SAAMI Case Spec and Trimming

[curisu]

I searched, but couldn't find any real information on the subject (here and out in google land).

I'm trying to get into .223 accuracy loads, and the first step (for me) is uniform case prep.

What is known:
SAAMI spec for a .223 case is 1.760 length.

Cases shot from my rifle (once shot, factory loads) measure anywhere from 1.740 to 1.755 or so on my digi-calipers.

Cases I pickup at the range, bought cleaned and "prepped" from folks here, etc. can measure anywhere from 1.73x to 1.780+.

After i run my cases through the resizer die on my progressive (properly headspaced with a Dillon .223 gauge), the length changes, adding a few thousands. Fine. I then run the re-prepped cases through my trimmer. I setup my case trimmer (CTS Trimmer) to trim to 1.760, get fairly close (1.7585 - 1.7600).

After trimming, I tested a handful of shells through the progressive again, and the resizing die seems to pull another couple thousands.

From my last test, I had a case resized, trimmed to 1.759 or so, and ran it through the resizer again. It pulled it to 1.761x.

Is this normal and is this dangerous now that the case is past the SAAMI chamber spec?

TIA for any input or corrections!
-chris

[GeoffLinder]

Trim to length for .223 is 1.750", 1.760 is approaching max case length. Cases always grow after re-sizing. Always trim AFTER re=sizing.

1st step and single most important step in accuracy loading is to use one batch/brand of cases. 2nd step is to sort cases by weight and discard any outside a 1.5-2 grain weight range. Use discards in similar weights to do initial testing. These two steps alone are best things you can do to start.

Proper case prep for max accuracy continues with trimming properly (use a Gracey or Giraud power trimmer), internal flash hole reaming (this removes excess metal in flash hole / case web area), neck shaving (this uniforms concentricity).

After all the metal work is done, re-weigh cases and sort into 1.5gr batches.

Now start working with powders, bullets and recipes.

Cases must have uniform internal volume to produce same pressure from case to case.

Palma shooters learned this a long time ago. They must use issued ammo so they weigh each round and only use ammo batches in small weight range. The bullet, powder and primer have almost no weight variance in comparison to the case. Cases are all same on outside so weight variances are pretty much all about internal construction and this is what controls case volume, case volume controls pressure.

[curisu]

Thanks for the input - good stuff to mull over. And more stuff to consider buying... wife will LOVE that.

Since the necking/resizing stretches the brass slightly, it seems to suggest that case-length consistency is not as important as brass weight/volume? Is accuracy more a factor of the concentricity and throat crimp force?

[30Cal]

Max length is 1.76". Usually the chamber has a little more room than that. If the bolt doesn't close easily, you're definitely too long.

Trim length is always 0.010" less than max.

Beyond sorting by headstamp, making accuracy gains with case prep is a comparatively huge time investment with a very low return.


BTW, I weighed a bunch of my brass and set 20 aside--the worst I could find. Those are what I use for my final load testing, after I've tuned everything else. Then I load my worst cases up with the worst powder variance I get from my measure. If it still shoots pretty good, then I know my worst possible is still adequate and I don't have to weight/measure or second guess the small stuff.

[GeoffLinder]

Thanks for the input - good stuff to mull over. And more stuff to consider buying... wife will LOVE that.

Since the necking/resizing stretches the brass slightly, it seems to suggest that case-length consistency is not as important as brass weight/volume? Is accuracy more a factor of the concentricity and throat crimp force?

Weight segregation AND trim length (trim quality too, need a square to case trim and a decent deburring/chamfer job to prevent scarring bullet jacket while loading) are both important, case volume slightly more so. But don't ignore either when trying for accuracy.

Crimp may or may not help OR hurt accuracy. Trim length consistency is important as it effects bullet release tension. The interference fit of bullet into sized case neck is all that is needed to hold bullet tight, crimp is mostly a reliability step that is added for commercial and military use. Sometimes a very slight taper crimp can help reduce the velocity standard deviation a tad, but thats usually not very noticeable on the target face except at very long ranges where the tiny things start to show themselves.

Just using good components in mixed lot, unweighed cases with a charge weight/velocity combo that hits your barrel node will in most cases be pretty darned accurate out to maybe 200+ yards. It's at 300+ that all the extra case prep and good loading practices begin to show themselves. At 400-500+ is where the tiny things get big..