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Introducing the 8mm KMP

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International Auto Mag
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    Posted: 27 Jun 2012 at 7:27pm
Introduction to the 8mm KMP

That’s right, two plus years in concept and with all the enthusiasm of the newly made Maynard Arms barrels, I decided to develop a cartridge that is just screaming to be made, the 8mm KMP.

Basic History:

To start with, all I got to say is, “why hasn’t this been done before?”  The beauty behind this cartridge is that there are no special cases that need to be developed/manufactured and no special dies or reamers to custom order. These are all standard off the shelf components that anyone can purchase. Here’s the simple history, I started with the historic and well known German World war II cartridge called 7.92x33, a.k.a. 8mm Kurz. The 8mm Kurz was originally developed for the Sturmgewehr line of assault rifles. The Sturmgewehr was most commonly known for one particular version, the MP44. Without getting into the complex history of the “Storm Rifle”, I will say this much, the 8mm Kurz is basically a shortened 8x57mm (8mm Mauser) cartridge developed for short and medium range warfare (street battles/urban fighting). The full-size 8mm Mauser seemed to be a little overkill, in most applications, therefore the 8mm Kurz was found to be the best cartridge for the modern battlefield and set the benchmark for most, or all the assault rifles of the world, i.e. the Kalashnikov and Stoner platforms, just to name a few.

The Cartridge Design:

The 8mm Kurz utilizes a similar case base to the 8mm Mauser (similar to a .308 WIN) and the case length is almost identical to a standard AMP cartridge (1.300-inch). At a glance, the spent case could almost be mistaken for a .357 AMP case, but with careful examination, the 8mm KMP (7.92x33mm) case body is more tapered then the straight-walled AMP case, plus the neck length is shorter than a .357AMP case.  

Projectile Options:

The options for pistol style 8mm bullets are limited to non-existent. Also, one should not confuse .32 pistol projectiles (.312 Cal) with 8mm rifle projectiles (.323 Cal).  The 8mm Kurz development chose the path the rifle took, using the larger diameter rifle bullet. One known pistol did use a .321 projectile (8mm Nambu), but this rare selection is under 90 grains in weight.  Even if one wanted to use .312 pistol projectiles, it’s hard to find one that’s more than 90 grain in weight. There are options to the bullet dilemma, for example, one can use a solid lead projectile (casting) or a specialized projectile can be machined from soft metal bar stock, like bronze, brass, copper etc. I’ve played with the odd and unorthodox way of reversing a rifle projectile and re-crimping it into the case. The method has been done on an “experimental basis” with varying degrees of accuracy. At this time, I only attempted this out of a 16-inch barreled rifle (MP44) with a few rounds of ammunition. The accuracy wasn’t that much worse then what the standard rifle ammo produced. The difficulty about the backwards bullet method is, you need a “boat tail” bullet and most bullets for 8mm are “Spitzer” shaped (flat-bottom). The Boat-tail design mimics a pistol bullet nose the best, which aids in the feed process. As mentioned before, I will look into manufacturing a batch of .323 heavy weight (150 gr. and up) pistol projectiles, and in the mean time I will continue to experiment with loading bullets backwards.

Ammunition Option:

Seeing that the Automag barrel will be chambered in the standard 7.92x33mm rifle cartridge, the free-bore and throat will theoretically chamber the standard 8mm Kurz “rifle” ammunition. Therefore, in theory, the Automag will fire the longer (1.88-inches overall) rifle ammunition.  Unfortunately, the Automag magazine only has the ability to accept a cartridge that measure 1.600-inches.  It’s hard to get anything longer into that magazine. I discovered this during the development of the .454 AMP (.454 Casull Rimless) ,which is 1.660 inches at its shortest length (with a light grain projectile), but one can “hand-load” a single 7.92x33 rifle round (using a 124-grain rifle projectile) through the ejection port, chamber it and fire it.  I believe this would be totally safe and the chamber pressure wouldn’t be much different than the .30 LMP or.357AMP cartridges as the muzzle velocity and energy are very similar for both.  For example, the 7.92x33 rifle round fires a 124 grain Spitzer or Boat-tail bullet at 2250 fps, with the muzzle energy of 1408 ft-lbf.  So, it’s possible to have a rifle round, as the first round in the chamber, followed up by magazine fed 8mm KMP pistol loads. Interesting, I think. 

Ammo Supply:

Ammo has been made since the early 1940’s, but most of this early ammo is either collectable and/or not reliable. Plus, they were made with steel cases. Recently, a large batch of ammo was produced in brass and reloadable. Hornady even repackaged a quantity of Serbian made (Prvi Partizan) ammo, but it usually sells for ninety cents a round! Save your money and shop. Either you can buy brass (fired or unfired) from Gunbroker or from reloading supply companies. You can even purchase cases of ammo from Cole’s Distributors, $549 per 1000 rounds. ($0.54 per round). Here’s a link:  http://www.coledistributing.com/MM046.ASP?pageno=32&aProds=FNM27,FNM27A,FNM27B,FNM27C,FNM27D 

Newly manufactured reloading dies can be bought for $60 and up. Just Google and find Hornady’s entry.  Example:  http://www.shtf-gear.com/reloading/dies-parts/die-set-7.92x33-kurz?cPath=1545&m1track=googlebase&utm_source=googlebase&utm_medium=cse&utm_campaign=export_feed 

Currently, Brian of Maynard Arms is making up a couple barrels for test purposes. If all pans out (there is no reason why it wouldn’t), the barrels will be available through Maynard Arms Co. and specially made .323 pistol bullets will be offered around the same time.  Stay tuned…
 
From left to rifle: 8mm KMP, .357 AMP, .40 KMP, .44 AMP
 
MP44 "Sturmgewehr" in 7.92x33mm (a.k.a. 8mm Kurz)
 
7.92x33mm rifle ammunition, made in Serbia for the FNM of Portugal.  Currently offered through Cole Dist.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ginsaw Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 7:50pm
Eric, I'm all for you on this. And just to be clear this time, you mean the original Auto Mag more so than the AM3-5, right? That's what it sounds like, unless Brian is doing barrels for the others.
 
We all salute the creative spirit, inventiveness and resoucefulness on producing or thinking up any new cartridges.
 
And I'm not even going to ask if another new AM round is needed, or if it's the classic case of an answer to a question that hasn't been asked. Because that's obvious. But it doesn't matter in the least. The fact that you've come up with something new and novel is plenty justification enough. Take that as a compliment in other words.
 
I'd say just one serious "but". I really think where attention IS needed with the AM is for someone with CNC capability to look at producing replacement parts. Is there any chance of that happening?
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Luc V. Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 8:17pm
'Quote'  but one can “hand-load” a single 7.92x33 rifle round (using a 124-grain rifle projectile) through the ejection port, chamber it and fire it. I believe this would be totally safe and the chamber pressure wouldn’t be much different than the .30 LMP or.357AMP cartridges as the muzzle velocity and energy are very similar for both. For example, the 7.92x33 rifle round fires a 124 grain Spitzer or Boat-tail bullet at 2250 fps, with the muzzle energy of 1408 ft-lbf. So, it’s possible to have a rifle round, as the first round in the chamber, followed up by magazine fed 8mm KMP pistol loads. Interesting, I think. 
Sorry to be a spoilsport, but I think it's not 'totally safe' to do this.
Most people here advise not to drop the bolt on a cartridge inside the chamber.
Bad for the extractor, slamfire ?
Just my 2cts.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KMP Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 8:51pm

Luc, I agree with you in most respects. If using military primed brass, the primers will probably be harder then commercial primers. Also, the Automag does have a firing pin return spring, just to aid from issues like that. Plus, I don't recommend anyone slamming the action shout, no matter the caliber. Finally, it's bad for the extractor? I agree, to a certain extent. Question: how does the Automag normally chamber a live round? Well, it loads a round first, then the bolt and the extractor comes over the rim and locks in place.  This is not like a 1911, where the round feeds under the extractor. Also, if the extractor was made of proper material, there shouldn’t be an issue. Ever since I made extractors from ATS-34, I never had one break again.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Auto Mag Whisperer Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 9:35pm
When Eric first told me of his idea, I jumped in with both feet.  I too am having an 8mm Auto Mag barrel made.  Ric Mutascio is our resident balistics expert and he has suggested some loads for this new round.  I don't find it hard to believe that this cartridge has never been used before.  There was only steel cased, mostly Berdan primed brass before the Serbians made up this modern 7.92 x 33 brass.  I'm casting my own bullets and even bought a cannelure tool.  165 Gr. RN.  Congratulations on a great idea Eric.  What happened to the 32 KMP ?     Bruce
 
Note the gas check to prevent lead fowling at speeds in excess of 1400 fps. 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KMP Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 11:31pm
Bruce,
 
I look forward in seeing what you're building in the near future. We need to compare notes/data soon.
 
The 32 KMP might get mixed up with the .312 pistol caliber, versus the 8mm KMP would better discribe the true bullet size (.323). Also, It's the first metric Automag cartridge Wink
 
Thanks
 
Eric
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Post Options Post Options   Thanks (0) Thanks(0)   Quote yenko03 Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 12:19am
how you do this grip Pleurer
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MAX GERA Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 2:03am
Originally posted by Luc V. Luc V. wrote:

'Q
Sorry to be a spoilsport, but I think it's not 'totally safe' to do this.
Most people here advise not to drop the bolt on a cartridge inside the chamber.
Bad for the extractor, slamfire ?
Just my 2cts.

Sorry to be a spoil-spoilsport, Luc.  I know that this is a popular myth... But, the Auto Mag unlike other pistols and rifles (on which the rim must slip under the extractor as the round is stripped from the magazine), due to it's fully enclosed bolt face, ALWAYS  behaves the same way: it DOES NOT matter if the round is fed from the magazine or if it's feed directly into the chamber. In both cases, the round MUST be fully chambered before it's enclosed by the bolt face AND the extractor always engages the rim afterwards.

I either case, the chance of a slam fire, due to a stuck firing pin is identical. 

When I test fire new barrels, I always simply drop a round in the chamber and  release the bolt.

So, here you have it, straight from the horse's mouth.

Max
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MAX GERA Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 2:26am
Originally posted by Auto Mag Whisperer Auto Mag Whisperer wrote:

  I too am having an 8mm Auto Mag barrel made. 

I guess the cat is out of the bag... Nevertheless Bruce, we'll keep them guessing about the looks!Wink

BTW, it's going to be a hell of a lot more than 1,400 FPS.Clap
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ginsaw Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 2:30am
For no particular reason I'm in the habit of releasing the AM bolt but holding it just long enough on the way forward to soften the punch a little, instead of just really letting it slam forward. There's never been a problem with that. Is that for the best, or does it matter not?
 
And is slamming it on a 1911 any legit risk, round in chamber or not?
 
I'm afraid I've been doing just that since the '60s and maybe just been lucky so far. A lot of times I've been hitting the slide release button with the thumb. Bad idea?
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