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So heres the concept. A 6.8 case cut down some to accomadate a 90 grain .224 Diameter bullet. I dont really know the steps in designing a wildcat. How could I figure out the pressure, Muzzle Velocity (20 inch Barrel),and what the case would needed to be trimmed to. Thanks for you help.
 

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Welcome to Shooters Forum, Spencer. :)

Here are the steps for designing a wildcat:

Get out your checkbook and sign several blank checks. Send one to the guy doing your drawing, one to the tool and die maker and another to your gunsmith. Don't worry about filling in the amount, they'll take care of that for you, once they get time to work on it. Expect it to take 2 years...if it's any less than that, terrific!

As for the pressure and velocity? If you do your homework, you'll have designed a slightly fatter, shorter 223 cartridge. Since you're almost certainly wanting to do this in an AR-15 action, your pressure will be limited to about 55,000psi and your muzzle velocity will be restricted by sticking so much bullet weight into a diminutive cartridge. In short: You'll be terribly disappointed. Accuracy will probably be good, but with the amount of money you spend, it won't be demonstrably better than either the 6.8 parent case or a standard 223/5.56.

If you just HAVE to do this, and we all suffer that affliction once in a while, the thing to do is cut down a 6.8 case, neck it to a 22-caliber neck (maybe using a 225 Win die?) and then measure the case volume. That case volume number is fed into QuickLoad and used to determine, to a large degree, the pressure and velocity you will likely get using different bullets and powders. Again, you'll be disappointed.

There's no free lunch when it comes to internal ballistics. To spin a 90-gr .224" bullet, you'll need a fast twist, which will mean slower MV. To seat a 90-gr .224" bullet into an already shortened 6.8SPC case, you'll have to stuff it down in there and lose powder capacity. When you're done, you MAY be driving it 100-150fps faster than you could from a 223 case, but what have you achieved?

If you want to have fun with wildcats, get a quality bolt-action or single-shot rifle. There is so much more fun to be had without all the restrictions of an AR. ;)
 

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Spencer -

Howdy !

Take a look @ Powley Computer .
The " Powley Computer " has drop-downs for things like case capacity, bullet length; etc .

When you mentioned potential need to trim the SPC case shorter, did you mean.... so that the new .224" cal wildcat' cartridge oal would work in an AR format ?

As an aside:

* You could do sample wildcat case forming using a .220 Swift FL die, that has its internals removed.
The Swift has same 23* shoulder angle and close to same shoulder diam an SPC case has.
You'd either cut a .220 Swift die shorter, to suit your needs; or make a " perch " for use w/ the stock-length die.

A " perch " is made out of a .308 Win shell holder, a 1/4 -20 flat head machine screw w/ head diam < .420"; and a 1/4-20 jam nut. The screw fits through the primer ram hole in the shellholder, and screw head height is set/held in-place using the jam nut located underside of the shellholder.

In-use, the perch is snapped into place in the press ( mine's a ROCKCHUCKER ), w/ any excess thread length positioned in the press arbor's primer ram slot.

Brass to be formed is placed atop the perch, and press arm operated to advance brass up-into the ( .220 Swift ) case forming die.

For your notional wildcat, case forming forces would be low; as both the die and case have mirror shoulder angles and shoulder diameters.

The Swift has a nominal .300" neck lg, while the 6.8 SPC neck lg is somewhat shorter @ around .272".
In that regard, the case would not need to be shortened, unless to ensure workability w/ a cartridge oal that needs to fit an AR; or other specific length magazine rifle application .

Should you form some trial cases, they could give you a real good idea of potential final case capacity; even in an un-fired state. If you are thinking about going w/ a sharp 30* or sharper shoulder angle, you'd probably still form your wildcat cases at a shallower shoulder angle; and then fire-form to final sharper angle.


With regards,
357Mag
 

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wildcat 3.0

Spencer -

Howdy, once more !

I forgot to mention the obvious....

Neck down of some 4 calibres to .224" final cal might result in some slight lengthening
of the 6.8 SPC case/case neck. It would be an easy trim, to final desired case oal.

It's fer sher the neck walls will thicken, some.

For the wildcat's case necks, you'd likely have to do an inside neck ream.
You'd also need to perform outside neck turning on the brass, to ensure wall thickness uniformity; and to provide final neck OD desired.


With regards,
357Mag
 
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