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Old 04-01-2013, 08:03 PM
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I'm getting back around to completing the "unique" version of my AR-15. A cartridge that will be used primarily suppressed but "full power" on occasion too.

Consistent with the way I look at things, I'm considering CASES, not calibers. Calibers are secondary, but not independent of the cases in question.

The three cases I am considering are the 7.92x33, 7.62x39, and the .223 Rem/5.56x45 NATO.

Obviously, if I'm considering the .223, I am planning on 'necking up' to something larger. The SMALLEST caliber I am considering is 8mm (.323).

There are some considerations as you might suspect. Two big ones are "bolt faces" and "magazines". The bolt face issue is easily resolved by using either the .223 or the x39 case. The AR-15 is made for the .223 so the bolt I got for it for that cartridge obviously will work for any .223-based cartridge. I purchased a bolt made for the 7.6x39 cartridge, so that is 'done'. The 7.92x33 has a "standard" large rifle head of nominal .473" (same as .308 Win, .8x57, .30-06, etc.) That's a bigger issue - but not insurmountable - for the AR-15. Just have to get your wallet out for a "specialty cartridge bolt".

Again, .223/5.56 mags will work for that case. By the same token, 7.62x39 mags will work for cartridges made from that case. Those mags will work for the 7.92x33 -based cartridges also. So the mag issue is essentially resolved. There WILL be some tweaking necessary for every cartridge regardless of the case used.

In the .223/5.56 case, the maximum caliber that case can be "blown out" to is .338. There is a "factory" cartridge on the .223/5.56 case in .338 called the ".338 Whisper." It is a very good suppressed cartridge using the 300-grain BTSP bullet. However, the "point" of that cartridge is PRIMARILY shooting paper. Some VERY impressively small groups have been shot at equally impressive ranges with that cartridge at subsonic muzzle velocities.

For the other two cases, the largest caliber that can be necked up to is .416. I kinda like that caliber. Of course I REALLY like 8mm too. So the caliber choices for the 7.92x33 and 7.62x39 cases are:

1) 8mm
2) .338
3) .358
4) .375, and
5) .416

Here are some pictures of dummy cartridges in these calibers.

Here are three on the .223/5.56 case. From left to right they two 8mm bullets and then a .338 bullet. The first 8mm bullet is a 240-grain cast bullet from a Lee mold. The second is the Hornady 195-grain "International". The .338 bullet is a 210-grain Nosler Partition. Note the slight shoulder on the 8mm cases.


These are the 7.92x33 cases. From left to right:
1) An 8mm cast bullet from a mold sent to me by Paul Hoskins, I think it's 150-grain,
2) A .338 200-grain Accubond,
3) A .358 225-grain Accubond,
4) A .375 260-grain Accubond, and
5) A 300-grain Hawk.


These are the 7.62x39 cases. From left to right they are:
1) 8mm 195-grain International
2) .338 180 Ballistic Tip
3) A .358 200-grain SPRN,
4) A .375-grain Accubond, and
5) A 300-grain Hawk.


And here's the rogue's gallery:


The bullets aren't TOO big a deal as I will choose the one that gives me what I want. Which brings me to the issue of just what is expected of a projectile moving at subsonic speeds and expected to be used on targets at ranges of 125 yd and less.

To start with, have another look at the 8mm cast bullet in the .223 case. It is as close to a cylinder as I could find. That's precisely what I think a subsonic projectile for use at short range should be - a cylinder. Ballistic coefficient is essentially unimportant below the speed of sound particularly at ranges under 150 yd. I want the best "hammer effect" that I can muster from a subsonic projectile. That is realized by having the largest frontal area as possible. Ideally, caliber diameter, which is a cylinder. However...

I have learned to appreciate feeding, especially in a semi-automatic rifle. Cylinders generally don't feed well from magazines in semi-auto or auto rifles. That's not to say they can't be made to, but in general, without tweaking to the magazines AND the feed ramp, cylinder shaped bullets are going to be a problem feeding.

There is another issue with the jacketed bullets. They are NOT going to perform as designed at subsonic impact velocities. (Another reason to use "cylinders".) Here's where things START to get 'sticky', and get worse as more elements are brought into consideration.

Since jacketed bullets aren't going to perform terminally as designed, kinetic energy becomes somewhat important. BUT... we're in the "handgun" velocity region, and experience says that if you have "big around" AND a blunt front, "lethality" is "good" as is penetration.

Recall that I have bullet swaging gear for 8mm bullets. Therefore, I can - and have - made 8mm jacketed cylinders that weight about 250-grains. That's "good". However, just "how good" is it relative to a 300 grain, .416, POINTED bullet? Or any other pointed bullet of larger caliber?

I've decided that the bolt face issue is going to exclude the x33 case from consideration for the AR-15. I'll relegate it to consideration for the AR-10 subsonic configuration.

Since I can get up to .416 on the x39 case, I am eliminating the .223/5.56 case from consideration, which obviously means that the x39 case gets "the nod". Now... what caliber???

You didn't think you were going to get away with no number crunching in one of MY threads, did you? I'll spare you 'til the next post.

Doesn't that x33 case make a seksy little cartridge?

Paul

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Tags
.223 rem, .338 caliber, .338 whisper, .358 caliber, .375 caliber, .416 caliber, 5.56x45 nato, 7.62x39, 7.92x33, 8mm, ar-15, bolt face, feeding problems, magazine, wildcat


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