Hypothetical Wildcatting - Bring on the cold water

[Desperatezulu]

Thanks for your responze Zulu, but you're going to have to explain your concerns either in much more detail or in much simpler words.

Why would a rimmed case be less resistant to pressure than a rimless one? Seems to me it should be the other way around as it has less un-supported area (because except for the case head itself, it's really the barrel that is taking the load as the case is far too weak and there is no extractor groove in a rimless case).

Just for clarity, the reloaring process would be controlled just as for a rimless bottleneck case, IE the sizing die would be set up to give minimal shoulder bump if at all.

Secondly, why do you feel that case head seperation is either more dangerous or more likely in a falling block? Again, it seems the opposite to me as firstly the action is much stiffer than a rear-locking bolt action and secondly the shooter's face is much better protected, as are his hands since the gas would mostly be vented out the top.

And thirdly, why do you feel such a combination would be less accurate?

Rimmed case headspaces off the rim - ergo the majority of the case is unsupported and thus subject to a lot of metal fatigue through each firing cycle. Just a more extreme version of what a belted case goes through.

My comment about the falling block is not a criticism of its inherent strength but more the access to a stuck case that has lost its head. The cartridge design creates the propensity towards case head separation, not the action.

I do not know the detailed science around inherent accuracy but all the leading benchrest calibres headspace off the shoulder and most of them have severe shoulder angles, plus short, fat powder columns. The 303 case is the antithesis of all this.
Maybe you get lucky with your wildcat but I'd put money on a design closer to a modern cartridge proportions outperforming yours in the accuracy stakes. And also case longevity.

[oafpatroll]

@zulu, I'm not getting the 'unsupportd because it head spaces off the rim thing. Isn't the amount of support or lack thereof a function of the case to chamber fit?

[Desperatezulu]

It is - and the case has to have a clearance gap or it jams in the chamber. The issue is where that clearance lies. The greater the clearance, the greater the case stretching. If the the stretching happens in the longitudinal direction (i.e. from primer end towards case mouth), there is thinning of the case wall and incipient case head separation begins.

With a conventional bottleneck case, the case headspaces on the shoulder - i.e. the case is 'pinched' between the bolt face at one end and the shoulder at the other. The case thus stretches circumferentially but not longitudinally (not beyond the 'headspace' itself, which is a few thou). Case separation is way less common in such designs.

Thinking about it differently, imagine you turned the rim off a 303 (or a revolver cartridge if you prefer). What would happen to that cartridge when you put it in the chamber? Assuming the firing pin could impact the primer, what do you think would happen to the case upon firing?

I seem to recall wikipedia has some nice diagrams on headspacing and different cartridge designs. That may explain the 'unsupported' concept better than my words

[oafpatroll]

It is - and the case has to have a clearance gap or it jams in the chamber. The issue is where that clearance lies. The greater the clearance, the greater the case stretching. If the the stretching happens in the longitudinal direction (i.e. from primer end towards case mouth), there is thinning of the case wall and incipient case head separation begins.

With a conventional bottleneck case, the case headspaces on the shoulder - i.e. the case is 'pinched' between the bolt face at one end and the shoulder at the other. The case thus stretches circumferentially but not longitudinally (not beyond the 'headspace' itself, which is a few thou). Case separation is way less common in such designs.

Thinking about it differently, imagine you turned the rim off a 303 (or a revolver cartridge if you prefer). What would happen to that cartridge when you put it in the chamber? Assuming the firing pin could impact the primer, what do you think would happen to the case upon firing?

I seem to recall wikipedia has some nice diagrams on headspacing and different cartridge designs. That may explain the 'unsupported' concept better than my words

Nope your words did it perfectly well, thanks.

[TStone]

Neither a rimmed- nor a belted case, with an adequate shoulder, need to headspace on the belt or rim, they can be sized to headspace on the shoulder.

For many years, I sized my .300 win mag cases to headspace on the shoulder, with excellent results. It will obviously not work on a .458 win mag or a .300 H&H but for everything with a shoulder it does. The same applies to rimmed cases, won't work on a .45-70 govt, will work on a 6mm/.303 Ackley.

[Heath Robinson]

Neither a rimmed- nor a belted case, with an adequate shoulder, need to headspace on the belt or rim, they can be sized to headspace on the shoulder.

For many years, I sized my .300 win mag cases to headspace on the shoulder, with excellent results. It will obviously not work on a .458 win mag or a .300 H&H but for everything with a shoulder it does. The same applies to rimmed cases, won't work on a .45-70 govt, will work on a 6mm/.303 Ackley.

Exactly.
Over-sizing is a problem, no matter what type of case. The only real difference is that a rimmed or belted case will actually fire whereas a rimless case in a push-feed action might not (if you over-size it enough). It's no harder to set the sizing die correctly for a rimmed case than for any other.

Oh and the Epps case has a 35 degree shoulder, very similar to most of the latest-and-greatest. I understand 35 degrees is about the sweet spot, because by 40 degrees it becomes very hard to forrm the cases.

As for access to get a stuck, headless case out, sorry, I don't agree with that either. Push a brass brush in, pull the case out. If that fails, the proper tool is pretty easy to make.

And as for a long, slim powder column: there are plenty of calibers that hav much longer-for-the-diameter cases that are revered for accuracy in the hunting rifle field. .25-06 and .270 just to name two.

[janfred]

Even spacing on the rim, the chamber on a new barrel can quite easily be cut to still give the required 3 thou headspace off the rim. As Tstone above, nothing stops you from using the shoulder for headspace. Did that for years with my .303...

Just because military rifles have generous chambers do not mean you cannot have a tight headspace on a civilian rifle.

[Desperatezulu]

As for access to get a stuck, headless case out, sorry, I don't agree with that either. Push a brass brush in, pull the case out. If that fails, the proper tool is pretty easy to make.

Clearly you haven't encountered this before. But all good. Hopefully your 6mm Epps works out with great accuracy and case life

[Heath Robinson]

Clearly you haven't encountered this before. But all good. Hopefully your 6mm Epps works out with great accuracy and case life

Not in a falling block, no. Nor in any other single-shot action.
But at one stage they were a virtually daily occurrence in my .44-40 lever action. Rossis were quite well known for coming from the factory "headspaced on burs". Brush trick worked with that most of the time as long as you are careful not to turn the brush. The trick was taught to me by Ben Musgrave. I eventually fixed it by getting a Andrew Kallman to weld up the locking shoulders in the bolt.
I have seen it several times in bolt actions as well, and again, the brush trick worked surprisingly often.

But again, the proper tool (as found in a complete R4 cleaning kit) is both easy to make and easy to use in just about any action.

[Heath Robinson]

I've been faffing with numbers a bit more.
I created a 3-D model of a cartridge case, which allows me to easily estimate the powder capacity. It's not 100% accurate as I really only have accurate dimensions of the outside of the case (and that excludes the dimensional effects of actually having been fired in a chamber) and the effects of tolerances, and my case walls are just a consistent straight taper from the base to the start of the neck (which isn't really accurate). But the volumes I am getting for standard cartridges are within the published ranges.

I have also calculated a powder column "slenderness ratio". This divides the length of the case (measured to the start of the neck) by the average diameter of the inside of the body from the base to the bottom of the shoulder. I guess a more accurate measure would be to compare the volume to the bottom of the case neck to the length but I don't think this is a critical number.

For interest's sake I included three calibers that are not under consideration at all, the two "benchmarks" being .243 Win and .243 AI, and a caliber that until very recently basically ruled F-open target shooting in the US, the .284 Shehane.

Although the Shehane obviously has considerably more capacity than any of the other cases being compared, what is interesting is that the "slenderness ratio" is within 2% of .243 and within 1% of .243AI. I realize that the dedicated benchrest calibers utilize much stubbier cases, but if it's good enough for F-class champions (who are regularly shooting sub-1/2MOA 20-shot groups at 1000 yards) it's more than good enough for what I want to achieve here.

At the moment there are two contenders. Both use the same body taper and shoulder angle as the .303 Epps (35 degree (or 70 degree included angle) shoulder and about 0.25mm body taper). One is based on the 6mm Musgrave, the other is based on 7.62 x 54R.

Compared to .243 AI, the 6mm Musgrave-based case has 5% more case capacity and a 10% higher "slenderness ratio" (higher being worse in this case).
I have two versions of the 7.62x54R-based case. One is full length, and one is trimmed down to 50mm. The shorter one has a 2% greater capacity than .243 AI and a 12.5% better "slenderness ratio", while the full-length version has 12% more case capacty and a 4% better "slenderness ratio".

Just to re-cap, the advantages of the 6mm Musgrave-based design is that my nostalgia is satisfied and that the cases (of which I already have a hundred) have the same headstamp as what would be written on the barrel and the license while probably achieving the same velocity as the benchmark at the cost of case quality and possibly accuracy.

Basing the caliber off the Russian case gives the possibility of further improving performance and the fact that Lapua makes cases (so the case quality issue goes away, hopefully taking with it the accuracy concerns) but I don't know whether those Lapua cases are even available locally.

Does anybody here load for 7.62 x 54R? Are any cases available?