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

[paul h]

I've known of a few wildcats that had very short necks, but personally I like at least 1 caliber of neck length. Bullet tension aids in consistent ignition of the powder.

For case taper, you really don't need much. Making the shoulder 0.010" smaller in dia than the case head will be fine.

I don't think twist rate has a significant affect on speed. 1-10 is not fast twist, there are 1-7 twist barrels and I know people using them with 22-250 ackleys and they don't have problems getting high velocity.

[Artful]

Most accurate calibers have at least 1 if not 1 1/2 caliber length necks to help alignment.

[plmitch]

This is an interesting project. Keep us updated on the progress.

[gzig5]

Is this a .243 or .224 caliber? With that case capacity you should be able to push a 55 gr close to 4000 fps in either caliber. Usually they would be 14 twist in .243 and 12-14 in .224 for that bullet weight. Depending on the bullets, you may end up over-revving them and have a few come apart. I've seen that happen in fast twist .223 remingtons using bullets with fragile jackets but that was with a 7" twist. You may get by with 10".

At least one caliber neck length if you don't want to inadvertently leave a long seated bullet in the bore. A short throat will allow you to chase the lands for a while. That much powder in a small caliber is going to be rough on the throat.

[flounderman]

I have a cheetah with a 1 in 10, I believe. I would get it 1 in 9 or 8 if I had it to do over. 69 gr bullets close to 3700 with no strain. It doesn't seem to handle the 75 gr very well. I don't see the purpose of shooting 55 gr bullets in a case that big. You can do almost as good with a swift. You are just destroying the barrel shooting light weight bullets in a large case. I had a 22 06 with a 1 in 10 barrel but back then the 63 gr was as heavy as was available. Friend had a 22 348 he shot a 90 gr 22 in. A rock at 400-500 yards would spurt smoke when you shot it. You get a 70 gr 22 bullet cruising at 3700 and it will shoot about as flat as anything and buck the wind. I don't like short necks or too square shoulders. You will need to ream and anneal the necks. You have enough case capacity with the existing confirmation and if you simply neck the case downj, you can always resize the body with an original die. The 224 middlestead is a 243 necked to 22 and you don't need to ream the necks. The cheetah, is a pain. necks need reaming and trimming and the shoulder is set back. I have a lot of wc 852 I bought cheap. might not be the ultimate, but over 3600 with a 65 or 70 grain satisfies me

[leftiye]

Ah gee! I've only got a 6mm Ackley improved. Ahm gonna cry. Thang uses as much of some powders as a 6mm-06, or 6mm/284. Produces surface wounds on jackrabbits with 70 grain bullets. Yuh've gotta go out and finish them off with your boot. I've had at least three deer not meet their maker because an invisible twig got in the bullet's way while the bullet was on its way to meet the deer. In other words it as insane as I've ever gotten, and it won't happen again.

[Idaho Sharpshooter]

A prime example of a modern wildcat shape is the 6.5x284 Winchester.
32-35 degree shoulder, .010" taper per inch, and a 1.25 caliber neck is the other popular blueprint. I got that from Keith Francis at JGS about thirty-two years ago.

Works...

Rich
Sua Sponte

[TRX]

A lot depends on what you want it for.

I'm working on one wildcat now. Essentially, it exists because I wanted a .300 Whisper, but I have a pile of .311" barrels. I also figure a subsonic bullet and cast bullets naturally go together. So the new cartridge is a .223 case with the neck section (essentially) of a .303 British grafted onto the front. The .303 has an unusual tapered leade that seems to be very cast bullet friendly. I'm also keeping the full length of the .225 case as a .5" long neck, and leaving it at swaged thickness instead of thinning it. This is because I want to cover as many grease grooves as possible on a long 200 grain bullet, and because I hope the thicker neck will mean I can reload without neck sizing, or sizing much. (there's a fair chance it the extra thickness might make the neck prone to crack; you don't know until you try)

Ballistically, there should be no meaningful difference between it and a .300 Whisper. The wildcat exists just to make the .300 friendlier to cast bullets, and because since most of my rifles are .311", I'm set up for appropriate-weight .311 bullets already.


My other wildcat is even simpler; I call it a "semi-wildcat". I have a Ross straight-pull rifle originally chambered in .303 British. The barrel needed to be replaced, and since it's a sporter there was no reason to stay with .303, other than the magazine, bolt face, and feed lips were set up for it. So I bought a 6.5mm barrel and made yet another 6.5-303 wildcat. I hope to get nice 6.5mm ballistics while hopefully having reliable feed.

The 6.5x54 Mannlicher is a rimless cartridge, but otherwise nearly identical to what I wanted. The case head size is compatible with the .303, so I bought a Mannlicher sizing die to shorten. Basically, ram the .303 brass into the Mannlichier die and voila! A 6.5x303.

I plan to use 6.5 Mannlicher loading data as a baseline. By modern standards it's nothing to brag about - only 2200-2400 FPS from moderate weight bullets. But that's perfectly fine; I don't need to knock my shoulder off every time I pull the trigger nowadays, and paper targets don't care about muzzle energy.


The point being, you need to have a specific purpose for a wildcat. Neither of these would have made good sense if I had to pay someone else to make reamers and dies. But I'm not expecting miracles from the finished rifles, either - just a bit more convenience from one, and a bit more accuracy from the other.

[badgeredd]

There are many commercial and even more wildcats out there with less than one caliber neck length. I personally prefer at least one caliber in length for the neck. It seems to me (rightly or not) that I've had far better luck with achieving very good accuracy with a longer neck. I do believe the neck tension is at least part of the reason. Body taper of 1/4 degree per side or 1/2 degree included also seems to be about as straight as a fellow wants in a high pressure cartridge. Twist will affect your bullet selection more adversely than attainable speed. One reason I say this is because of the RPMs a bullet can withstand will obviously affect your accuracy. The faster the twist, the higher the RPMs at a given velocity. The other reason that comes to mind is the slower the twist the more limited you'll be in your choice of bullets within a length window, ergo weight. Consider the early .244 Remington vs. the 243 Winchester as an example.

Edd

[texassako]

I have tossed around the idea of wildcatting a Mosin. In my research I saw that zhat has a 6x54r reamer and there are a few 6.5 Vostok biathalon rifle owners floating around the forums as well. It is a backburner project right now; so I never got around to asking how either of those shoots.