A question about headspace.

[effinnewguy]

I bought an RCBS precision mic for .308 Win. I was reading the instructions that came with the box and was informed of the minimum-maximum headspace for .308 which is 1.6300-1.6400

I measured 10 unsized cases that were fired from my rifle. Here are the measurements (the figures below +1.6300):

.002
.001
.0015
.000
.002
.001
.003
.002
.001
.002

Getting an average from above measurements gives me a chamber headspace of 1.63175 which I will round up to 1.632, since the instructions say I can/should bump the shoulder back .001-.002

Am I doing this right? Is my chamber headspace the ANSI minimum of 1.6300? Because bumping the shoulder back from the figure above does give me that figure to work from. Thanks all.

 

[jdgray]

Re: A question about headspace.

I like shooting the brass until it just starts chambering stiff, by neck sizing only. Then I measure in a headspace tool I made that reads off the shoulder, then I slowly turn my body die in until I get .001-.002" headspacing. Once adjusted, I made reference marks on my die and press, so the die is always in the correct location.

 

[Deadshot2]

Re: A question about headspace.

I've noticed the same thing on my .308. Fired cases measure with less "headspace" than the "minimum SAAMI" spec. When I first noticed this I chambered a couple of different factory rounds and there was no issue.

I wrote it off to my measurement method. To really find the "headspace" of your rifle a gauge set works best, not measuring the fired cases. You could also cast the chamber with cerrosafe.

Just shoot away and then use the precision mic when setting a new die for shoulder bump. I have all my dies set up in L-n-L bushings so they remain in adjustment from use to use.

Follow jdgray's method of just shooting the case until it gets snug when chambering then adjust for "bump". It usually takes a couple of firings to really stretch out the case.

 

[country888]

Re: A question about headspace.

Yes your measurments look correct. I use the Hornady lock n load gauges and measure 5 fired cases, then bump the shoulders back .002 and shoot.

 

[effinnewguy]

Re: A question about headspace.

Thanks for the replies.

Now I'm trying to figure out seating depth and their instructions are driving me nuts. I was able to use the freebore tool but am lost from that point on.

 

[stangfish]

Re: A question about headspace.

What do you mean lost?

 

[kaseyfield]

Re: A question about headspace.

Lands should be same style method..

 

[jdgray]

Re: A question about headspace.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: effinnewguy</div><div class="ubbcode-body">Thanks for the replies.

Now I'm trying to figure out seating depth and their instructions are driving me nuts. I was able to use the freebore tool but am lost from that point on. </div></div>

Lightly neck size a case, so a bullet will slip in easy by hand but not fall out, and chamber it in your rifle seating the bullet with the lands. Carefully extract and measure....

 

[Fuzzball]

Re: A question about headspace.

Your case gage is an excellant comparitive tool but it's not measuring headspace (that's the chamber), it's only measuring the case shoulder lengths.

Forget 'averageing' anything, your cases have expanded to full chamber size and shrunk back some amount depending on how hard/springy the alloy is - and that varies by work hardening each time it's cycled. The only case length you need to be concerned about is the longest because shorter ones will obviously slide right in.

Size your cases so the longest sized shoulder matches the longest fired shoulder length and forget any more set back; that's often posted "conventional wisdom" BS. The original shrinkage insures your matching reloads will easily chamber and fire with minimum stretching.

The RCBS Case Mic is great but that "freebore tool" is, for most of us, useless. Thankfully, we can do what needs to be done quite well without it. Most factory rifles and common bullets shoot best from 20 thou off the rifling to as much as 5 times that much so all we really is an approximate max OAL to the lands to start. If we start our shooting seating/OAL experiments maybe 15 thou off that max length and work back in 5 to 10 thou steps we will be quite safe and will usually find a good seating OAL pretty quickly.

Good luck!