I am considering a 6mm wildcat based on the .357Max case. Specifically for use in a Martini-Cadet action. I've recently rebarreled the little Cadet to .357Max, using a cheap, surplus barrel from GPC. It may not shoot, but the barrel was cheap enough that I could experiment to see if the .35Max case was suitable for the Cadet action. (It is.)
My reasoning is thus: With .243 dia bullets in the 90-100gr range, velocity would be sufficient for a relatively short range whitetail round, and would also suffice for a varmint rig with the lighter bullets. Considering where I live, it would get much more use as a whitetail round. Questions: Twist rate? Has it been done before? Expected velocities with various bullets? Overall feaseability?
Comments welcome.

I looked at something like this but with a 7mm.
You can use 6mm TCU data and twist rate. It's almost the same case just rimed

I have a 22 RC Maxi, which is a 357 Max cas run into a .222 die. It is basically a .222 Rimmed with a short neck. (This is built on a small Martini). The idea when it was developed was to use cheap 357 Max cases.

If I had it to do over, I would simply chamber for the 222 Rimmed. You can get the cases and making the .357 based is a bear. If I were you, I would simply buy .222 Rimmed cases and neck up(an easy job). You could go for an improved case form if you want. I think the cost and effort to get the 222 Rimmed cases would offset the problem of necking the 357 case down so far. Head an rim are the same and the 222 has a slightly longer neck.

Appreciate the comments, folks. Already two good ideas!

All you would have to do is shorten a 6TCU die set and you are ready to load, I have a 7mm Super Mag Contender that uses the Max case necked down in a 7TCU die, and it comes out with a very short neck. I also make my 256WinMag from the max case, it will take a lot more pressure than the 357Mag case.
Albert

I'm not knowledgable in regards to what the M-C action will handle either with dimensions or pressure, but I'd look elsewhere for brass. The .357 Max brass supply is doomed. How about brass from the .444 Marlin or 30-30? The 6mm/30-30 Ackley essentially duplicates the .243 Win in ballistics. Luck to you in any case.(no pun intended)

Art S. I was interested in your reply. I've been thinking of a 222 Maxi on the small martini. I wondered how successful the chambering is in real life, i.e., how difficult the cases are to reform to 222 format. I was balancing using the 357 max case against the 222 Rimmed. At the moment the balance is coming down on the side of the 357 max case because the 222 Rimmed brass from Bertram in Australia is not particularly easy to get and is of very poor quality - the latter comment coming from a very respectable accuracy gunsmith - he actually said it is so bad that I should forget it - mind you he also said that reforming the maxi cases was a total drag.
There is another solution, one I've seen on another small martini 222 rimmed, which is to use 5.6x50 cases cut down. And these cases from RWS or Hirtenberger appear to be about the same price as Bertram's, and obviously, they are of vg quality.

First. A question for Art S. I've measured Remington brand 222R and 357max cases for wall thickness, approx q.1275" from case mouth. The 222 case varied from 0.0136 - 0.0107" and the 357max case varied from 0.0087 - 0.0104". The max case was much more regular than the 222 case which suggests less prep and more accurate ammo? So using 357max cases may not just be a case of cheapness but also enhanced accuracy. Comments?
Second part is the thickness of the 357 brass. It is thinner than 222 brass. Measured a Remington brand Hornet case wall at the same point and it is about 0.0075". Commonly considered to be thin and tricky. Was there any problem with the 357max brass in this respect?
Third part. 'The Handloaders Manual of Cartridge Conversions' by J Donnelly says that the 222 rimmed can be made from 357max brass by annealing the neck and F/L sizing in 222R die. Is it as easy as that?

Rupert

It is as easy as that, but that's not very easy.

This is a huge neckdown. You must anneal really soft first. When you get soft enough to go that far, you simply crush about half the cases. You need to work it down slightly. Anneal, form about halfway up the shoulder, anneal, form to the bottom of the neck, anneal, finish. You get the idea. Use a ton of lube. If you don't go super slow, then you will lose about 50% of the cases. If the 222 Rimmed cases are low quality, then I would try the 5.6x50 route. That sounds like much less work.

Another point is that after necking down this far, the necks are really thick and generally uneven as to length, so a lot of work is required to thin and clean up, and you lose a little more of an already short neck.

One final point is that the reference you mention is not correct. When you neck down the Maximum case, you don't get a 222 Rimmed. You get a similar case with a much shorter neck. This cuts powder capacity slightly but more importantly does away with the one design point that made the 222 a bench rest marvel in its day (the long neck to accurately align bullets). If you chamber with a standard reamer, the Maxi derived cases will position the bullet far, far from the lands. If you use a short neck reamer, standard 222R cases will not fit. On my rifle, the smith used a pilotless reamer and then used an N&T reamer to cut the throat and neck.

My suggestion would be to cut a standard 222Rimmed chamber. You can use the Maxi cases in a pinch or if you find a load that is accurate, but I would make the 5.56x50 derived cases or the regular factory cases my primary. I am going to have my neck and throat reamed to do this.

Pedastal, why not just go for it. Just use 6 TCU dies and neck the 357 max down, a 6 Super Mag, the 7 has been done but don't see any reason the 6 won't work. As for the short neck, I couldn't tell you why it wouldn't work in the Martitni as well???
Jeff
BTW, all the 7 super mags I have seen performed very well.

Art S. Much appreciated info.

jsh
It appears that is the thing to do. This has been a very interesting discussion. In answer to a previous poster i.e. 30-30 brass, the head size it a bit larger than I'd feel comfortable with in a MC. Same for .44 Mag... The barrel thread shank is only .750" Some of the case forming agony might be migitated by going in stages: say .30 carbine, 7TCU, then 6TCU. And annealing as required. I have found with my .17 Ackley Bee, even with proper form dies, I need to anneal twice-the last time after the case is formed, and before fire forming. Otherwise, the necks start splitting after as little as 2 reloadings.