headspace, SAAMI, measurement and a lot of confusion

[solst_ice]

Hi guys,


According the SAAMI specs for the .308Win, the min headspace of the chamber is 1.630", the maximum 1.640". The max headspace of a cartridge is specified at 1.634", the minimum at 1.227" ( max - 0.007) . The datum line for all the measurements is located at 0.400" .

Last week, a friend of mine and I did extensive velocity testing of different .308 loads with Hornady 168gr HPBT bullets. The cases we used had been fire-formed in our rifles. My buddy was using S&B brass in his Tikka T3X Tac A1, I got Lapua brass in an AIAX ( equipped with a barrel made by Lothar Walther ). I never had an issue feeding the rounds, but my buddy could not close the Tikka's bolt sometimes. The 1st thought that came into my mind was about seating depth of the bullet. We measured them, but found all of them consistent at an OAL of 2.894", which was the desired OAL. ( 2.268" measured with the Hornady OAL gauge at the ogive). At home, I pulled the bullet, removed the powder and measured the length of the cases, which was around 2.006" - 2.008". That should be fine as well. After that, I used the Hornady headspace gauge with the D400 insert to measure the headspace of the cases at th datum line. The readings were around 1.622 - 1.625. This is still below the SAAMI minimum. We finally verified the cases in the Tikka's chamber and they did not fit. ( Bolt did not close). We had the suspicion, that the digital measuring gauge could be wrong and crosschecked with another one. Same values here. I measured the caliber of the D400 insert, which was right at .399 - .0400 . Next, I took same .308 go / nogo gauges and measured those:

Manson .308 NOGO: 1.625
Manson .308 GO: 1.620
Manson .308 Field: 1.628


Forster .308 NOGO: 1.622
Forster .308 GO: 1.616

This is weird. Nearly all of those are below SAAMI minimum for the cardridge.
I also measured my fireformed Lapua brass which gave me around 1.619" .

I also took a Redding FL body die and did a FL sizing of the S&B brass, that did not match Tikka's chamber. After resizing, the bold could be closed, but they measured 1.620.

I know, it takes some time to get used to the Hornady OAL gauge. But I've measured hundreds of cases using this item. The result don't change.
Does someone have any hint, what I'm doing wrong?


Best regards,

Frank

 

[spife7980]

The hornady bushings have a chamfer on the inside edge which leads to the differences, they are only comparators and not absolute measurements which is why your gauges all measure less than one would have thought.

http://forum.accurateshooter.com/th...dspace-gauges-any-good.3923217/#post-36955545

Measure fired brass from the tikka and adjust the die so that the shoulders measure .002 less after sizing. If you are sizing that down and still have the issue measure the base of those cases and see if they are getting sized down any.

 

[Sheldon N]

What Spife said.

It's a comparator tool, not for absolute measurements. Ignore the go-gauges, and just measure what fired brass comes out of your rifle at. Look for the case with a measurement of the longest headspace, not all will be quite as large as your chamber until you start to get to 2-3 firings on the brass. Then you want to adjust your sizing die so that shoulders are pushed back about 0.002 from that largest case measurement. Secondary step you can take when setting up sizing die is to make sure there's no effort on bolt close for empty brass. To do this you need to pull firing pin spring, remove ejector plunger and spring, then clip the brass case under the extractor and feed it into the chamber. You'll be able to feel if there's any resistance on bolt close or if the bolt handle drops freely. I set up my sizing dies to find that sweet spot where you have no resistance on bolt close but are not bumping the shoulder any more than necessary. It happens for me around 0.0015 to 0.002 shoulder bump.

Second thing to check is distance to lands of your bullet. You say that your desired OAL was 2.894" with 2.268 at the ogive, but you don't mention any reference of distance to the lands. You need to find the actual distance to the lands and know how far off the bullet is for that chamber. It's possible that the resistance on bolt close that you're feeling is bullet jamming into lands, not any issues with the brass headspace.

You can check distance to lands with a Hornady OAL case and OAL tool, or you can use a method involving "feel" with a bolt stripped as referenced above. Here's a youtube video explaining in more detail.



Final thing to consider when moving brass between chambers or when setting up sizing dies is the base diameter of the case just above the rim. That can be a possible source of interference in the chamber, depending on if your chamber is large or small, if your sizing die affects the base of the case properly, and what size chamber the brass was fired in previously. Measuring the base of the case as you go through the above steps (fired, sized, comparing chambers w/ fired brass from each) will help you know if this is part of the issue.

Two more videos that may be helpful...

 

[J!m]

Good post Sheldon.

 

[solst_ice]

Hi Spife, hi Sheldon,

thanks a lot! That answers my question perfectly!
@sheldon: You're right, I should have given the distance to the lands also. It was 2.271 measured with the Hornady OAL gauge. We were 0.003" away from the lands.
Thank you so much again.

Best regards,

Frank