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Making Reduced Capacity Cartridge Cases
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Old 08-13-2017, 06:46 PM
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Default Making Reduced Capacity Cartridge Cases

Those that have been reading about my exploits efforts to get a GOOD, subsonic, .22 caliber bullet load will know that such a goal is not easy to achieve. (In spite of what the "internet" says.) I have come to the conclusion that the challenge is consistent ignition and burning. Even with the small case of the .22 Hornet, the fast powders don't occupy enough of the space in the case to ensure consistent ignition and burning for good precision.

Yesterday, Hunterbug sent me a picture of a cartridge that a German company is fabricating.


I think this is some .30 caliber cartridge, but for the time being, pretend it's a Hornet case that you are looking at.

It's obvious what's 'up' here. This company is making these for the purpose of shooting projectiles at subsonic speeds without having to add "filler" or some other 'stuff' that goes out the barrel when you shoot it. Hunterbug immediately thought of my subsonic adventures and sent the picture.

At the moment, I'm focused on .22 caliber subsonic, not the 'big' stuff, so my first thought was the Hornet. The final load I settled on was 4.85 grains of W-296. And that's NOT subsonic! That charge of powder is only 53% of case capacity, so I pack 1/2 of a single ply of one square toilet paper on top of the powder charge. (See here for details: http://thehunterslife.com/forums/sho...t=19530&page=2)

As you can see from the picture, the manufacturers turn the cases from "solid" round stock. I don't want to do that. I WILL do that, but I don't want to START there. Instead, my first thought was to fill the case with lead, and drill out the 'chamber'. I also considered aluminum and brass. I discussed this with j0e_bl0ggs, and he suggested "CerroSafe", the eutectic alloy that is commonly used to case chambers. (I contains 42.5% Bismuth, 8.5% cadmium, 37.7% lead, 11.3% tin.) I'm going to start with CerroSafe". If that works, I'll probably turn my own solid cases from brass round stock. Although, I might use any of the above metals melted and poured in existing cases. In order of likelihood: lead, aluminum, brass. They melt at the following respective temperatures (f/c): lead - 621/327; aluminum - 1221/660; brass - 1652/900 to 1724/940. I have the means to melt each of those, but as you can see, the brass has to get pretty hot. I'm pretty sure the hassles associated with melting and pouring brass would be more than those associated with turning cases from bar stock.

'Armed' with that idea, I spent the afternoon fiddling with QuickLOAD and various chamber volumes. I should digress a moment here to discuss "decapping". As you can see in the above image, one would have to have a special decapping rod to deprime the reduced capacity (hereafter, RC) cases. However, since the Hornet case is so small, there's really no need to make a separate (long) flash-hole. Instead, I'll simply use a drill/reamer of appropriate diameter and drill/ream down to the bottom of the case, using the diameter of the drill/reamer to control case capacity. This works especially well in the Hornet case because the drill/reamer cannot be larger than the case mouth - 0.223".

As it turns out, reaming right to the bottom of the inside of the case with a 5.5mm (0.2165") reamer is just about the best choice. That reduces the case capacity from ~14.5 grains of water to about 12.5 grains. Once the 70-grain bullet is seated, the case capacity is reduced to 7.75 grains of water. Again sparing you the 'gory details' of all of the calculations, that case capacity filled with 2.4 grains of TrailBoss, represents 100% of case capacity, gives 100% burn of powder in the barrel, generates ~21.5 kPSI max chamber pressure, and yields a MV of ~1100 f/s. None of the other powders did a whole lot better than in the 'large' capacity case.

I haven't shot any yet. In fact, when I finish this post, I'm going to make three. I'll report what happens when I shoot them in this thread.

Paul
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