Quite some time ago I embarked on a quest to wildcat an 8mm "Kurz" following the lines of what the Germans did with developing the 8x33 Kurz from the 8x57. My idea was to come up with something more powerful than the 8x33 but still fit the standard .473 case head/bolt configuration. With that parameter in mind I didn't want to necessarily design something so unique that I have to invest in manufacturing custom dies, particularly if the performance results turned out to be only so-so.

My design criteria were:
·.473 case head (rim)
·52,000 psi chamber pressure
·Push bullet weights from 125 to 195 grains
·Large enough case to not be pressure sensitive to minor changes in charge weights
·Feed reasonably in a short bolt action rifle
·Caliber length case neck

My solution was to use 284 Win brass that enables the use of a .473 rim but a fatter .500 case head diameter. I fitted that 284 brass into a cut down 8mm Rem Mag full length die to obtain the 8mm neck (.320 length) and 25 degree shoulder. The 284 case head diameter of .500 comes to rest almost half of the length of what would have been the original die before chopping it down.

 




Pictured are the cut down 8mm Rem Mag Trim die, FL die and seating die. Loaded cartridges are with the 150 grain Hornady bullets at OAL of 2.600. My case necks turned out a bit short due to my learning experience in sizing and fire-forming but more on that later.



Cartridge specs:
·.473 rim, .500 diameter case head
·.488 diameter shoulder
·25 degree shoulder
·.320 neck length
·Max COL 1.891
·58 grains water capacity

More pics and experiences to come on case sizing and forming. I should mention the test rifle chambered for this round is a short action barrel-nut Savage with a Shilen 10 twist 24 inch barrel. More on that too later.

Nels

Nels, I have been waiting a long time to see your project bear fruit.
Well done mate, I will be looking forward to seeing more posts and information coming forward about this your "Super Kurz" cartridge.

Could you please do a pic to compare your cartridge in size with, say, a .308?
Also what sort of velocity, energy etc are you expecting to get?
I think you have posted this before, a long time ago and it is buried within the THL archives.......

Here's a pic:
 

 
Before I get into performance expectations let me explain a bit why it took so long to come up with the final dimensions. I was stuck for a while trying to determine the case length. Based on industry dimensions I calculated one length but by cold-forming brass I'd get a big variance. True dimensions need to be obtained by fire-forming, but then I need a reamer to cut the chamber. How do I spec a reamer without really knowing the final dimensions? I was really chasing my tail on this one.
 
I turned the problem over to Dave Manson of Manson Reamers, along with my dies and cold-formed brass. He promptly whipped out a solution and ground me a reamer and go-gauge. The go-gauge is an invaluable tool to have to set head space and reloading dies and I am really glad I invested in this one.
 
Now with the reamer I got a Shilen chrome moly sporter barrel chamber cut, then I was able to headspace it by threading it into the Savage receiver until it stopped on the go-gauge and torqued down the barrel nut. Pretty simple.
 

 
Now I could fire-form the brass and get true dimensions, weigh cases, and obtain water capacity.
 

 
1.6cc of Red Dot, remainder corn meal and a drop of Elmer's wood glue. In case the initial shoulders are set back too far, I lubed the cases liberally with case lube to ensure full expansion. I learned that one the hard way.
 
Now knowing case capacity and plugging in the dimensions into QuickLoad I could then better predict load performance. For a 24 inch barrel and 52ksi chamber pressure QuickLoad gives me some pretty optomistic numbers. For example:
 
125 grain bullet over Win 748 - 3100+ FPS
150 grain bullet over Win 748 - 2900+ FPS
195 grain bullet over H BL-C2 - 2500+ FPS
 
To date I've only fired a set of 150 grain bullets down the barrel over a modest charge in the range of 46ksi to begin breaking the barrel in and getting the scope zero'd in.
 
Results so far are encouraging; the fat rounds feed fine in the short action Savage with a blind magazine. However, it only holds 3 down plus one in the chamber. I was able to zero the scope in quickly and the barrel appears to be settling in nicely.
 
Nels

Quote from: 22hornet;107631Nels, I have been waiting a long time to see your project bear fruit.
Well done mate, I will be looking forward to seeing more posts and information coming forward about this your "Super Kurz" cartridge.

Could you please do a pic to compare your cartridge in size with, say, a .308?
Also what sort of velocity, energy etc are you expecting to get?

Ditto!

Really looking forward to more of the tale.

Paul

One thing I left out in the previous cartridge forming operation was the use of a shell holder extension. This is a Redding and fits the Lee press.



Without it I'd be hard pressed (no pun intended) to do any forming or reloading this "Kurz" using cut-down dies.



A point in regard to QuickLoad is the fact that when I plug in this cartridge dimensions it calculates a weighting factor of .47. All the bottle-necked cartridges I've seen in QuickLoad are no less than .50. That may be one of the reasons it's providing optimistic load performance.

Nels

Quote from: Nelsdou;107679A point in regard to QuickLoad is the fact that when I plug in this cartridge dimensions it calculates a weighting factor of .47. All the bottle-necked cartridges I've seen in QuickLoad are no less than .50. That may be one of the reasons it's providing optimistic load performance.
 
Nels

What do you mean by weighting factor?

Hornet, you're making me do some homework, but that's OK, it's keeping me honest.

Out of the QL Manual, it describes the weighing or weighting factor as:

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Cool!

My view of the weighting factor is also that it is a 'fudge factor' implemented by the author of QL to make the predicted values closer fit actual values. I am unfamiliar with the Sebert's Factor.

The "weighting factor" is something I 'fiddle with' in QL to get the results to exactly fit a specific rifle of mine's actual output - AS LONG AS I don't move the weighting factor more than about 0.15 units. I think this factor, as I apply it, MAY account for how 'easy' or 'hard' a specific powder gets out of the  case at ignition, AND how 'smooth/rough' a specific rifle's bore is.

I'm just guessing at the causes, but if I adjust the fudge factor so that I get QL estimates that are very close to actual results, when I change components, I get results from QL that are closer to the new actual results.

It will be very interesting to see how well QL predicts the MV of this rifle when you finally get to shoot it. When I first started doing those comparisons, the one QL variable I always seemed to forget to adjust correctly was temperature.

Living vicariously through your posts,
Paul

PS - I note in my version of QL, the standard .308 Win case has an overflow capacity of 56 grains of water, and I see that your case has 58. Was that by design? In other words, did that volume influence your case length specs? You may have already explained this point in your earlier threads, but I don't recall.

Paul

The volume is what it is from the mating of the 284 case into the top half of the 8 Rem Mag.

In trying to spec out the final dimensions from the case drawings I made the asssumption the taper of the 8 Rem Mag from shoulder to case head was linear when in reality it is not. That error lead me to believe the Super Kurz cartridge length would be much shorter, as the .500 284 case head would drive deeper into the 8 Rem Mag die.  My calcs were somewhere around 1.690 for COL and hence a smaller volume.

But when I actually started cold-forming brass they came out considerbly longer and thats when I turned to the professional services of toolmaker Dave Manson.  I could have "forced" the dimensions for the reamer to be 1.690 COL but I wanted the cut down dies to work as-is and thus live with the resulting volume.

So far so good.  Re-sizing the fired brass through the dies went slick with minimal working of the cases. Now that I've got the scope roughly zero'd in my next load set will be for the 150s over H BL-C2 and walk the barrel starting at node 5 up in increments to node 4.

Nels

There are some mouths watering at THL Nels. When it comes to 8mm's I think you've hit on a winner with the Super Kurz. When formal production commences I'll be ordering cases and a savage barrel so keep us posted. well done.
 
Cheers
Jorge

Quote from: Jorge in Oz;107752There are some mouths watering at THL Nels. When it comes to 8mm's I think you've hit on a winner with the Super Kurz. When formal production commences I'll be ordering cases and a savage barrel so keep us posted. well done.
 
Cheers
Jorge

Have to agree with you Jorge. This is one cartridge that I am very interested in.
Current funds won't allow me to chamber a rifle in this cartridge yet, but later on maybe.....

I'm still unclear about the problems you had forming the cases Nels. Was it the case overall lenght that was too long or the measurement from the shoulder to the base? Was this due to the brass springing back after forming?
Do you think there is a way to form the cases with a non custom die and not have to fireform them later on?
And do you have to ream the necks of the cases? I take it that the shoulder of the .284 case is reformed into the neck of the SuperKurz?

Thanks everyone for the encouragement. :)

I'll get some photos arranged shortly that will help explain.

Righto about the thick case neck walls that have to be reamed. I supose having a trim die made or cut with the reamer then ream the necks would work but I liked having the assurance by fire-forming the overall cartridge would fit perfectly to the chamber that it is being fired in. Also too, one of my goals was to avoid any special or custom dies so that basically anyone could duplicate what I'm doing with an 8mm Rem Mag FL die, a trim die, a seater die and a hacksaw.

Nels

Here are the dies:


Left to right the cut down 8mm Rem Mag FL die, an 8mm Rem Mag die uncut,  the uncut 8mm Rem Mag trim die, the 284 brass after running through the cut down trim die, the cut down trim die, and a cut down seater die.
 
Notice how the 284 brass drops into the uncut Rem Mag trim die until its case head brings it to rest against the ID of the die:
 

 
Now notice the OD dimension of the unsized 284 brass:
 

 
Max OD should be .500.  When I started sizing brass with the dies before having a go-gauge, the varience in the unfired brass dictated the sized length because I didn't know where to set the stop and pressed until the brass itself stopped me. The individual pieces I formed that way all varied considerably in length. Once Dave Manson figured out the correct case length for me and ground the reamer and go-gauge, from there it was a piece of cake.
 
Here I set up the trim die and stop with the go-gauge:
 

 
Then push the shoulder back the proper amount and trim off the neck with a hacksaw and file.
 

 
Then fire-form and ream the case neck ID using a Forester tool:
 

 
And finish with case neck annealing and FL resize.  Doing it this way gives me a finished cartridge neck OD with loaded bullet of .349~.350 for a chamber neck ID of .352.  A check with blueing on the necks confirmed adequate clearance.
 
Here's a pic with a round in the Savage magazine that fits a bit more snug than the original 308 Win but feeds without a hitch.
 

 
And the almost finished rifle.  I swapped the original mounts with a Predator one piece base and rings.
 

 
Range results coming this weekend.

Thanks for the extra pics Nels. That makes everything much clearer now.

I like your style. Using and modifying ready made components. Great idea!

Your rifle is looking good too. I see that you still have some room in the magazine to seat out longer projectiles.
Did you use a Stevens 200 rifle as the donor or is this a Savage action and stock?

Just another thought. If you are using cut down 8mm Rem Mag dies then I guess it would also be possible to use an 8mm Rem Mag reamer to cut the chamber too? Just use the reamer to the depth that you need to achieve the desired case length. Or am I on the wrong track?

This is a Savage Model 11 I picked up on a sale that was chambered in 308 Win and has the accu-trigger which I don't believe the Stevens has. The key is the barrel nut system that permits fairly easy barrel swapping so bolt actions by Savage/Stevens, Marlin, and Mossberg were on my short list.

Using a 8 Rem Mag reamer and limit the cutting depth was part of the inital plan but I dropped it in favor of a custom reamer based on the advise of more than one gunsmith and I'm glad I did.  The 8 Rem Mag reamer drawing I obtained indicated it would cut a chamber neck diameter of .356~.357 which I think is excessive, particuarly for this short cartridge. I went with .352 which is still plenty big. The other aspect of the custom reamer is you can specify the freebore/lead based on the bullets you plan to use. That's the only two dimensions I took exception to from the 8 Rem Mag reamer specs but I considered them important ones to me.

A note about the Savage bolt head and extractor system. Feeding the Super Kurz into the chamber so far works very well but the extractor design has a couple of issues. It has a small spring loaded "tooth" that bites over the rim that doesn't grab on the rebated 284 rim as well as it does the standard 308 so when working the bolt back the cartridge always extracts but sometimes pops off in the magazine and doesn't clear the action. The plunger pin has also been sticky in its play but a slight honing should solve that.  When I was fire-forming cartridges and manually inserting them in the bolt face I managed to send the extractor tooth capture ball flying into space not once but twice and miraculously found it both times.  Just not robust as a Mauser claw.  I guess these small parts (ball, spring, extractor tooth) are spare part items every Savage shooter must keep on hand.

Nels