Wildcat Design: Minimum Neck Length, Minimum Case Taper and Twist Rate

[jkpq45]

OK folks, designing a wildcat along the lines of 240 Gibbs, but using the 7.62x54R case as a parent vs. .30-06. Using Nosler Varmageddon (tipped, flat-base) 55-grain j-words with polymer tips, 0.750" long. On to the questions:

1. What's my minimum cartridge neck length? Am I going to get in trouble if I grab the bullet with too little neck? I'm envisioning seating to the bottom of the neck--why not? After all, I'm turning the reamer....

2. Regarding case taper, is there a minimum goal for good extraction? I assume straight-walled isn't going to be a good idea as the goal is reasonably high-pressure firings (mid-60,000 PSIg range). The parent 7.62x54R tapers ~1.89% (0.029" taper over 1.533" distance). The reasonable comparable 240 Gibbs only tapers 0.84% (0.018" taper over 2.145" distance).

3. How much is twist rate going to affect speed? I'm thinking my barrel has a 1-10 twist (not sure as it hasn't arrived yet). Is the "fast twist" for the tiny bullet going to detract a bunch from speed?

Thanks for all the advice!
jkpq45

 

[Jim K]

Why? The only reason I can see for basing a wildcat on the old 7.62x54R would be to use it in a M-N rifle which would not be the world's greatest platform for any kind of varmint rifle.

OK, now tell me I am an ignoramus who does not understand creativity and to go pound sand.

Jim

 

[NCsmitty]

You may be proving to tackle an exercise in futility, but I encourage you do it anyway.
Rimmed cases can be wildcatted as easily as a rimless case, but their use may be limited to fewer actions due to their often large rim dimensions.
The Russian rim is exceptionally large, but anything is possible. Perhaps an NEF/H&R singleshot would be something to look at to convert.

I've got a cartridge drawing of the 6.3x53R Finnish, I believe based on the stock 7.62x54R necked down to 25 caliber, that you can envision and work from for ideas.

Most 6mm Rem/243 Win use 1in9.125" and handle 55gr weights quite well, so your 1in10" twist should work too. The question remains about the life of the barrel at the velocities you may anticipate.


NCsmitty

 

[25cschaefer]

Theoretically, less body taper leads to less bolt thrust; this is one of the major design characteristics of an Ackley Improved cartridge. Less bolt thrust means less wear on your gun keeping it accurate longer and somewhat safer.

Yes, more taper would make it easier to extract but, if you keep your gun clean and aren't pushing the limits of your platform you shouldn't have a problem. The Russians put that taper in the case because it needed to work 110% of the time no matter what; dirty powder, dirty gun, ect.

I would be concerned with bullet set back with too little neck, especially if you were building a bolt action that feed from a magazine. Generally, longer necks are going to give you more case life, you can reload them more. My suggestion would be to look and some popular 6mm or 6.5mm cases and copy their neck lengths.

As far as twist rate, longer bullets need a faster rate of twist than short ones. Think a 45 cal muzzle loader shooting a round ball with a 1 in 48" vs. a 45 cal shooting modern conicals with a 1 in 16". yes, velocities and bullet material play into this a little but generally speaking a heavier (longer) bullet needs more twist. Here is a good website:
http://kwk.us/twist.html

 

[Clark]

25cschaefer
Theoretically, less body taper leads to less bolt thrust; this is one of the major design characteristics of an Ackley Improved cartridge. Less bolt thrust means less wear on your gun keeping it accurate longer and somewhat safer.

I have read a good part of a million gun forum posts over the last 20 years, many before the first www gun forum.
I once read a funny response to that idea, I can't remember where.

~~" that is like putting your brass radiator in front of your bumper, to protect the bumper in case of a wreck."

Back to the OP.
I am looking at making a Mosin Nagant in 257 Roberts Ackley Improved Rimmed. But I just realized that the rim must fit tightly into the bolt face and the case head has a bevel. So, like you, I will probably be using the 7.62x54R brass. They say that short necks make for more accuracy with less neck tension. But the world record accurate group was with a 222, that has a long neck. I think whenever I make a wild cat, I find that every short cut has a penalty. Make the cartridge look as much as possible like other cartridges.

 

[jkpq45]

Why? The only reason I can see for basing a wildcat on the old 7.62x54R would be to use it in a M-N rifle which would not be the world's greatest platform for any kind of varmint rifle.

To be perfectly honest, I have brass that isn't being used in the form of 7.62x54R--the .30-06 brass I have will be for a .30-06AI; didn't feel like making a .240 Gibbs.

I would be concerned with bullet set back with too little neck, especially if you were building a bolt action that feed from a magazine. Generally, longer necks are going to give you more case life, you can reload them more. My suggestion would be to look and some popular 6mm or 6.5mm cases and copy their neck lengths.

Point well taken, sir. Most of the 6mm/.243 caliber cartridges I've seen hold on to the projectile with a neck at least the length of the diameter of the cartridge; I'll probably shoot for 0.300" or so neck length as a fine balance.

Rimmed cases can be wildcatted as easily as a rimless case, but their use may be limited to fewer actions due to their often large rim dimensions.
The Russian rim is exceptionally large, but anything is possible. Perhaps an NEF/H&R singleshot would be something to look at to convert.

I have a very strong Brazillian single-shot 12 gauge I may chamber for this cartridge--otherwise, I'll rework a Mosin action I've been meaning to butcher.

You may be proving to tackle an exercise in futility, but I encourage you do it anyway.

Roger that! Proceeding with every intention of keeping you all posted on progress.

 

[Clark]

jkpq45
I have a very strong Brazillian single-shot 12 gauge I may chamber for this cartridge--otherwise, I'll rework a Mosin action I've been meaning to butcher.

I have converted a number of break action single shot shotguns into rifles this year. There is bunch of arithmetic to get them to lock up tight and have the firing pin hole line up with the center of the bore. I have yet to make an extractor. I have killed a lot of animals just dropping a brass rod down the bore to extract the empty, so I can get a second shot.

 

[jkpq45]

Clark--what flavor of single-shot do you use? H&R? Any chances you have plans for that DIY barrel lug it looks like you crafted?

Also--hard to make out the engraving on the back of your barrel. Looks like 500 something... making a .499 Hubel Express?

 

[Clark]

That is my wildcat 50 CB Short.... just a 56-50 or 50/70 Gov case shortened to 1/2".

I have used Iver Johnson, H&R, and a couple other, just lying around or cheap at gun shows.