I used the sinclair chamber length gage insert in a modified case as directed in the instructions leaflet. (though its not recommended for gas guns)

I removed my ar10 extractor from the bolt to enable me to insert and retract the bcg without the force of the casing getting kicked to the side.

I found that if the insert and case body measured 2.047 the bolt close easily and the insert wouldn't move and deeper into the case. If the insert and case body was anything over 2.048 the insert moved deeper into the case and the bolt was hard to close.

I assume that this means that my throat to case head measurement (overall chamber length) is 2.047.

If I take a measurement from the datum point to the case head (1.622) subtract it from that 2.047, that would give me the exact throat / leade of my chamber, right?    (.425)

I would reduce that by how much, to come up with with an ideal neck length?

Would I then take my ideal resized case measurement (from datum point to case head) and add it to this new neck length measurement and have my ideal trim to length?

I modified my .223 RCBS precision mike to measure the neck relief step of my chamber.  Turn a flathead screw down to .250 dia. on the head.  Replace the dummy bullet supplied by RCBS with this screw.  Take out the extractor claw and ejector pin from the bolt.  Insert the modified tool and close the bolt and push back on the delrin piece (white plastic).  Extract and measure the overall length (bolt face to neck relief dia in the chamber) using digital or dial calipers.  Do the measurement several times to get an avg, std dev, and extreme spread.  Use the statistical overall length minimum or -3 sigma to trim the case.

Unfamiliar with this term. got it. Standard Deviation...

This is 3 standard deviations below the avg value (a lower limit of the measurements).  It marks a 99.9% probability that the actual chamber dimension will be longer than that dimension (lower limit value) in the total population of measurements if you measured a large number of times, assuming a normal or gaussian distribution of the population of measurements (bell curve).

Mathematician I'm not. And I'm afraid to dig deeper here.

Are you saying, take 10 sample measurements, calculate the average, calculate the extreme spread, calculate the standard deviation, find the lowest overall length measurement and reduce it 3 standard deviations?

Minus 3 sigma takes care of the fact that you have not made a very large number of measurements.  It could very well end up that (avg minus 3 sigma) is less than the least point of extreme spread of your measurements.  Large extreme spread and/or wide data scatter will increase sigma value.  Small spread (sharp distribution curve) and/or narrow data scatter will reduce sigma.

And, AeroE makes a very good point.  If you are cranking out ammo for every man's AR15 rifle, use the 1.76+.00 -.01 for the trim length to assure a good fit.   That dim. comes direct from a 5.56 x 45 mil spec case drawing.  See below.  Also, as someone pointed out earlier, if you are roll crimping the case mouth into a cannelure, it is desireable to minimize the variation on case trim length as much as you can.

I'm sorry but I'm failing to grasp the reasoning for measuring this. If you are using a standard chamber (not a wild cat caliber) what advantage would you gain if you found you could actually trim your brass to .010" longer than the standard? I could see maybe in a short neck caliber such as .300 Win. Mag. where you may gain a little more neck length and it may help hold the bullet better but that is why they used the .338/300 and similar rounds for competition.

I used to make .280 Rem cases out of GI 06' brass for my silhouette rifle. Granted this is not a "Bench Rest" type of competition but the rifle would consistently shoot less than MAO. The fact that the converted 06' brass was only about 2.500" to 2.505" and the trim length for .280 is IIRC 2.530" never seemed to have an adverse effect on the rifles accuracy. I had the throat checked by a gunsmith before selling him the rifle and was told it was in good condition.

The firing pin strike can reduce the head to datum line measurement (push the shoulder back). This will let more neck protrude into the chamber, as much as .006" on a  223/5.56   Edit/add Rifle was a Savage Axis. Another person on this forum tested  with brass annealed & not. Both showed shoulder set back in his Savage bolt gun.

It has always intrigued me how a firing pin strike could drive the case that hard, but I suppose it could happen.  When you think about it,  that is similar what happens in a FL resizing die.

I was thinking about setting up an experiment, but I have the light springs in my colt and bushy fire control group so that would bias the results.  Also, part of that .006 is probably elastic and springs back, and part may be inelastic and remain as a permanent offset in the case length.  Anyway, the point is well taken.

Here are the results of my measurement techique described above based on 10 measurements for each rifle.

5.56mm NATO chamber,  casing neck relief step to closed bolt face

Bushmaster chamber...Colt chamber

1.7690........................1.7665
1.7685........................1.7645
1.7640........................1.7630
1.7690........................1.7630
1.7690........................1.7655
1.7675........................1.7650
1.7700........................1.7670
1.7705........................1.7655
1.7705........................1.7655
1.7685........................1.7670

Average=1.7687...........Average=1.7653
SD = 0.0019................SD = 0.0014

Low 3SD=1.7630..........Low 3SD=1.7609

So, for me and my AR15s, all cases get trimmed to 1.76+ .00 / -.01

I am equally intrigued (and surprised someone has not jumped in yet and restated) that "BlackHills does not trim to 1.76+00 / -01.  My unfired Black Hills cases run [ETC:] 1.80 1.78, etc."  But, that is another thread, I suppose.  

See: http://www.ar15.com/archive/topic.html?b=6&f=42&t=265440

And from the Molon report on Black Hills:

So where is the discrepancy coming from? Do ammo manufactures have more leeway (because of Saami spec) than the established trim lengths used by reloading data publishers? Or is the Black Hills ammo sold as 5.56 ammo and NOT .223 ammo? I can't see them taking such a chance. Jamming a case mouth into the rifling and causing a serious over pressure event IS one of the REAL safety concerns in reloading. Sorry for the sarcasm but I've seen too many "set-back" threads lately dealing with rifle bullets/ammo.

IIRC, the 1.76 +00 / -01 case length dimension is a .mil requirement, so it would be applicable to 5.56.

The IMI 5.56 MM, M855 in my stash is Trim length =1.756".
The XM193 M193 in my stash is Trim length = 1.755".

I will trim to standard. And I read it yet again.



I thought I would be able to control more overall length that way.  

I have manuals, I read the manuals, and go back to them often. However, as I'm still learning, I need to go over these concepts more than once.

So I ask my questions here and learn even if it means swallowing some pride. I've yet to ask a question and not receive more about it than I anticipated.

As a follow up, there are warnings in reloading manuals to never use cartridge cases that were used for reduced loads again with full power loads. These cases will steadily get shorter in cartridge headspace length each time they are fired because of the firing pin driving the case forward. I have seen cases that became .001 shorter each time you pulled the trigger. You can get away with it on a rimmed case like the 30-30 Winchester but not on modern rimless bottle neck cartridge cases.

As I was reading that I was thinking well I'm safe. Most of my plinking ammo is 7.62x54R but thanks for the heads up. I do have some rimless loads too. Funny thing is I always had kept that brass seperate and marked "plinking loads only". But for other reasons. Probably a good thing too because one of the rifles is a .308 with a Wolf extra power firing pin spring.

This thread kind of jumped topics a little. It started out being about case lengths then picked up this headspace thing too.