Making ammo for the 1896 Bergmann No. 3

My friend and employer, Brandon, has a habit of finding weird, old guns and having me and my coworkers make them go bang again. The latest of our adventures was an 1896 Bergmann No. 3 - a now-obscure model of one of the earliest semi automatic handguns.

This gun was originally chambered in 6.5mm Bergmann, an early and mildly humorous looking entry in the chain of small arms ammunition that lead us to today's calibers. Speaking of today's calibers, this one fires an 80 grain bullet at 750 FPS, adding up to exactly 100 ft-lbs of energy, right in the ballpark of .22LR.
I wrote "today's calibers" and then immediately realized that despite still being used today, .22LR is actually older, from 1887. The two different approaches for achieving the same muzzle energy are remarkably different.

One of the first things you may notice is the lack of an extractor groove. As far as I understand, the first patterns of Bergmann pistols lacked an extractor entirely. These guns run on a direct blowback, unlocked breech system. An interesting quirk of direct blowback is that you don't actually need an extractor. The casing is generally pretty firmly pushed against the bolt (it is the thing pushing it open, after all) and finds it's way out of the gun just fine. There is a downside however - unloading. Removing a cartridge from the chamber of an extractor-less firearm is a frustrating task, and one experiencing it immediately understands why it was added on later models.
Which brings us to a quirk of this particular example of gun - it is a later model. So why doesn't the cartridge have an extractor groove? Two reasons: one, the extractor is snapped off. Two, the barrel on this gun is from an early pattern example. In other words, it's an early pattern barrel shoved on a late pattern frame. Oh, the joys of working on a 130 year old pistol.
It turns out they changed more than just adding an extractor groove when they revised the cartridge. We painstakingly got our hands on a single live round which was, of course, the wrong pattern for our barrel. Everything from the diameter of the bullet, to the taper of the case, to the length of the shoulder was all entirely different. 
So we set about reverse engineering the chamber. A series of precision ball bearings, along with a depth micrometer, allowed us to map the case body very precisely. Other details, such as the shouder and bullet diameter, were achieved with a Cerrosafe casting. 

 Picture of solidworks ball bearing measurements here

Once we had a map of the chamber, making a model of a casing was fairly straightforward. We (meaning poor Nik) looked through a ton of CIP and SAAMI chamber and cartridge drawings to figure out the typical amount of clearance given in different areas of the chamber, and we drew a casing based on these clearances. We had to make a pretty major departure on cartridge OAL to get the gun to actually headspace. In a direct blowback gun set up like this, I set the headspace as the bolt stopping on the case head with 0.005" of clearance from the barrel face (which is the impact surface on this gun).
From there, we set up brass bar stock in my CNC mill (we don't have a CNC lathe yet) and machined brass dummies, tweaking the headspace length and shoulder shape until everything was happy.

Picture of shoulder contact here

I then made a very bad guess at the thickness of the brass casing internally, and hollowed out the model to approximate the internal cavity of a real casing. Taking the dimensions of the casing from Solidworks, especially the volume calculations, I went over to Gordon's Reloading Tool and set up a custom caliber. If you don't know, GRT is a free internal ballistic simulator with a large library of different types of gunpowder (some better than others - we'll get to that).
After defining the cartridge and chamber, it was time to choose a powder. The original powder is a long discontinued type, somewhere in between black powder and smokeless. The biggest hurdle with choosing a powder in this instance is case fill. If you have a big open cavity with only a little bit of gun powder in it, the primer flashing over the top can ignite all of it at once, causing a detonation instead of a burn.
We dug through the library entries in GRT until we found something with a pressure curve that felt right, and a low bulk density (IE the gunpowder is very airy and takes up a lot of space). 
According to GRT, we needed 1.7 grains of Vihtavuori N32C powder, most commonly used for Cowboy action loads. In typical fashion, this powder was also discontinued. Nothing else in GRT was even remotely close to viable. Many people reccommened Trail Boss, and while I think it would probably work, it's not available for the simulations and I really didn't want to break this gun. Even using this fancy Vihtavuori powder, we were still sitting at a case fill of about 40-45% - right on the edge of the danger zone for flash ignition. More on this later.
I asked around and got insanely lucky - my friend Josh happened to have an unopened canister of the N32C sitting on his reloading bench. Thanks Josh!

Picture of the weird powder here

Pressing onwards, we machined a few of the casings. Using a lolipop end mill to get the undercut inside, and a slitting saw to part them off.

Picture of casing here

A strange discovery we made while mapping the chamnber was that the bullet diameter changed quite substantially. The old original bullet is actually 6.5mm /  0.256". The grooves of the rifling in this pistol come in at 0.267", a difference of 0.011"! That's enough to blow something up, if you even manage to get it to chamber.
Typically for an old cast bullet cartridge, the bullet diameter is specified as about 0.001" under the groove diameter to limit pressure spikes. Of course, a 0.266" 80-grain cast bullet was not something we could find anywhere. Thankfully Lee Reloading sells blank bullet molds, so we just made a mold and cast our own. 
("just" I say, as though it didn't take days)


Now that we had all of the components, it was time to load a round and see what happened.
This is where the case fill problem comes in. After talking to quite a few reloaders, we decided to use Cream of Wheat to fill the empty space in the cartridge and hold the powder firmly against the rear of the casing. Cream of Wheat doesn't really add or subtract from the burn rate of the powder, and mostly makes it's way out of the barrel when fired. 

Well, shit.

The first round fired, and ejected (so I thought). The chronograph read the FPS as 740, very close to the target! So I chambered another round, not knowing that the first one had actually split. I then immediately discovered that the Bergmann 1896 No. 3 does not have an out-of-battery safety. The new cartridge was sitting on the remnants of the old one in the chamber, and fired out of battery, rupturing it.

Luckily, the gun and my hand were entirely unharmed. 

After this humiliating defeat, I decided that we really were obligated to cut open the original cartridge. I initially didn't want to deface history like that, but at this point it was either cut the ammo or risk hurting the gun again.

Don't mind the long proportion, I couldn't be bothered to push the bullet all the way back in before taking the picture.
Cutting open the ammo confirmed my suspicion - the gunpowder cavity was a good bit further away from the case head than mine, and the case walls were roughly twice as thick.