calculating performance of fictional rifle rounds

As the name suggests, its for a round for a science fiction kind of thing. How can I get a rough estimate of a fictional round of roughly modern levels of technology? Really, I am here for just creative purposes. Most of what I know was from other hand loaders and other professionals.

Anyhow, the rounds I'm thinking of are rather crude, consisting of .323 caliber straight walled, untapered cartridges, headspaced entirely at the mouth. So of course, they would have to be very long for their diameter, proportional to the very powerful sharps rounds such as the .45-120 or .50-140. Though of course, overall smaller. The three rounds I'm thinking of are of course divided by the engagement ranges or size of game.

Though what I'm thinking long and hard about is the longest one. I'm thinking of a around that would fire a 160 or so grain spitzer bullet at 2500 feet per second for 2200 foot pounds of energy. How long would the brass case have to be? I'd suspect pretty long, but not precisely how long.
 
The basic cartridge you are looking for as a baseline would be the .32 Ideal, a.k.a, the 32-25-150, meaning a 32 caliber bullet with 25 grains of black powder and a 150 grain bullet. The only issue is that it is designed for low black powder pressures of about 17,000 psi. You would not have that pressure limit in a modern gun as long as the brass held up. I have no information about the case head thickness, but since you are assuming modern technology, and if you could shoot it in something like a Ruger #1, which is strong and doesn't have unsupported brass, then you should be able to run it in the 50,000 psi range, like the 45-70 is in that rifle. If you got a 28" barrel (Ruger's longest standard length in that rifle), using H110/296 magnum pistol powder, that would get you to about 2050 fps without altering or lengthening the cartridge further.

To get to 2500 fps in a standard 24" test barrel length, using QuickLOAD, it looks like you need to lengthen the cartridge case from 1.770" to 2.500" and make the overall length about 3.20". That would allow a 160 grain Reichenberg Spezial Jagd bullet seated one caliber into the mouth to reach 2500 fps in a 24" test barrel propelled by about 35.5 grains of H335 powder. There are assumptions involved in this that could throw it off by a little. I assumed a 0.15" thick head. If you made the cases a modern design for 50,000 psi so it would work in most bolt action rifles or in a pump action, you'd be looking at a head probably closer to 0.175" minimum thickness, so you could make the case 2.525" long and the overall length 3.225" to allow for that. If the chambers are widened a little at the rear to allow the long straight case to feed from a magazine, the brass also might have to have some extra wall thickening just forward of the head that might make you use 2.6" of case length and 3.350" of overall length. By way of comparison, a .30-06 has a maximum length of 3.340", and has a maximum case length of 2.498". So you are right in that same ballpark for lengths.

The bottom line is you will have a range of possibilities, depending on the firearm design, magazine feed, ability to make modern brass, etc. The main thing for establishing length was getting enough to lower the expansion ratio to where you could use a little bit slower powder at a high enough charge weight to make more gas overall and get some late bore acceleration.
 
You'll need the case capacity of a cartridge capable of pushing a 160gr bullet at 2,500 fps.

You could do that with a 32 SPC if you bumped the pressure a smidge, it will launch a 165gr bullet at 2,410fps at the muzzle. That's 2.9 cm cubed volume. I'll bump that to 3 cubic centimeters for good measure.

To calculate 2.9cm cubed with a radius of 4mm (8mm internal diameter), 0.35x0.35cm = 0.16 cm2. 3.0 cubic centimeters/pi time 0.16 = 5.9 cm.

Add in another 5mm for case head and rounding errors and you have an 8x64 straight walled rifle round. That's 30-06 length, and I'd really want at least two degrees of taper on a straight walled cartridge that long (prevents sticking issues on extraction).

This is assuming that the cartridge is operating at the 32 SPC level of pressure. Bump it up a bit and I'm sure you could shorten the round a tad.

Jimro
 
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You see. I dont want the rifle round to be perfect. It isn't. I want it to be quirky. You see, the country, or commonwealth that's using it developed and advanced it in a family of weaponry. Really, this rifle cartridge is based off the dimensions of a sturdy, thick cased paper cartridge, thick with wax to keep the moisture out and to keep the thing durable- a 9.271mm bullet. The thick paper case was not nitrated. They then experimented with metallic cases forming a rimmed, cartridge with a heeled bullet. This was also when they began to experiment with high pressure propellants, during a time when metal quality was not all that time, as it was after all, war time- it was made from either low quality brass, or soft iron. Their rationale was not to make the cartridge taper to improve reliability, but to make it extremely thick. In other words, the cartridge case thickness at the mouth was half a millimeter, twice that of the 5.56mm. And the thickness near the web was double that, a single millimeter.

Decades later they decided to work on magazine fed weapons, they realized they needed a rimless cartridge. Seeing they had so much of the rimmed ones, they literally just took them, and shaved off the rim and machined in extraction heads. Their automatic firearms look weird in the inside. The feed ramp was long and shallow, and the magazine placed higher. The chambers always had to be of low tolerance. Steel became especially favorable as chamber pressures increased to modern levels- approaching 60,000 psi. Steel doesn't expand as much as brass, so I assume they're more reliable.

I know this will result in a not so accurate rifle that will run extremely dirty. My question now is, will the aligned magazine, long feed ramp, loose tolerances and steel cases make magazine fed rifles chambered in this long, untapered thing feed reliably?
 
Giving you a serious answer,I suggest you study the 9.3 by 74 R cartridge.
It is generally straight walled,though with significant taper,increasing case volume.
It is a rimmed cartridge,headspacing on the rim.Bullet dia is .366.If memory serves,ita a 286 gr bullet at about 2400 fps.
The 74 mm case length,about 3 in,is pretty darn long.
At some point,the process of drawing brass longer has limitations.
Extraction of a very long straight small bore will be an issue at some point.
Anyway,if I understand your request for proportions,the 9.3x74 approaches the extreme.It may give you something to work with.

Feeding from the magazine,etc.
I'm afraid my imagination and yours do not work quite the same.
You might want to make a layout on grid paper and work with manipulating cutouts.
I think you will find it very challenging to get a non tapered case resembling a cigarette to feed upward,even at a shallow angle,into a straight chamber.

I think you will have a problem with "going around the corner" into the chamber.Taper makes it work.

Maybe if your historical progression began with something like an ice cream cone wound waxed paper cartridge...

You also might ponder shotshells.Old ones were waxed paper,with the brass base.Like a .410
 
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I think a fictional round fired from a fictional rifle can be anything you want it to be since you can also use fictional propellants.
 
I see nothing wrong with loose tolerances and a long feed ramp.

I do see an issue with using brass or cast iron for small arms without using steel for the bolt, barrel, and chamber. Brass or iron was last used for major weapon systems in the US Civil War. Brass was the frame for some lever rifles, and some artillery was made from cast iron such as the Parrot Rifle (artillery piece).

Using Cast Iron for small arms is a bad idea because it shatters. The Parrot Gun got away with this because it used really thick cast iron, and the lever action brass framed rifles used steel chambers and barrels.

Even today the aluminum receivers of an AR-15 or AR-10 is just there to hold the barrel extension in place, all the parts that touch the ammunition are ordnance grade steel.

There is a cartridge, the 400 Whelen, or the 411 Hawk, which is a low taper essentially straight walled cartridge. They function just fine out of bolt action rifles. A rimmed 458 Win Mag, or longer 458 Lott, is also essentially a straight low taper cartridge which functions fine in bolt action rifles.

For semi auto rifles such as a Browning BAR (hunting, not WWII machine gun) they will also function just fine.

It is easy in my mind to see a gas operated semi-auto rifle built around the fictional cartridge you describe. A stretched out AK variant would be relatively easy, just have to have a longer bolt and gas piston to travel further to snatch up the next cartridge in the magazine. Or stretch out an M1 Carbine (the Ingram corporation stretched out the M1 carbine action to handle even 7.62x51 NATO, so it is possible).

The Swedes routinely used a larger body diameter in their small ring Mausers to make chambering the 6.5x55 cartridge easier (although they kept headspace within tolerance). This leads to bulges on the sides of some 6.5x55 brass, and the reason I don't full length resize my 6.5 brass (over works the body of the brass which leads to premature failure).

Your fictional 8x64 straight cartridge probably wouldn't do well at 60k PSI, which is why I used the 32 SPC as the basis, long small taper rounds tend to stick more than shorter higher taper rounds (one of the big reasons the 7.62x39 is so reliable is the large amount of taper). I would keep the pressures around 45k or under for reliability. This is easily done with even primitive powder technologies (such as cordite or flake powders).

Hope this is helpful.

Jimro
 
Also, I didn't say that the weaponry themselves were made out of aluminium and cast iron. I said the cartridge cases were made from them.

Okay then cases are extremely thick, with a lip diameter of 9.271mm, and 10.3mm at the rim.
 
OK.I'm not saying this to be a critic,I'm trying to share more useful knowledge.

A really critical task of the cartridge case is to seal the gas pressure in the chamber.
At this point,the ideal is the thin walled brass case...at least thin at the neck and forward parts of the case.
As pressure begins to build,the cartridge case obturates...expands,to make a gas seal that keeps the hot gas where it belongs.
Like out of your face.
It is amazing how fast the steel of the firearm,like a bolt face,will be cut by the hot gas if it leaks.
 
Also, I didn't say that the weaponry themselves were made out of aluminium and cast iron. I said the cartridge cases were made from them.

Consider this a pennies worth of free editing advice then.

The subject of this paragraph is magazine fed weapons.

Decades later they decided to work on magazine fed weapons, they realized they needed a rimless cartridge. Seeing they had so much of the rimmed ones, they literally just took them, and shaved off the rim and machined in extraction heads. Their automatic firearms look weird in the inside. The feed ramp was long and shallow, and the magazine placed higher. The chambers always had to be of low tolerance. Steel became especially favorable as chamber pressures increased to modern levels- approaching 60,000 psi. Steel doesn't expand as much as brass, so I assume they're more reliable.

You'll just have to forgive me for assuming that the steel in the paragraph referred to the firearm (the subject of the paragraph) and no the cartridge (the subject of the preceding paragraph). To further explain why it is reasonable for me to assume that your words referred to the firearm and not the cartridge, there are examples of cast iron and brass chambered and barreled weapons from the Civil War and post Civil War era. The brass barreled weapons were generally 22 rimfires made as cheap as possible, but even in the last two decades you could still find brass barreled air rifles for the same reason.

It is also true that cartridges have been made from paper, brass, steel, and aluminum. It is known that you can increase chamber pressure using steel cases (the 7.62x54r is a good example of this) but because of the lack of springback in soft steel you then run into extraction issues, even in cartridges with significant taper. Hence the reasons for copper washing or zinc plating steel cartridges, to create a slicker surface between the steel case and the steel chamber (in addition to corrosion control for the cartridge).

So if you are going to use steel for a case, keep the pressures low enough that it doesn't cause sticking, or coat the steel with something that lowers the friction to avoid sticking.

Jimro
 
Okay, I'll add taper to the cartridge at this point, shrinking the case mouth thickness to acceptable levels.

But in a last attempt. What if the straight walled bullet was perfectly lined up with the barrel, meaning no ramp has to be used. Just a straight shot into the chamber.
 
If the rim were fed onto the bolt face and held in place by an extractor claw of some sort, yes it is possible, no need for a long feed ramp.

You would want some sort of cone surface around the chamber just in case a cartridge was slightly misaligned it could be guided into the chamber.

I know it isn't exactly like your fictional setting (it seems industrial refinement is less than 1930s US tech), but the M1 carbine did use a straight walled rimless cartridge and did so quite well. The fictional cartridge you are designing is probably longer, and in larger diameter, but I don't see why it wouldn't work. The length of an M1 Garand action feeding ammunition from a box magazine like an M1 Carbine.

Jimro
 
Just to say Jimro, I really appreciate your help.

And yes, the rounds I'm thinking of are longer. The first is full powered, the second is a pistol round. And of course the final is an intermediate cartridge for use in assault rifles.

Also is that statement on untapered, or tapered cartridges.
 
Not to nitpick,but many (not all ) "straight walled" cartridges ,including the 30 Carbine,are slightly tapered.
Part of that is about ease of extraction,but as the bullet comes up the feed ramp,and enters the chamber,angling upward,at some point the bullet ogive hits the upper surface of the chamber.Atruly straight case must be short,like the 45 ACP.Note the 45 ACP 1911 has a fair portion of the lower chamber unsupported.
At the time the ogive of the bullet contacts the upper chamber wall,the cartridge case is (hopefully) being released by the mag feedlips so the case head may ride up the bolt face and become coaxial with the chamber.
Also,at about this same point,the lower edge of the chamber is on the verge of contacting the side of the cartridge brass.This is a critical point.Intrference here results in a stoppage
The longer the cartridge,and the steeper the feed angle,the more taper is required to avoid interference.
Watch a semi-tractor trailer rig turn a tight corner in the city and you can watch a similar dilemma in action.
Now,there is another way.If you look to the tube magazine technology,you will see the "lifter"
One of the most innovative,simple,and effective is the Ithaca 37 shotgun.
It has no side ejection port.
The cartridge is released from the mag tube,the case head,the rear of the case,rides up the lifters to the breech face.
As the lifter arms raise the mouth of the cartridge to the chamber,they spread!.
They pass around the cartridge,to a position above the cartridge.
After firing,upon opening the action,the lifter arms snap downward,acting as ejectors,flinging the hull out the bottom of the receiver.
Simultaniously,the cartridge stops are tripped,releasing another round to the lifter arms.
Amazing design!!I really like the Ithaca 37
 
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Breadbasketbomb,

I hope to read your fiction someday. If you are putting this much effort into the ammunition you'll probably avoid the gun handling mistakes that some authors make describing action scenes.

Jimro
 
Well, these are the final designs I came to. And well, not to be awkward, But I have changed the initial wanted velocity, because I assumed that long range obsession would carry through for these people.

Calibers are 8.241mm

The full powered cartridge is now a 74.168mm long cartridge. 9.271mm neck diameter, 10.301mm rim diameter. Straight walled, bullet seated 1.5 calibers deep. 200 grain bullet.

The intermediate cartridge is 43.264 inches long, also with a 9.271mm neck Diameter. However it has a 9.872mm rim diameter, and is seated only 1 caliber deep. 120 grain bullet
 
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