Friday, February 21, 2014

Military brass is thicker? (A repost)

(I'm having a discussion with someone regarding thickness variations between commercial and military brass, and thus... this repost.)

Carteach is in the process of load development for Liberty, the M1 Garand. This entails shooting... lots and lots of shooting... but that's only part of the happiness. It also requires lots of hours blissfully spent at the reloading bench.

Part of the process involves case preparation. On this topic, books have been written... but not by The Fat Man. Here, just one small topic is being addressed.

In choosing which cases to run the load development with, the entire 30-06 inventory in play. That means over 1000 cases of different date and manufacture. Amongst them, a fair number of military casings.

The thing is, there is a 'Given' in the reloading world. It's been engraved in stone for so many generations that it's accepted as a golden rule. That is: Military brass is thicker and heavier than commercial brass. That means the interior volume of the case is smaller, so therefor the charge weight must be lowered or excessive pressures will result.

But.... is this rule so golden?

Today Carteach took a bullet for the team, and weighed several hundred cases, both commercial and military. The results were unexpected. Weighing the cases is a traditional way of separating out cases into lots of like-volume. The theory being that all sized cases have outside dimensions pretty much the same, so any weight variations must point towards differing case thickness, and thus differing internal volume. (As one intelligent reader pointed out to The Fat Man... metal density comes into play as well. He proposed a simple test for internal volume, consisting of filling the cases with water before weighing them. This makes sense, as the weight on the water filling the case will vary depending on internal volume of the case. This sounds like a wonderful excuse for further testing, and another article here....)

The bulk of the cases were divided into three major types. Greek HXP, Lake City, and Commercial Remington and/or Federal.

Here's the unusual part.... on average, the commercial brass was heavier than all the military. Both Lake City and the commercial were heavier than the Greek HXP military. This indicates the HXP does not have a smaller internal volume, but in fact has a larger one. This changes the whole ball game when loading with the Greek HXP brass.

That chart below shows a clearer picture of what was found:

Now, I know this isn't exactly going to be wildly interesting for those not deep into the intricacies of hand loading.... but for those us who share that particular addiction, it's fascinating.


Randy said...

Years back when I was reloading 223 fairly often I noticed that the same load in commercial cases was a little bit faster than in surplus cases. (LC if I remember right) Some more checking showed that the difference was visible when both charged cases were standing side by side. So don't assume, measure.

Carteach said...

in my discussion with the young man, I advised him to buy a case of commercial 223 and shoot it up in his rifle. After that, weigh into piles by weight. The biggest pile is his load of matched brass for that rifle.

that's when the real work begins.

Hartley said...

Since the real point of this exercise is to determine relative case volume, why not measure volume for a few representative cases? It would probably be time-consuming to do a lot of them, but how about selecting a few particularly heavy and light representative cases and see how the internal volume compares? If using water or other liquid is a problem, you could use powder, say a nice small-grain ball powder, to determine relative volume.

Windy Wilson said...

Interesting. In 2006, when I competed in the CMP games at Camp Pendleton, the old guys there talked about how HXP was considered "hotter" than Lake City. From what Randy says, if it used the same load as Lake City, and had the same thinness of the case wall as you found with the .223, it might account for the difference.