Very minimal case expansion on 308 Win

[edhead35]

So I just got my RCBS precision Mic and my Hornady bullet comparator and headspace comparators in today.

If SAAMI minimum is 1.630 and that is where the RCBS mic is set at zero, then I found something I thought was strange.

Two bolt action rifles. I measured fired cases from both and with both the mic and the Hornady headspace gages, the fired cases measure within .001/.002 of each other across all cases in both groups from both rifles. This is the case across 30-40 total fired cases between the two rifles.

To add to this oddity, I measured the boxes of never fired factory ammo that fired cases came from, and they are SAAMI min, or -.001/-.002 under SAAMI min. The fired cases are +.001/+.002. Has anyone see fired cases only grow .002 to .004 after firing, let alone from two different rifes?

I then ran the RCBS FL sizing die, setup per instructions and it doesn't touch the shoulder.

I am going to fire two new cases, one from each rifle and confirm my measurements, but I have had this brass bagged and labeled since I fired them.

 

[Bullet Bill]

I'm curious, what ammo, brass, loads are you using. I only neck size my .308 and with factory brass like Federal and Remington I never have a need to full length size. When I use the brass from my NATO 7.62 ammunition (not LC brass) it is usually outside of spec after only one or two firings and needs to be full length sized to even chamber. All of my loads are near starting loads and no where near "hot".

 

[edhead35]

One set is once fired Hornady match brass from a factory box of Hornady superformance ammo. The other is Winchester and Remington brass from factory ammo, once fired.

 

[EddieCoyle]

I think you have your SAAMI mins mixed up. The SAAMI min for headspace in a .308 chamber is 1.630, with a max of 1.640.

The SAAMI spec for the headspace length on a .308 cartridge is 1.634" (-0.007"). This means that the SAAMI 0 for a .308 cartridge is 1.634", and it can be as much as 0.007" shorter and still be within spec. The 0 on the RCBS Precision Mic is 1.634"

The amount that fired cases 'grow' after firing is determined by the difference between the size of the chamber and the headspace length of the cartridges before firing. If they're close, you're not going to see much of a change.

What you're seeing is pretty normal for decent bolt action rifles.

What brand of shell holder are you using with your RCBS dies?

 

[edhead35]

What brand of shell holder are you using with your RCBS dies?

RCBS FL size die and RCBS shell holder.

The 0 on the RCBS Precision Mic is 1.634"

The RCBS precision mic instructions state that mic 0 is SAAMI chamber min, which is 1.6300 for 308 win
http://www.rcbs.com/downloads/instructions/PrecisionMic.pdf

What you're seeing is pretty normal for decent bolt action rifles.

I would have thought that the chambers would have been on the higher side of the 1.630/1.640 range for reliability of feeding, than for them to be say 1.632 plus whatever little bit the case shrunk down post firing. I am happy that they are so close to the min, because brass abuse will be minimal on the first firing of new cases.

The SAAMI spec for the headspace length on a .308 cartridge is 1.634" (-0.007"). This means that the SAAMI 0 for a .308 cartridge is 1.634", and it can be as much as 0.007" shorter and still be within spec. The 0 on the RCBS Precision Mic is 1.634"

So if a cartridge is at SAAMI cartridge max, and a chamber is at SAAMI chamber min, what happens to the .004"? Will the gun not chamber now, or is there enough excess to the breech face that creates a clearance situation? I would have expected there to be at least .001 difference from the limits.

 

[edhead35]

I also realize that in reality that manufacturers wouldnt be aiming to cause this interference, but I am thinking more about QC issues and some brass at max gets through or a chamber gets through on the mins.

 

[EddieCoyle]

I must've misread. I thought the precision mic's 0 was the cartridge min. You learn something new every day.

So if a cartridge is at SAAMI cartridge max, and a chamber is at SAAMI chamber min, what happens to the .004"?

The round won't chamber.

 

[JackO]

I think you have your SAAMI mins mixed up. The SAAMI min for headspace in a .308 chamber is 1.630, with a max of 1.640.

The SAAMI spec for the headspace length on a .308 cartridge is 1.634" (-0.007"). This means that the SAAMI 0 for a .308 cartridge is 1.634", and it can be as much as 0.007" shorter and still be within spec. The 0 on the RCBS Precision Mic is 1.634"

I just have checked the SAAMI spec for 308 Win cartridge and chamber. You are absolutely correct on both numbers. Although the RCBS Precision Mic (per RCBS instructions) is set to 1.630 zero, i.e. RCBS Mic is zeroed at the SAAMI chamber minimum.

To answer the OP question, what happens if case is SAAMI max (1.604), and chamber is SAAMI min (1.600) - I guess, it does not fit !

 

[edhead35]

I must've misread. I thought the precision mic's 0 was the cartridge min. You learn something new every day.



The round won't chamber.

I just wanted to be sure I wasn't imagining things. We fight with a couple engineers at work all the time because they will design a male feature
.5000 +0/-. 0002
And the female feature
.5000 +.0002/-0

when the parts come in at the max material Condition, or .5000 exactly in this case the line to line condition means it no goes into as we say. If it absolutely has to fit, you need a slight factor of clearance.

 

[Slowdive]

I also realize that in reality that manufacturers wouldnt be aiming to cause this interference, but I am thinking more about QC issues and some brass at max gets through or a chamber gets through on the mins.

I've measured unfired brass / factory @ 1.620", 1.618" or less w/ the Hornady kit which is way under the mins. My fired brass vary's by rifle, but it's closer to 1.630", usually lots of trimming involved after the first firing.

 

[Steve Marshall]

One thing all those "precision cartridge mic thingies" are missing, is calibration. Surely some of you guys have both mics and headspace gages?

 

[EddieCoyle]

One thing all those "precision cartridge mic thingies" are missing, is calibration. Surely some of you guys have both mics and headspace gages?

The RCBS Precision Mics I have are pretty much on the money. There's no calibration with the Hornady gage. It's a relative measurement.

 

[edhead35]

I ended up picking up a new set of Hornady dies. Hornady single stage press, Hornady shell holder, hornady dies.

When I setup the die to touch the shell holder at the top of the stroke before it cams over, the Hornady die doesn't touch the shoulder, and the fired brass is chamber SAAMI min, RCBS mic 0. I lapped the shell holder down some, and got the shoulder bumped back .002-.003 without too much issue.

I noticed something else. I sized some WIN and R-P brass, and it was all within .001 of each other at .002-.003 back as I stated above. When I sized some Hornady match brass, it came out .003 longer on the headspace dimension pretty much right at SAAMI min. Any idea why this would happen for a different brass manufacturer?

 

[EddieCoyle]

Some brass is springier than others.

 

[edhead35]

Some brass is springier than others.

Thanks for all of your replies everyone. Jumping into rifle finally and it just opens a whole new can of worms!!!

ETA:I also got my first case stuck with homemade case lube. 10:1 IPA and lanolin. I suspect I didnt let the cases dry long enough. By the time I got the stuck one out, the rest ran like butter.

 

[Steve Marshall]

At the risk of ruffling feathers, did you check the RCBS Mic against headspace gages. One that I measured was .006" off.

 

[EddieCoyle]

At the risk of ruffling feathers, did you check the RCBS Mic against headspace gages. One that I measured was .006" off.

I checked one in .223 (which is the only caliber for which I have both a Precision Mic and headspace gage). It was within 0.001". It possible that they can disagree and both still be right.

Even if it does not match the gage, it's still useful for a relative measurement: Measure the fired case, and set the sizing die to make it a hair shorter.