Why not larger caliber rimfire cartridges?

[Burt Blade]

I recently read an article (somewhere in the great cyber-swamp of the Internet) describing manufacturing replacement rimfire cartridges for use in relics such as the Henry lever-action rifle or Colt open-top (1872) revolver in the original .44 Henry rimfire cartridge. The basic method is:

Locate a modern centerfire cartridge of simiar dimensions, to use as a foundation.

Deprime and clean.

Solder a plug in the primer pocket.

Ssand the case head flat.

Trim to length, shave the rim down, etc, so it is dimensionally correct for the desired antique cartridge

Bore an off-center countersunk hole in the plugged case head, such that a .22 blank fits in place with a portion of the rim at the outside edge of the modified cartridge.

Press-fit the blank as a primer. (Might need an adhesive to hold it.)

(carefully) Load powder, seat the bullet, and crimp.

Place the assembled "rimfire" round in the chamber of the weapon, positioned so that the rimfire hammer will strike the .22 blank rim, firing it and igniting the main charge.


This works fairly well for a revolver, but limits repeating rifles to single-loaded rounds placed directly in the chamber.

And it does work. Considering the value of unfired antique rimfire cartridges in shootable condition, all the work listed above is cost-effective if you really want to shoot your antique.

 

[Trent]

Well i am actually longing for proper .22 centerfires, something along the lines of a .22LRC (LRC=Long Rifle Centerfire).

Seen the 5.7x28mm?

Great little round, and reloadable (although you have to be careful!)

Shoots a 40 gr V-Max at 2100 fps from a rifle barrel.

Savage makes a bolt action for it, if you want something that shoots it besides the FN FiveSeven handgun, AR 57 upper, or PS90.

 

[Flechette]

Another issue: With larger calibers, you'll need more priming compound distributed around the rim. Priming compounds are part of the health issues we face with shooting, and are obviously pretty sensitive in the manufacturing processes. Keeping the amount used to a minimum is a good idea.

Why would the priming compound pose a health risk? I understand that older primers were corrosive but the new ones are not. In any case, once the cartridge is fired wouldn't all of the primer compound be consumed?

 

[Flechette]

Enlarging rimfires also runs into a strength of materials problem. The larger the chamber (and hence the cartridge), the thicker the material needed to contain the same pressures.

To illustrate, assuming a .22LR cartridge develops, say, 20,000 psi, the thin brass can hold that in a "vessel" the size of a 22 LR case.

But put that same pressure in something the size of a .380 cartridge with the same wall thickness and it will rupture. The tensile strength required to hold it dictates a much thicker vessel wall.

Force = pressure times area.

Since the outward pressure is vectored into stretching (tensioning) the case wall, it can be seen that the tensile forces on the wall are sort of multiplied by the internal area, that is to say, the diameter of the case. The bigger the case, the greater the area the pressure has to work on. In turn, then, this results in greater tensile forces on the cartridge case wall.

A simple analogy:

It's like blowing up a party balloon. You've surely noticed how hard it is to expand it at first, but as its volume (and therefore its internal area) increases, it gets easier, at least until it's ready to pop.

While it is true that the chamber supports the cartridge, in any firearm, there are inevitably areas of the case that are unsupported. So scale that same principle down to these unsupported areas, and you have the same tensile strength problems, albeit on a smaller area..

Imagine a rimfire 5" artillery shell. It could not be made out of brass the thickness of a .22 case. (Apart from melting and handling problems.) Unless it were absolutely 100% supported in the chamber, it would rupture at any small unsupported area.

I always wondered about the new "Short Magnum" cases, where they retain the same internal powder volume, but with a bigger diameter case to make up for the shortening. The tensile forces on those bigger diameter cases (and therefore the chamber walls) must be enormously increased, compared to a conventional long case of the same volume, where the diameter is smaller.

Terry, 230RN

Thanks for the good explaination. I understand the tensile problem- I think that this is why really large cannons, like on battleships, forgo the use of a case altogether and simply load bags of powder into the breech.

However, if reloadability is not an issue (like current rimfires) then could a case be designed that deformed albeit in a predicatble way? For example, paper shotgun cartridges work just fine, even though they cannot hold much tensile forces at all.

I envision a brass case end (like a shotgun cartridge) with a rimfire primer around the base and a polymer/plastic cartridge case body. The plastic body would expand upon firing but contract after the pressure is gone, allowing easy extraction.

The brass case end would be solid (no hole for a primer to sit in, and therefore no primer seating related problems) and be supported by a recessed bolt face.

Would this not be a viable and less expensive manufacturing method than currently used? It uses a lot less brass (getting more expensive these days) and could be upscaled to say, a 9mm cartridge.

 

[Vern Humphrey]

Bore an off-center countersunk hole in the plugged case head, such that a .22 blank fits in place with a portion of the rim at the outside edge of the modified cartridge.

You've just described the Hammond Game Getter, which is available for virtually any cartridge you can name. Mine is in .30-06. and I keep on in my shirt pocket while sitting on a deer stand -- when the squirrels start to over-run me, I slip in my Hammond Game Getter.

 

[PercyShelley]

230RN, pressure is force divided by area, thus "pounds per square inch." Other than that though your explanation sounds reasonable. Wall thickness of a pressure vessel scales linearly with diameter for a given pressure:

http://en.wikipedia.org/wiki/Pressure_vessel#Scaling_of_stress_in_walls_of_vessel

 

[bigfatdave]

Why would the priming compound pose a health risk? I understand that older primers were corrosive but the new ones are not. In any case, once the cartridge is fired wouldn't all of the primer compound be consumed?

the issue is lead and heavy metals, burning the compounds that contain them doesn't "consume" them, it just changes what chemical they're bound up into.

 

[Owen Sparks]

I have a few obsolete rim fire rounds that I have picked up at gun shows and various places. They all appear to have copper rather than brass cases and are probably all loaded with black powder.

 

[Jim Watson]

Dixie used to sell substitute cases for the big rimfires that took a .22 blank as a primer.
You have to line it up with the firing pin, doubt it would work in repeater mode in a Spencer or Henry, but at least you could make the old gun go Bang on ceremonial occasions.

There were several designs of inside primed centerfires ca 1870.
Maybe a modern version of that would allow cheaper ammo in existing calibers, at least in non-magnum pistol calibers.
I don't think it would be MUCH cheaper, the equipment and processing for regular Boxer primed cases are so refined.

 

[230RN]

Percy Shelley:

230RN, pressure is force divided by area, thus "pounds per square inch." Other than that though your explanation sounds reasonable. Wall thickness of a pressure vessel scales linearly with diameter for a given pressure:

http://en.wikipedia.org/wiki/Pressur...alls_of_vessel

I said force = pressure times area, not "pressure is force divided by area."

But I was looking at the forces on the case walls involved in firing a cartridge, which inevitably translate into tensile forces on unsupported portions of the case.

You are correct that pressure = force divided by area, which is just a transposition of terms:

f = p X a

p = f / a

a = f / p


According to the way I understand that last statement, the breech walls of a 16" naval rifle, operating at around 50kPSI, need not be thicker than the walls of a .30-06 sporting rifle running about the same pressure. This is obviously not the case:

(Public Domain)


I therefore must be misreading it.

And as you can see, I was in much better shape back in ought-44.

Terry, 230RN

ETA: OK, I gotcha now. I was misreading that last clause of yours, and it supports my remarks about Short Magnums. I presume they have likewise scaled up the breech walls on those to match the increased case diameters.

 

[PercyShelley]

Oh yep. You had F=P * A. Cannot read today to save my life.

And yes, that's what it looks like; for a given tensile strength of breech material and a given cartridge pressure, the ratio of the minimum breech diameter to the firing chamber diameter should remain constant.

Hmmm...

So, on a 6.8 SPC AR-15, you're keeping pressure about the same, and losing firing chamber wall material and gaining cartridge cross-section. Are AR-15 firing chambers just massively overbuilt for 5.56? Maybe because they have to be wide enough to support the rotary locking barrel extension?

Is there an engineer in the house who actually knows how to use Lame's Equation (I certainly don't)?

Anyway, back on topic, I wonder if it's possible to make a rimfire cartridge/chamber design where the back of the cartridge is not left unsupported. Maybe something like an Oerlikon/Becker design only... with a rim? Not seeing how the geometry would work right away, but perhaps something like that would allow you to make large rimfire cartridges without the runaway case head thickness problem.

 

[Jim Watson]

I don't doubt that a sharp designer could come up with a breech that would handle bigbore rimfires. But, as already said, that would require tooling up for a new line of guns and ammo which would wipe out the cost savings.

 

[Jim K]

The Next Generation gave the answer in Post #2. Folks keep confusing caliber with pressure; the .58 trapdoor cartridge has a big caliber but very low pressure; so do the .44 Henry and the others mentioned. The only modern rimfire of moderately high pressure was the 5mm Remington at 37k; the .22 LR runs 24k, with the .22 WRM the same.

Jim

 

[Owen Sparks]

There were rimfire cartridges made in all sorts of calibers from .22 on up to .58. They were practically all straight walled and developed during the black powder era when pressures were much lower. The new smokless powder proved too much for them.

 

[Trent]

Good lord. Wonder what admiral I'd have to bribe to let me shoot that naval cannon once? Just once!!!! It can be at an unoccupied island, or an empty barge, or hell, just the open sea!

That had to be a heck of a feeling.

 

[230RN]

^ Trent

Here ya go. But the firing is probably just a button-pushing or automatic operation in some fire control room elsewhere than the turrets. I'd rather be in the turret to watch.

In the color pic, the red line on the wall is the furthest point to which the cannon recoils. The bags of powder have red lines on them to indicate which end has the black powder primer in each bag. The guy nearest the breech has a bigassed glove on to wipe the "mushroom head breech seal" off after each shot.

The copper pipe on the breech is to blast either nitrogen or CO2 through the cannon to clear out any possible remaining sparks from the burnt powder bags and other possible junk before the next charge is introduced. In many pics of the firing, you can see a little puff coming out of the cannon after the actual shot, which is that cleaning puff.

That's the way I understand it, but I have no doubt that there are differences depending on the time and the ships involved.

There are many old artillery men on this board who can correct or confirm what I said above.

Much info here:

http://www.ussiowa.org/pics/turrets/index.htm

Terry, 230RN