I occansionaly read some of the benchrest crowd's developments because I like to pick up some revelant tips and tricks.  Always seems to be some benchrest R&D effort going on that's interesting to read about.

Over the last few years the PPC and BR rounds looked interesting but generally looked highly specilized to be just paper punchers with little other utility.  But some the later ones appear to be a bit more "snappier", such with those in 30 caliber.

Reflecting back on history, the first of smaller but lethal modern rounds started with the 7.92x33 "Kurz".  H'mm, if one wanted to re-invent the Kurz today with today's brass and powder choices, what would it be?  If one could do some extrapolation from those the BR guys are using, I'd think about necking up a 30 BR or the 1.5 Barnes to 8mm.  Use the 125 grain Hornady bullet and 308 Win brass as the base.  If using a parameter like case capacity to bore ratio as a guide from the PPCs and BR cartridges, the 1.5 Barnes necked up to 8mm might actually need to be about 1.6 inches in length or so.  (Interesting to note that is about the length of the 7.62x39.)

I haven't calculated the optimum barrel twist for the 125 Hornady, but my guess it would be slower than the standard military Mauser twist that would also aid in efficiency.

My quick run-through of 8mm wildcats doesn't show anything "invented" like this other than the 8x51, but I could be wrong.  Anywho, just some musings about what an "optimized" Kurz might look like today.  Maybe a project for one of my old mausers.

Nels

I wuz thinking about - and still am - building a set of rifles on a specific caliber instead of a specific case. Of course my first caliber of choice was the .323, and I intended to start at the '06 case at 63mm and go down in 6mm steps to 57, 51, 45, and 39. All of which except the 39 are cartridges 'long in the tooth' already. However, I did not consider "optimizing" the cases.
 
"a parameter like case capacity to bore ratio"
 
Is this case capacity to bore diameter or bore volume? The reason I ask, is that of course bore volume would be a function of barrel length, so case capacity could easily be matched by changing barrel length. So... I assume this refers to the case capacity-to-bore-diameter ratio.
 
I do like that 125-grain 8mm bullet, and hope the short-throated 8mm x .376 Steyr I've made shoots it well.
 
Using QuickLoad to 'make' a .323 cartridge based on a Mauser case but only 39mm long, I get the following theoretical numbers:
 
Case capacity would be in the neighborhood of 43 to 45 grains of water.
Seating the bullet .67 calibers deep (.216"), and using 33.8 grains (99% of capacity) of Rel-7, a muzzle velocity of 2927 f/s might be realized in a 24" bbl. at a chamber pressure of 56,565 PSI (CIP Mauser pressures).
 
Paul

That's pretty neat of QL to be able come up with some estimates.  With what I read the 30BR pushing a 125 grain bullet to 3000 fps in a slow twist long barrel, I was optimistically guessing the 8mm 125 might go 2700ish fps in a similar cartridge but shorter barrel.  Interesting too is that QL came up with the same amount of powder that the 30BR shoots.

Still, I'm amazed at the velocity posssible on such a meager amount of powder.  Looks like a 150 grain spitzer might also be within the realm of this theoretical "Kurz".

Nels

QuoteLooks like a 150 grain spitzer might also be within the realm of this theoretical "Kurz".

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Oh I think so. Taking the "African" perspective of 2400 f/s being a 'special' velocity particularly suitable for penetration, a 150 can reach that easily in  24" bbl. If you want to push it to 2700 f/s in a 24 in barrel, 36 grains of Ramshot X-Terminator will supposedly do it. That's likely to be a coompressed charge as the loading % is 106. Do-able without doubt, but it will require a slight bit of compression. Still below CIP 56,565 PSI for Mausers at 56,000.

Increase the bbl length to 26", and the load to get to 2700 f/s is 35.35 grains (105% of capacity), at 53,000 PSI.

Of course these are truly only estimates, but I feel confident that the numbers are almost certainly within 100 fps of reality.

Sighted in for a 6" target, a 150 Hornady Spitzer doing 2700 at the muzzle will be almost 8" low at 300 yds, doing 1929 f/s and carrying a little over 1200 ft-lbs of energy. Zero is 220 and max-point-blank-range (-3") is 257.

A heckuva "deer cartridge" for something like a Ruger Ranch Gun.

Paul

Ackley told me years ago it is always easier to get more velocity out of the larger bore than a smaller bore if bullet weights are the same. The bigger bore has more "piston" area for the pressure to work on. In checking the 7mm Mauser against the 8mm Mauser I found this to be true.  ............Paul H

Next phase in the "Super Kurz Daydream":

I landed a few pieces of 7mm Rem BR brass and some 125 grain Hornady bullets.  Necked a few pieces of brass up to 30 cal then to 8mm.  Getting to 30 cal wasn't too bad but getting to 8mm is almost impossible without getting the necks off center and making a "doughnut" at the base of the neck.  To do it right I believe fire-forming would work better and have the brass flow "out" and not "in" from the mandrell.  Case length before forming was 1.510"; after getting to 8mm; 1.490".  I also measured water capacity in grains after forming just to see what I could get and it was 35~36 grains of water coming just up into the case neck (useable capacity-not fired).

Anywho, got a couple sized "close-nuff" to seat some 125 Hornadys to get an idea of what I'm working with.  Kinda dinky but kina qute next to a regular 308 Win case:


And another shot to get a feel for "size".


Next I cycled these through a k98 chambered in 7.62 Nato and they fed and extracted without a hitch.  I just have to rig up a "block" of some sort and modify the follower to keep the cartridges from shifting too far forward in the magazine to ensure the bolt face "catches" the shell.

Now I did think about forming my own custom length cartridges from 308 brass, but necking up the 7mmBR brass gives a me a quick prototype to evaluate.  I would like to have the cases a 'smidge longer to get about 41 grains capacity but more on that when I get to a custom reamer.

Now for a barrel.  My best guess is to purchase a barrel blank bored .323 because it will be custom reamed anyway or, getting a LW chambered for 8x33 kurz and reaming it.  Twist wise, it seems all I find are 9 to 10.  For 125 to 150 grain bullets that twist seems excessive when I run the numbers, and this was using the Lilja calculator.  I was hoping for maybe finding something in 12.

I'd welcome any ideas on "shortcuts" or  '"whoa there!" before I pass the point of no return.

Nels

Very cool there Nels.
 
Y'know... I've yet to do it myself, but I have a couple of books with drawings and pictures of a fire forming device that seems really easy to make. Essentially it is just a resizing die with a way to close the 'breech'. I'll find the books, post a couple of pictures and list the citations.
 
The 7mm BR case seems like a good start to me, and in the end may prove a better choice than .308 cases due to the slightly thicker walls on the 08 case. Also, if you 'shrink' a .308 case wall you're almost certainly going to have to turn the necks. 'Swelling' a 7mm BR will only thin the neck. I prefer the latter in most instances.
 
What about the .30 BR case to start with? It doesn't have the same shoulder as the BR, but firing will form that. That would reduce the total distance you had to enlarge the neck by 0.24" (.308 to .323 vs .284 to .323). 24 out of 39 is a big difference.
 
Finally with regard to forming Kurz cases from 7mm BR cases, did you anneal the necks first? I didn't used to do that automatically, but when blowing out the .348 Win to .50 Alaskan it made a difference. I would expect it to help with the 7mm brass.
 
As for twist rates, I can appreciate a desire to 'fine tune' things, but I doubt a 1:9 or 1:10 twist is going to effect - in a measureable way - the internal or external ballistics of this cartridge and bullet. If you are concerned about the extra energy used in rotating the bullet faster than it has to be, I 'did the math' on that a couple of years ago and even in the most aggregious cases, the cost in over-rotation was less than 3%. I'll look up the numbers for 8mm, but I would expect it to be at or below 1%. If your concern is precession, well... I can't help you there. Do keep in mind muzzle velocity on this cartridge with the 125. With a 1:10 twist and a muzzle velocity of 2900 f/s the rpms generated are 208,800. Most folks that worry about precession don't do so 'til the rpms get over 300,000. I don't ever worry about it.
 
Again, very cool stuff. You know how much I like to see people 'fiddle around'. Besides, you're breakin trail for my brace of 8mm cartridges from 39 to 63 mms. :)
 
Paul

Gee, reading the info at this site, http://www.6mmbr.com/30BR.html I'd hafta say that if the .30 is so much better than the 7mm, then the 8mm - with the same 125-brain bullets - should be that much better than the .30...
 
Yikes!:eek:
 
I took all the info I could from the above site, and redid the numbers for the "Super Kurz". Using the recommended H4198 powder and a 24" bbl, QL gives a MV of 2880 f/s at a chamber pressure of 56,553 PSI. The BC of the Hornady 125 is listed at .248 - about the same as the 112-grain .30 caliber specialty bullets listed in the above article. Using those numbers, QL predicts the following trajectory:
 

 
I suppose you could up the BC by having custom 125-grain .323s made. Might get it to .300 without too much effort.
 
I think the guys in the article are getting higher velocities (3050) in part because they are running their loads up to the SAAMI max listed for the 7mm BR of 60,000 PSI. More importantly, I suspect it's just QL's typical 'conservativeness'.
 
Your idea sure looks good on paper Nels.
 
Paul
 
Oh yeah.
 
I did read all the comments on twist rates, with the recommendation for the .30 caliber being between 1:15 and 1:18, "with the advantage going to 1:17 or 18". I find it difficult (but not impossible - to argue with people that are actually shooting bullets as opposed to those that are simply solving equations (me). After all, they are the ones producing the 0.122" groups. And 0.122" groups are pretty tough to argue with :). Ialso note that one of the reasons for the slow twists is reduced torque at the bench. Since I've never shot a barrel with a twist slower that 1:12 in a centerfire rifle, I can't comment on the significance of that in the hunting field. Given that 'those guys' critisize the .30 for its recoil, I'm guessing that I wouldn't find torque an issue.
 
I can only say that maybe the twist rate isn't the only variable associated with the improved groups. It's so difficult to "change a variable and test again" without putting on a new barrel. And when you put on a new barrel, even discounting all the hassle and cost associated with that, it's really difficult to be certain that one variable is all that has changed in the new barrel.
 
While it might sound like I'm advocating for faster twist rates, that's not the case. What I am advocating for is 'simplicity', as long as it doesn't have an adverse effect on the ultimate goal. Nonetheless, these folk's opinions would suggest that 'simplicity' (getting a 'regular', over-the-counter twist rate) does indeed effect that ultimate goal - maximum accuracy. Like I said, I'll not argue against real-life results... but I will 'keep looking' for why real-life apparently disputes theory - theory that otherwise predicts reality well.
 
Paul
 
I looked up the "cost" of using a 1:9 vs a 1:18 twist rate. The 1:9 twist rate uses 0.48% (.0048) more of the powder's energy than a 1:18 twist rate does. At 2900 f/s that translates to about a 14 f/s increase in velocity.
 
I will add that the twist rate calculator that I use has the minimum twist rate to "maximally stabilize" a 125-grain .323" bullet that is 0.838" long doing 2900 f/s at 1:31.5. Looks like 1:18 is a long way from that, and 1:9 even farther. Of course at 1:31.5, you couldn't shoot any bullet longer (heavier) that 0.838" (125-grains). A 1:31 twist rate would theoretically increase MV by 17 f/s over a 1:9 twist rate.
 
It also says the minimum twist rate required to "maximally stabilize" a 195 grain bullet doing 2456 f/s is 1:19.77. That means that one could use the 1:18 twist suggested by the above shooters, and shoot anything less than 200-grains. I like that, because that 195 grain .323 bullet has some great ballistics even in the 39mm case.
 
Paul

Appreciate the input, Paul.  It puts my mind at ease about twist, particularly on the fast side.

One thing I had been toying with is building a k98 in military trim to field the Super Kurz, much like a buddy I know that built one for the 8x33 Kurz, but on the practical side for hunting, a modern short action would take better advantage of the cartridge design. Specially if the estimated performance shines at higher pressures.

I have an appointment to see my 'smith tomorrow to talk "options".

Nels

I was thinking along the same lines - "modern action" - Nels. I was thinking something like a No. 1 would surely allow one to squeeze as much MV out of that case as possible.
 
Paul

Today, I discussed some ideas with a 'smith.  One of several  options he showed me was a single shot Savage bench rest setup in 6.5-284.  I'm not interested in a BR, but the Savage does have some interesting features like a more friendly way to switch barrels and adjust headspace.

Ah, yes a Ruger. This will give me a good excuse to go check a No. 1 out.

Nels

Nels, I can vouch for the Ruger being the strongest and safest rifle available for handling high pressure. Much safer than any bolt gun. Just have the extractor and slot made 1/8th in. wide or less. If testing a dubious load, keep your hand off the fore end piece but I doubt you will have any problem.   ..........Paul H

Update:

While perusing thru the Handloader's Manual of Cartridge Conversions looking at options on brass, the cartridges based on the 7mmBR or similar are the 1.5x308 in 30 caliber and the 338 Whisper, so my idea of the 8mm Super Kurz falls right between the 1.5x308 and the 338 Whisper, with the 338 Whisper being intended for subsonic use.  That gives me hope anyway, that a custom reamer wouldn't be totally oddball.

I floated an idea about turning down a heavy 8mm barrel blank with a 'smith who indicated that turning down a lot of metal could be expensive.  The Savage setup or finding a barrel in 7.92x33 to ream out is starting to look more appealing.

Nels

I decideed to come back over to this thread to continue the discussion in the context of the 7.5x55 case shortened to your 'Kurz' lengths and case volumes. This is a continuation of the conversation at "Tell me what you think of this ..."
 
So...
 
I cut off a 7.5x55 case to exactly 39mm and weighed it. Adjusting shoulder angle (maintaining the .30 BR's 30 degrees) and length to shoulder, I got the case capacity to 40.67 grains. Again, keeping the 24" barrel (the barrels of the K-31s I have are 25.675" in length from bolt face to muzzle), and H4198 that the BR experts claim is best, QL predicts the 125 Hornady will be doing 2863 as it leaves the muzzle. Now the 125 Hornady doesn't have a particularly good BC at .246 so it doesn't carry that velocity very well. However, if you were to have someone (Berger or NwCP maybe) make a boat-tailed 8mm in 125 grains with a BC of .400-ish (should be easy to do - I expect .500 might be doable), this cartridge might be a good bench gun. As a hunting cartridge, that Hornady 125 carries over 1000 ft-lbs to 300 yds. Up the BC to .400 and a 125 grain bullet carries 1400 ft-lbs out to 300.
 
Ql says another 50 f/s can be had by using Rel-7, upping the MV to 2913. This also reduces the load density from 105% for H4198 to 99% for Rel-7. The external ballistics are about as much better as you would expect if you increases the MV of any bullet by 50 f/s.
 
By setting the zero range at 82 yards, the bullet never rises above the line of sight. Doing this gives an idea of how far the bullet will fall with a  "level" aim and a 'scope mounted 1.5" above the center-line of the bore. I figured you might be interested in this from a target shooting perspective. So sighted, the 125 Hornady bullet has the following trajectory from 100 to 600 yds:
 
100 . . -0.1"
200 . . -4.2"
300 . . -15.4"
400 . . -36.3"
500 . . -70.6"
600 . . -123"
 
From a hunting perspective, sighted in for a 6" target yields a zero range of 227 yds, a Max Point Blank Range of 265 and the following trajectory and energy out to 300 yds:
 
100 . . 2.7", 1764 ft-lbs
150 . . 2.9", 1645 ft-lbs
200 . . 1.5", 1348 ft-lbs
250 . . -1.7", 1171 ft-lbs
300 . . -6.8", 1012 ft-lbs
 
Again, with a more aerodynamic bullet, the numbers improve, increasing the 300 yd energy to 1461 ft-lbs.
 
All just so much "paper whipping", but it does give some idea of potential. In a single-shot action like a No. 1, the pressures can be increased substantially. I'm not sure the increase in performance would be worth the 'beating' the action would take.
 
Paul

Paul, this is impressive performance indeed from such a small case. It would be an ideal deer and black bear cartridge for most hunting conditions. It would leave the 30/30 in the dark. Gives me an idea for the Turk Mauser I bought recently.  .............Paul H