Hypothetical Wildcatting - Bring on the cold water

[Heath Robinson]

So I have this dream/fascination/infatuation/goal. It will probably never actually happen because finances and because I don't actually need it, but when has those two things ever killed a dream?

I would like to build a classically-styled Springbok and maybe Jackal rifle.
I want to do it on a Ruger No.1 or similar falling-block action, and in a caliber that maximizes maximum point blank range (yes, I know, it's an outmoded concept in these days of laser rangefinders), and I want to do it in a rimmed case.

Also I have long wanted a 6mm Musgrave. There is a lot of history between my family and the Musgrave family. And I am thinking that might not be a bad place to start, since .243 is a pretty popular caliber for exactly these types of uses.

So my understanding is that the 6mm Musgrave is about 100ft/s slower than the .243 Win, but it has a greater case capacity and the slower velocity is due to the lower max pressure which is limited to make cases last in a Lee Enfield action. 6mm Musgrave has about 5% more case capacity than .243 Win but the max pressure is 49 000 PSI whereas the .243's max pressure is 62 000 PSI? So in theory, if you run it in a stronger action (like the Ruger No.1 or similar), the 6mm Musgrave should be able to at least match the .243 without compromising either case life or operator safety?

But then I was thinking, why not "Epps improve" the 6mm Musgrave? Dimensions are a bit scarce, but if what I found is correct, the Epps conversion increases the .303's case capacity by about 12.5%.
Applying the Epps' body taper and shoulder angle to the 6mm Musgrave but movign the shoulder back by 0.5mm (just to make it easier to stay out of a donut) would give an increase of nearly 13% case capacity over standard 6mm Musgrave or 8.5% over .243 Ackley.

Thus again, would it be reasonable to assume that a 6mm Musgrave-Epps in a strong action would be able to match .243 Ackley?

So the questions are:
1) Is there an existing caliber for which cases and other components are reasonably available that does this already?
2) Is there somebody locally who can make reamers, or can a reamer be imported from the US without too much hassle? Any idea what this would cost?
3) What have I missed, what are the drawbacks etc? I realize barrel life won't be spectacular but the rifle probably wouldn't see a whole lot of use. And yes, I know it won't be a cheap exercise...

Bring on the cold water chaps.

[oafpatroll]

I'm an awful long way out of my typist lane here so take this at no more than 2c worth. I'd be surprised to find that a 303 case in good condition fired in an action like a No1's couldn't handle substantially more pressure than it could in a LE. Even in the LE, WW1 era proof loads were at least 50% overpressure if memory serves.

Very much looking forward to roofkyking on this project if you go ahead with it.

[Heath Robinson]

Oaf, not too sure how much time you spend on youtube, but there is an old video by Bloke on the Range where he and some other guy re-chambered a Lee Enfield action to .300 Win Mag and then abused it to try and find the limits. If I remember correctly, it handled a couple of shots of factory ammo without anything actually breaking.
However, the Lee being a rear-locking action I would think that pushing it at all would lead to case head separations after very few firings.
With the scarcity of 6mm Musgrave cases and even .303 cases, case life would be something I would not want to compromise.

But on the other hand I believe that since the primer pocket defines the thickness of the case head and since I am pretty sure that no ammo factory makes cases for different calibers from different alloys, I would think that the true pressure limit in any given rifle would be a factor of action strength and case head diameter and the SAAMI or CIP maximum pressure becomes effectively irrelevant?

[TStone]

I do not like the 6mm Musgrave, I have only seen one that was reasonably accurate (sub moa) and that was a German drilling, 16ga/16ga and 6mm Musgrave, that had been custom built by a German gunsmith. So, my opinion is definitely a bit prejudiced. The 6mm Epps might be different and you will probably have, to have a custom reamer made.

The Australians did a lot of wildcatting of the .303 British case, so Australian forums might be a good place to start. If you want to be a little more adventurous, how about necking down some of your 7x57R cases to 6mm, that will basically give you a rimmed 6mm Remington which, in a strong single shot action, will be faster than a .243 win.

Or what about a 9.3x74R case necked down to 6mm, .257 or 6.5mm? That will give you plenty of case capacity and cases are available.

[oafpatroll]

I'm familiar with the Bloke, a consumate rabbit holist, and his mates. Pretty sure that's where I got the MOD/War Office document that specified the proof load specs. I'll try and dig it out and send it for reference. I get what you are saying about minimum case head thickness being defined by primer dimensions but that wouldn't constrain case wall thickness and given how much variation I've seen across case volumes from different manufacturers or even batches for specific calibres I'm pretty sure those are variable. Have no idea how important or not that is.

[oafpatroll]

Or what about a 9.3x74R case necked down to 6mm, .257 or 6.5mm? That will give you plenty of case capacity and cases are available.

That sounds like a winner to me.

[Heath Robinson]

Hmm. Interesting thoughts @TStone. I do have a question on the inaccurate 6mm rifles you have experience with: were they all sporterised Lee Enfields, or were some based on other (more rigid) actions?

So many rabbitholes. Johan Loubser (ex Somchem chief ballistician) stated publicly that in his opinion/experience there was no such thing as either an accurate or an inaccurate caliber. It was all down to the quality of the rifle and the quality/consistency of the ammunition and the components thereof (thus primarily cases and bullets) and finding the right load. Hornady's chief ballistician is on record saying almost the opposite. Their position is that there are design features that make a caliber more or less accurate, and that fiddling with the powder charge and seating depth is virtually pointless.

Other theories I have seen are that the following cartridge design factors may contribute to accuracy:
1) Shoulder angle
2) Powder column length (shorter supposedly being better)
3) Flash-hole size
4) Where the bullet sits in the neck
5) general tolerances

Now I don't know which theory is correct, nor whether any of the above factors really do play a role (except that we can probably all agree that basic tolerances are probably important).

The base diameter of the case (IE the diameter just ahead of the rim) of all the parent cases mentioned so far (.303, 7x57R and 9.3x74R) are virtually identical.
Starting from that basis, all the other factors are to some extent under the case designer's control except for the case wall thickness in the area below the neck. That's down to getting or choosing well-made cases.

To sum up the above, during the process of changing the 6mm Musgrave to the 6mm Musgrave-Epps, the shoulder angle and powder column length is already changed, and the chamber dimensions can be specified such that neck turning is necessary, thus normalising the case wall thickness in the most critical area, and using any of the other named cases to achieve the same ballistic results will result in an effectively-identical case.

Now while I said that I have enough 7x57R cases, I don't have enough for this as well.
Thus the only advantage of starting with 9.3 x 74R I can see would be that you have the ability to make the case longer and get even more velocity.
This being contrasted to the one advantage of starting with a 6mm Musgrave being that the headstamp says basically the same thing as the barrel and license. Now I know that in theory the headstamp shouldn't matter, but I don't like pushing my luck with putting the phrases "common sense" and "reasonable" in the same sentence as the acronym "SAPS".

[TStone]

Apart from the drilling mentioned above, there were various Lee Enfield actions, a P14 and a Martini single shot. Apart from the drilling, which consistently shot moa groups, all rifles shot 2-3moa groups.

[TStone]

According to this, the 6mm/303 Epps does exist.

6mm MUSGRAVE - Cartridge Collector

Over the years, I have contemplated various wildcat cartridges but always ended up settling for more practical factory choices. but if I had the time and money, I think I would build a rifle in an obscure wildcat cartridge just for the hell of it. A .30/.357 in a Martini cadet action have always tickled my fancy.

[Heath Robinson]

According to this, the 6mm/303 Epps does exist.

6mm MUSGRAVE - Cartridge Collector

Over the years, I have contemplated various wildcat cartridges but always ended up settling for more practical factory choices. but if I had the time and money, I think I would build a rifle in an obscure wildcat cartridge just for the hell of it. A .30/.357 in a Martini cadet action have always tickled my fancy.

Thanks, that is interesting.
Must say I am not surprised that it's been done before.

I'm a little concerned about that P14 and the Martini.

But again, changing it to an Epps would blow the shoulder out to 35 degrees, so if shoulder angle is a factor in accuracy the process of "Epping" should solve it. And if powder column length is an issue, then cartridges like .270 Win and .22-250 should also be dismal, and we all know they aren't. And all the other factors I have seen mentioned are inherent in the base case and not much can be done about them except maybe to go to a different base case, but I don't know of any rimmed cartridges with a specific reputation for great accuracy that would be suitable?

Re a Martini cadet based wildcat or uncommon chamberings, I have Cadets in .22LR, .22 Hornet and .310 Cadet, and a full-sized Martini in .577-450 (but a sporting rifle not a sporterized military rifle). The hornet is not particularly accurate but the barrel is in very poor condition. The .310 is about as accurate as I can still shoot peep sights and the .22LR is probably the most accurate rifle I own. It has shot many 1/2MOA groups at 100m despite wind.