bolt close with no feeling of added pressure, one will close the bolt and feel some added weight, the third case will almost not let the bolt close. {Generally FGMM is pretty good about producing once fired cases that fit this description.}

I use an RCBS case micrometer, but any suitable tool works well.

Anyways, sort the cases by sholder length on your measurement device, and find a small number that you can feel on the bolt but it will still close AND have the property that the measurements are very close on the sholder. It is these cases that you want to use as reference points and size cases 0.001 to 0.002 shorter.

Also note: I have "not had good luck" pushing the sholders back 0.001. This is within the springiness of the case brass itself and little sizing gets done. In fact, I have had cases grow when trying to push the sholder back 0.001 (shrinking case web cahses sholder to move forward). By the time you get to 0.002, you are pushing on the brass hard enough for it to flow into a new shape (defined by the die) and it "works better".

 

[SPAK]

Re: measuring headspace on once fired brass

Added note:

once fired .30-06 mil surp greek hxp brass measures 1.952" to the shoulder.

My once fired Win brass using 48gr IMR 4320 and 180gr jacketed bullet measures 1.948-1.949" to the shoulder.

2nd firing of Win brass using 58gr RL 22 and 208gr Amax measures 1.952-1.953" to shoulder.

Which measurement should I use?

 

[JALVI]

Re: measuring headspace on once fired brass

I just take the measurements from 5 fired cases divide by five, and that gives me an average measurement to start and bump that 0.0015 - 0.002 back. Check after the next firing just for S&G's. With this current batch of Lapua brass I'm on the 16th firing.

YMMV, just my .02.

 

[SPAK]

Re: measuring headspace on once fired brass

Mitch, should I be feeling the bolt closure with some effort with the resized case or should there be no effort at all when properly sized?

 

[MitchAlsup]

Re: measuring headspace on once fired brass

When properly sized, the only force you should feel is the friction of the cam and the push of the ejection plunger. If you remove the plunger and the cam is nicely polished, the bolt will almost fall into place. The case, itself, should have at least 0.001 radial clearance and 0.002 longitudinal clearance. This ensures no binding of the case at any point in the chamber.

 

[308sako]

Re: measuring headspace on once fired brass

deleted in protest!

 

[bdh308]

Re: measuring headspace on once fired brass

So it is possible to see variances in your once fired brass? Meaning the shoulders can be different in measurments. Thats what I got confused on but could never get an answer. Some of my brand new once fired lapua brass measured different at the shoulders and when I went back to check the new cases out of the box there was at least .001 to .004 difference. To really set up my Redding FL bush die I had to measure several cases to actually set up the die which sucked ass. Not alone ruin some brass along the way. Im honestly thinking about neck sizing the first 2 to 3 firings then using the fl sizer. I was just told that using FL die and bumping the shoulders back from the beginning is the way to go but hell, it takes forever to try and setup your die cause of the variances. I could never get my die setup properly because of this and it has become more work than just busting out the old lee collet and size the neck, prime and throw your powder charge and shoot the SOB.

 

[SPAK]

Re: measuring headspace on once fired brass

ok. here's some more info. I shot some virgin win brass yesterday, and there looks like a lot of stretching just ahead of the web. One has incipent case head separation (I can feel the line inside of the case with a paper clip) Measurement to shoulders varied from 1.949-1.952"

The load: .30-06 180gr spbt win brass (1.936" to shoulders new) 47.5gr IMR 4320 seated .02" off the lands. Max load is published at 49gr. I've shot up to 49gr but never had an incipent case head separation. Could it be just a bad piece of brass?

side note:

(Bolt is hard to close until the shoulders are bumped to 1.950" I placed a sharpie marker on the shoulders to see if there is any contact, I don't know if this works that well but you can still see a faint circle around the shoulder in the marker at 1.950" when chambering and it dissappears at 1.949")

 

[MitchAlsup]

Re: measuring headspace on once fired brass

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bdh308</div><div class="ubbcode-body">Some of my brand new once fired lapua brass measured different at the shoulders and when I went back to check the new cases out of the box there was at least .001 to .004 difference.</div></div>

Perfectly normal.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">To really set up my Redding FL bush die I had to measure several cases to actually set up the die which sucked ass.</div></div>

It is the price one pays for precision. In order to know, one has to measure, several times, before and after. Once you know, then the number of measurements can go down.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> Not alone ruin some brass along the way. Im honestly thinking about neck sizing the first 2 to 3 firings then using the fl sizer. I was just told that using FL die and bumping the shoulders back from the beginning is the way to go but hell, it takes forever to try and setup your die cause of the variances. I could never get my die setup properly because of this and it has become more work than just busting out the old lee collet and size the neck, prime and throw your powder charge and shoot the SOB. </div></div>

For ultimate accuracy, FL sizing is required. However if you are willing to allow the groups to open up 0.05" (from 0.3" to 0.35") then you can increase brass life by 3X-5X with neck only sizing.

It does NOT take forever to set up the die. What happens is the different pieces of brass get to different hardness levels, and will size differently in a die. Therefore, there is no ONE setting where the sizing die is setup perfectly. Thus, you measure before sizing and after sizing, and it its not right, you run it back over the lube pad and size it again until it IS right. Precision requires you to be a pain in the but to the reloading process and not let ANYTHING get out of spec. This requires measurements, lots of them, and it pays off once you finally isolate the anomoly that is making your ammo "not as good" as it could be.

ANd then there is the use of the press itself. If you load a case in the press, whang the lever arm down, and whang it back up, you will NEVER get the cases sized to the level of perfection required. You need to feel the case as it gets sized. So the lever arm goes down until you feel the case merge with the die, then you feel the sizing happening as you SLOWLY add pressure to the lever arm and hold the case at full compression for a second or so and then slowly remove the case from the press. Note: you slowly ad pressure, you should not be cognizant of the arm moving, just of the pressure being applied--that is sizing is a pressure induced phenomonom, not a movement induced phenomonom.