A way to increase the bore length but not the barrel of a gun?

[manmade]

I saw an airgun that shot arrows. The arrows was placed over the barrel instead of inside it. I realized that this will make the effective length of the barrel increase, without increasing the length of the actual barrel.

I made a picture to explain what I mean:

I hope you get my explanation. To me it seem to be an overall advantage. Does it work like this, or what’s the explanation why this concept haven’t been favored over conventional bullets? Does it only work with air and not gunpowder?

 

[Baluncore]

Does it work like this, or what’s the explanation why this concept haven’t been favored over conventional bullets?

The rifle was originally designed to fire a small, fast, bullet a long distance, in a straight line. The volume of propellant gasses, produced safely in the chamber, are only sufficient to fill the barrel.

With your external sleeve arrow, the volume of gas that can be produced safely will be insufficient to fill the barrel, and the tube chamber of the arrow, so the arrow will fly too slowly.

External sights and attachments on the barrel will need to be removed before firing the arrow.

"Rifle grenades" usually have a rod that fits part way into the barrel of the gun.
https://en.wikipedia.org/wiki/Rifle_grenade

If the arrow propellant was placed in the arrow tube, at the back of the head, then ignited by the rifle fire, the device would be called a "spigot mortar". https://en.wikipedia.org/wiki/Mortar_(weapon)#Spigot_mortar

 

[manmade]

With your external sleeve arrow, the volume of gas that can be produced safely will be insufficient to fill the barrel, and the tube chamber of the arrow, so the arrow will fly too slowly.

Why is that? Look at this picture:

The same gunpowder charge is used. Then look at this picture when it’s fired:

There’s not much difference in term of barrel volume between the normal gun barrel and the short (half the length) gun barrel + hollow arrow. If the arrow and the bullet have the same weight, they should have very similar velocity. If a normal bullet where to be fired from the first barrel, that is half the length of the second, that wouldn’t be the case at all. The bullet fired from the short barrel wouldn’t have time to stabilize either, while the hollow arrow has as much time to stabilize as the bullet fired from the long barrel.

 

[Frabjous]

1) Your projectile will weigh more.
2) Your effective barrel diameter will be larger so pressure will decay faster.
3) Tumbling will be a problem.
4) You have to load two things not one.
5) In the normal gun, your unburnt powder takes up an unrealisticly large percentage of the barrel. Correcting this will not help your design.
6) Flight performance will be different.

 

[Baluncore]

Why is that? Look at this picture:

The wall of a rifle barrel is usually thicker than the bore. A shotgun can have a thinner wall at the muzzle than the bore.

A barrel is tapered externally along its length. There will be blowback of the propellant charge as the projectile departs. Maybe it is better to use a spigot mortar.

Because the projectile has a greater radius than the barrel, it must be thicker to handle the increased hoop stress. The wall of the projectile as shown, is too thin, it will burst.

The same gunpowder charge is used. Then look at this picture when it’s fired:

The barrel cannot be inserted into the projectile if the projectile is filled with propellant, like a bomb. You need to draw the initial configuration, showing the placement of the propellant charge, before it is fired.

 

[Hill]

I think the initial configuration is more like this:

than like this:

By the time bullet gets to the midpoint it is already accelerated.

 

[manmade]

1) Your projectile will weigh more
2) Your effective barrel diameter will be larger so pressure will decay faster
3) Tumbling will be a problem
4) You have to load two things not one
5) In the normal gun, your unburnt powder takes up an unrealisticly large percentage of the barrel. Correcting this will not help your design.

1, not necessary, it depends on the material it’s made of. Short crossbow bolts are about the same weight as a 9mm bullet, and those bolts are over 4 inch long, the projectile in my design is only 2 inch. However since it will contain pressure, does it have to be as strong as the barrel? Or will the pressure curve be low enough and the force not as directed towards the walls at the point when it reaches the hollow projectile, to allow it to be weaker? If it has to be made of steel to be strong enough, depending on thickness and length the weight might be an issue. Looking at how thin revolver chambers are, I think they could be made thin enough for weight not being an issue.

2, yes, I don’t think this will be a problem though, and if a charge would be to small, it can simply be increased to compensate for the larger diameter.

3, the arrow airgun was very accurate, it used front heavy arrows with fletching. Will the short barrel in my gunpowder design make it less accurate?

4, this is true. But why didn’t they use this concept in muzzleloaded pistols? Black powder burn slower than smokeless, so they had to use longer barrels back then compared to today. For small black powder pistols with low velocity due to short barrel, like the Philadelphia derringer, this concept would’ve been a great improvement?

5, look at a snubnose revolver for example, the cartridge basically take up that much space. The chamber is as long as the barrel. With fast smokeless powder it’s not as big of an issue as it was with black powder. However the concept of my design still holds true?

 

[manmade]

The wall of a rifle barrel is usually thicker than the bore. A shotgun can have a thinner wall at the muzzle than the bore.

A barrel is tapered externally along its length. There will be blowback of the propellant charge as the projectile departs. Maybe it is better to use a spigot mortar.

Because the projectile has a greater radius than the barrel, it must be thicker to handle the increased hoop stress. The wall of the projectile as shown, is too thin, it will burst.

The barrel cannot be inserted into the projectile if the projectile is filled with propellant, like a bomb. You need to draw the initial configuration, showing the placement of the propellant charge, before it is fired.

I don’t think this concept will work for rifles, it’s for handguns. I’ve seen revolver cylinders having less than .004 inch wall thickness between chambers. I’ve thought about blowback, this might be a big issue. But won’t the force of the propellant charge be directed more towards the front, since it’s pushing out the projectile, or will it act as a rocket engine?

Won’t the pressure decrease rather than increase when it fills the larger inside diameter of the projectile?

The propellant have to be inserted into the barrel. With a reversed design with the barrel containing a hollow projectile the projectile could hold the propellant, but I thought such a design would require a stronger projectile since the pressure peak at the chamber before it starts moving? If the projectile is to weak it might expand like a case.

In either way, a blank could be used as propellant, or the gunpowder could be poured from the muzzle and a percussion cap at the rear be used to ignite it.

 

[Baluncore]

I’ve seen revolver cylinders having less than .004 inch wall thickness between chambers.

The revolver accommodates all the chambers inside a sold cylinder. There is a brass cartridge case protecting that thin wall from the gas pressure, and there is usually another brass case on the other side, of that same thin wall.

If such thin walls alone, could handle the hoop stress due to propellant gas pressure, then guns could be built much lighter.

But won’t the force of the propellant charge be directed more towards the front, since it’s pushing out the projectile, or will it act as a rocket engine?

After the recoil has broken your wrist, the propellant gas will blow back in your face, as it continues to expand, pushing the projectile forward.

Won’t the pressure decrease rather than increase when it fills the larger inside diameter of the projectile?

Yes, the pressure will be less, but to withstand the same internal pressure, requires a thicker wall in bigger diameter pipe.

 

[manmade]

The revolver accommodates all the chambers inside a sold cylinder. There is a brass cartridge case protecting that thin wall from the gas pressure, and there is usually another brass case on the other side, of that same thin wall.

If such thin walls alone, could handle the hoop stress due to propellant gas pressure, then guns could be built much lighter.

Does the honeycomb configuration of a solid revolver cylinder increase its strength? I imagine that it will rupture at its weakest spot anyway, which is the locking slot? I also wounder how much a thin brass cartridge would really add to its strength?

 

[BWV]

a rifle is defined as a grooved barrel to impart spin to the projectile, this is what makes modern guns more accurate than old smoothbore muskets. A scoped WW1 era rifle can shoot accurately at up 700-800 meters. How will the projectile travel farther and more accurately with the scheme in the OP?

 

[Baluncore]

Does the honeycomb configuration of a solid revolver cylinder increase its strength?

Yes. The outer cylindrical band of metal, around the outside, holds it all together.

I also wounder how much a thin brass cartridge would really add to its strength?

The brass will make a difference because it will spread the pressure over a wider contact, to include the places where the chambers are further apart and the wall is thicker.
The brass will also prevent erosion by the burning powder, of the thin wall. That is not a problem in the barrel where the walls are thicker.

 

[manmade]

Yes. The outer cylindrical band of metal, around the outside, holds it all together.

The brass will make a difference because it will spread the pressure over a wider contact, to include the places where the chambers are further apart and the wall is thicker.
The brass will also prevent erosion by the burning powder, of the thin wall. That is not a problem in the barrel where the walls are thicker.

I get that a solid cylinder will hold together each chamber, but not why such a cylinder would be better at retaining the pressure inside the chamber at its weakest spot.

Look at this cylinder for example:

If you put one round inside this cylinder and fired it, why would that be different to firing one round in a single chamber of the exact same steel and wall thickness?

The brass case expands inside the chamber, without the chamber supporting it, it would explode. It doesn’t retain the pressure at all. Yes the extra material of the thin brass walls will increase the strength slightly, but not to any relevant degree nor will it distribute the pressure from the weakest part to the strongest, why would it? It’s just an extra layer of material increasing the thickness to each part of the chamber equally?

Look at this burst cylinder:

As expected it bursted at its weakest spot, which is the locking slots. And the locking slots of the two chambers nearby were compressed. While the thick parts of the cylinder are still intact. The thicker parts of the chamber didn’t prevent the thin parts from bursting despite being one solid piece.

 

[manmade]

After the recoil has broken your wrist, the propellant gas will blow back in your face, as it continues to expand, pushing the projectile forward.

Why would the recoil break my wrist if the arrow is of the same weight as a bullet that don’t break my wrist? If the weight of each projectile and gunpowder charge are equal, the recoil should be equal too?

I think blow back from the propellant charge might be the biggest issue with this design. Especially with a 2 inch short barrel. But I don’t know to which extent that will be a problem with a pistol charge of gunpowder? And the propellant is also blowing forwards out of the barrel, not only backwards out of the arrow. It’s like a hybrid of a muzzleflash and a rocket flame crashing together. Maybe taking out eachother?

 

[Baluncore]

The brass case expands inside the chamber, without the chamber supporting it, it would explode. It doesn’t retain the pressure at all.

Big barrels are built up from shrunk on tubes. The theory of elasticity applies. They can be stronger because the material is elastic. The brass sleeve will pull back slightly from the chamber wall after being fired. It therefore must have been providing some reduction in pressure against the chamber wall.

Look at this burst cylinder:

Obviously, too much powder was used, or the powder used was too fast.

Why would the recoil break my wrist if the arrow is of the same weight as a bullet that don’t break my wrist? If the weight of each projectile and gunpowder charge are equal, the recoil should be equal too?

I was basing my statement on your earlier pictures that overloaded the cartridge and projectile. The higher drag will slightly increase the push-back.

All things being equal you say; then I see no advantage in firing a high drag projectile, when a bullet will fly further, faster, and more accurately. You have now defined your idea out of contention.

 

[Rive]

Look at this cylinder for example:

Modern guns has more sturdy approach for higher push. That old style thing had limits.

Also, did you consider that while a conventional gun has only the 'business end' as a dangerous point, your idea would make all the length of the barrel (covered by the 'bullet', what's supposed to depart fast) dangerous?

 

[jrmichler]

or what’s the explanation why this concept haven’t been favored over conventional bullets?

Another reason is related to the ballistic coefficient (BC). Compare the range of a spire point boat tail bullet (high BC) to a shotgun slug (low BC). Then compare to your idea. Start by designing a bullet with a typical weight, say 200 grains. Keep in mind that the inner diameter of the bullet must be larger than the outside diameter of the gun barrel. Calculate the drag at a typical velocity of, say, 2500 ft/sec. Calculate if the drag force will collapse the bullet immediately after leaving the gun barrel. And then you will answer your own question.

Good search terms for the drag calculation: bullet drag coefficient.

 

[berkeman]

I’ve seen revolver cylinders having less than .004 inch wall thickness between chambers.

Link?

 

[cormsby]

I’ve seen revolver cylinders having less than .004 inch wall thickness between chambers.

I do not think so. One of my (many) Summer jobs was at a gun shop, where I cleaned *lots* of revolver cylinders, and none looked as though they had a mere 0.004-inch (only 0.1mm, or 100nm) between chambers. Even 1.0mm (ten times as much) seems inadequate.

 

[manmade]

All things being equal you say; then I see no advantage in firing a high drag projectile, when a bullet will fly further, faster, and more accurately. You have now defined your idea out of contention.

The advantage is that the barrel can be shorter, since the projectile itself will act as a barrel.

 

[manmade]

I do not think so. One of my (many) Summer jobs was at a gun shop, where I cleaned *lots* of revolver cylinders, and none looked as though they had a mere 0.004-inch (only 0.1mm, or 100nm) between chambers. Even 1.0mm (ten times as much) seems inadequate.

Link?

Sorry, I meant 0.04 inch.

 

[Baluncore]

The advantage is that the barrel can be shorter, since the projectile itself will act as a barrel.

But the projectile will not travel far, it is a high drag body, more like a shuttlecock than a bullet.

The accuracy of the gun is partly decided by the way the propellant gas flows as the projectile separates from the muzzle of the barrel. That is non-obvious, but the crown must be very carefully controlled with a rifle barrel to ensure a clean separation. An external tube projectile will deviate unpredictably, as it will be randomly turned by the imbalance of the first gas escape during separation.
https://en.wikipedia.org/wiki/Transitional_ballistics#The_transitional_period

 

[Nik_2213]

OP, I think you need to look at rifle grenades, their various types' pros & cons...
https://military-history.fandom.com/wiki/Rifle_grenade