Interpreting SAAMI Specs

[gregmark24]

In attempting to use a Hornady headspace comparator gauge to verify SAAMI specs, I find that I'm confused with the specs. Take the 6.5 Creedmoor as an example. The SAAMI spec from base of the case to the 0.400" dia point on the shoulder is 1.5438" - 0.0070". Let's say it is on the long end at 1.5438". The next illustration shows the chamber, with the spec from breech to that same 0.400" dia point as a minimum of 1.541" and a maximum of 1.551". Let's say it is on the shorter side at 1.541". Doesn't this indicate that, worst case, a case at 1.5438" is longer than the close-bolted chamber of 1.541" by 2.8 thousandths?

As a separate note, I think I've figured out how to get from the comparator gauge to actual lengths. The full length of my inserted gauge (red cut-out to silver colored insert face) is 2.004". However, there is a chamfer between the measured face of that gauge to the actual 0.400" hole. I THINK that chamfer is 0.010". So, in my case, to get from the measured comparator length (say, for a spent 6.5 CM case, is 3.538"), take the 3.538", subtract 2.004", and add back 0.010" for the chamfer loss, and get a true measurement of 1.544". Factory match grade ammo is 3.536" on the comparator--minus 2.004 plus 0.010" = 1.542, and within spec. I need to bump the head back by 0.002"

 

[can1010]

save yourself a lot of headaches by remembering Hornady's comparator is for comparing before and after measurements not for confirming saami measurements. if you're trying to measure a saami chamber for headspace you need headspace gauges.

 

[gregmark24]

What about the SAAMI specs and possible interference with brass that is too long for chamber?

 

[can1010]

the brass is sized to fit your chamber, no two reamers cut the exact same specs. now if your talking the overall length of your case just use book specs. for oal. and trim length.

Brownells sells a chamber length gauge that will tell you how long your brass can be before it starts to crimp into the bullet but that isnt necessary just use book specs. in your load manual.

 

[Ray Gross]

Doesn't this indicate that, worst case, a case at 1.5438" is longer than the close-bolted chamber of 1.541" by 2.8 thousandths?

Yes. It is normal among SAAMI specs for max cartridge headspace to exceed minimum chamber headspace. Keep in mind that there is a significant gap at the other extreme, from min cartridge to max chamber.

I specifically asked a couple of the SAAMI engineers about this at last years SHOT Show and was told that they had established that velocity SD's where significantly reduced for ammo that was tightly fitted into the chamber. As I recall, it had to do with the more consistent primer ignition.

I don't believe the intent was ever that commercial manufacturers would make ammo larger/longer than the chamber. In fact, I would suggest that the ammo manufacturers would (and do) run their manufacturing tolerances inside of the SAAMI specs, and below the minimum chamber specs. Indeed, that is exactly what you see when using commercial ammo and commercial chambers.