Bad science in fiction Hall of Shame

[razark]

50 minutes ago, ARS said:

Is it true that longer barrel makes the bullet more accurate because the grooves inside the barrel allows the bullet to spin further, thereby increasing it's stability and accuracy? How about a shorter barrel with tighter grooves? (Makes the bullet spin inside the barrel with the number of rotation comparable with longer barrel, but at shorter barrel length)

Doesn't the longer barrel allow for more complete combustion, thus greater speed, and therefore greater accuracy?

[ARS]

Assuming the shorter barrel has enough length for complete combustion, and has grooves designed to provide the same number of bullet rotation compared with longer barrel, and the difference is only in length, does this produce a different ballistic trajectory compared with using longer barrel?

[mikegarrison]

A longer barrel (up to a point) means a faster projectile. F=MA, and as long as there is still some F from the pressure behind the bullet being higher than the pressure in front of the bullet, the bullet will accelerate.

But at some barrel length the expansion of the gases created by the gunpowder will reduce the pressure to the point where it is no longer greater than atmospheric. At that point the acceleration from the gases stops and all that remains is drag slowing the bullet down. So the barrel should end before that point is reached.

[magnemoe]

2 hours ago, mikegarrison said:

A longer barrel (up to a point) means a faster projectile. F=MA, and as long as there is still some F from the pressure behind the bullet being higher than the pressure in front of the bullet, the bullet will accelerate.

But at some barrel length the expansion of the gases created by the gunpowder will reduce the pressure to the point where it is no longer greater than atmospheric. At that point the acceleration from the gases stops and all that remains is drag slowing the bullet down. So the barrel should end before that point is reached.

True, but think that would require an unpractical long barrel. 

Main issue with an short barrel is slower projectile who will drop faster reducing accuracy at range. 
 

[ARS]

I read a story about guns and at one point, there's a sniper that used a heavily modified anti-material rifle with absurdly long barrel (around 50 to 70 meter in length) and used ammunition with a caliber of at least 20mm, if not more. It's said that he can snipe the target (humans or armored vehicles) using that sniper rifle from more than 11 kilometer away. Assuming we create a weapon like this, is this claim possible? (Obviously it's impractical)

[Bill Phil]

Just now, ARS said:

I read a story about guns and at one point, there's a sniper that used a heavily modified anti-material rifle with absurdly long barrel (around 50 to 70 meter in length) and used ammunition with a caliber of at least 20mm, if not more. It's said that he can snipe the target (humans or armored vehicles) using that sniper rifle from more than 11 kilometer away. Assuming we create a weapon like this, is this claim possible? (Obviously it's impractical)

Range is a function of velocity and angle. And some other factors.

https://en.m.wikipedia.org/wiki/Range_of_a_projectile

A long barrel will probably get more velocity, especially if the powder charge is large. 

[kerbiloid]

8 hours ago, ARS said:

Is it true that longer barrel makes the bullet more accurate because the grooves inside the barrel allows the bullet to spin further, thereby increasing it's stability and accuracy? How about a shorter barrel with tighter grooves? (Makes the bullet spin inside the barrel with the number of rotation comparable with longer barrel, but at shorter barrel length)

Too fast rotation is also bad.

[YNM]

9 hours ago, ARS said:

Is it true that longer barrel makes the bullet more accurate because the grooves inside the barrel allows the bullet to spin further, thereby increasing it's stability and accuracy?

No, what matters is the rate of spinning.

(continuation)

The reason why long barrels are used is to maximize the convertion between "explosion power" and acceleration - it's a little bit like rocket engine bells. Too long a barrel would make increased drag.

Edited March 16, 2018 by YNM

[p1t1o]

@everyone but particularly @ARS

On barrel length:

There is a maximum speed that you can propel a round via gas pressure. It is limited by the speed of sound in the medium - not in air, but in the combustion gases.

You cant push the round faster than the gases want to expand naturally. 

Note that a pile of 100tons of propellant does not produce a shockwave that goes any faster than that from 100pounds of propellant.

With explosive propellants this tops out around Mach 3-4.

 

To fire projectiles faster you have to use propellants with much higher sound velocity, so you use a "light gas gun" which uses explosive to compress hydrogen, this hydrogen can then be used to propel projectiles up to around Mach 20-25, commonly used in hypervelocity or high pressure physics research.

 

To fire rounds further, you fire a heavier round.

 

So, the gun with a 50-70m barrel that can snipe out to 11km is total BS.

I mean, a 20mm round might be able to travel 11km, but not out of a 50m barrel, and not particularly accurately.

I would imagine that due to, amongst other things, air not being a smooth field (pressure variations, wind, turbulence etc.) that theres an upper limit to accuracy that can be achieved with unguided projectiles. You cant just keep arbitrarily scaling up weapon parameters infinitely.

[YNM]

6 minutes ago, p1t1o said:

So, the gun with a 50-70m barrel that can snipe out to 11km is total BS.

I mean, a 20mm round might be able to travel 11km, but not out of a 50m barrel, and not particularly accurately.

The other problem is that at least on a perfect "flat" (read : ellipsoidal) Earth surface, you can only see for around 6 km from eye height level.  You'd need some leverage, or you need to shoot "up".

Not to mention winds.

[wumpus]

43 minutes ago, YNM said:

The other problem is that at least on a perfect "flat" (read : ellipsoidal) Earth surface, you can only see for around 6 km from eye height level.  You'd need some leverage, or you need to shoot "up".

Not to mention winds.

I'd think the bigger issue is trying to wave around a 50-70m barrel.  The only use I can imagine is for sniping the rear lines of WWI trenches from an equally rear line.  Such a monster would quickly be taken out by artillery.  Even with a 5-7m barrel, I'd strongly suspect that all the limitations would be in the sniper and the bullet, and that at that distance shear chance will overwhelm any bullet velocity (even if you had a two-stage gun.  And I can't imagine the second stage not pulling the barrel at all).

The Paris Gun was 34m.  I'm not sure it could hit a target smaller than Paris (although it did from 130km away).

[YNM]

13 minutes ago, wumpus said:

I'd think the bigger issue is trying to wave around a 50-70m barrel. 

Well, they had the V-3.

[Bill Phil]

1 hour ago, YNM said:

The other problem is that at least on a perfect "flat" (read : ellipsoidal) Earth surface, you can only see for around 6 km from eye height level.  You'd need some leverage, or you need to shoot "up".

Not to mention winds.

https://en.m.wikipedia.org/wiki/Observation_balloon

That or some other method of target finding.

[YNM]

2 minutes ago, Bill Phil said:

That or some other method of target finding.

Just fly up your enemy's back then... Oh, it's an Air Force ! XD

[Bill Phil]

2 hours ago, p1t1o said:

@everyone but particularly @ARS

On barrel length:

There is a maximum speed that you can propel a round via gas pressure. It is limited by the speed of sound in the medium - not in air, but in the combustion gases.

You cant push the round faster than the gases want to expand naturally. 

Note that a pile of 100tons of propellant does not produce a shockwave that goes any faster than that from 100pounds of propellant.

With explosive propellants this tops out around Mach 3-4.

 

To fire projectiles faster you have to use propellants with much higher sound velocity, so you use a "light gas gun" which uses explosive to compress hydrogen, this hydrogen can then be used to propel projectiles up to around Mach 20-25, commonly used in hypervelocity or high pressure physics research.

 

To fire rounds further, you fire a heavier round.

 

So, the gun with a 50-70m barrel that can snipe out to 11km is total BS.

I mean, a 20mm round might be able to travel 11km, but not out of a 50m barrel, and not particularly accurately.

I would imagine that due to, amongst other things, air not being a smooth field (pressure variations, wind, turbulence etc.) that theres an upper limit to accuracy that can be achieved with unguided projectiles. You cant just keep arbitrarily scaling up weapon parameters infinitely.

Funnily enough the Martlet 2, which was fired out of a lengthened battleship gun, managed muzzle velocities around Mach 6, with a range much in excess of 11 km, and altitudes exceeding the Karman Line. I'm not sure which propellant they used, though. Certainly not a light gas gun. Pretty light projectile, too. Well, at least not nearly as heavy as a battleship gun shell.

The 20 mm could be depleted uranium, making it much heavier than a convential 20mm shell. Gun could be a light gas gun with a large charge. Depends on how it's portrayed, really.

Edited March 16, 2018 by Bill Phil

[p1t1o]

51 minutes ago, Bill Phil said:

Funnily enough the Martlet 2, which was fired out of a lengthened battleship gun, managed muzzle velocities around Mach 6, with a range much in excess of 11 km, and altitudes exceeding the Karman Line. I'm not sure which propellant they used, though. Certainly not a light gas gun. Pretty light projectile, too. Well, at least not nearly as heavy as a battleship gun shell.

Rocket-boosting the projectile is cheating

 

***EDIT***

I looked up some paperwork on this and apparently the early Martlet-2 was not rocket boosted (until Martel 2G1 - pictured above, 2nd from bottom) and achieved the specs you mentioned. I also looked up the propellant used, and it turned out to be fairly standard, mostly nitroglycerine and nitrocellulose, basically cordite, similar to most firearms even today.

What I said about mach limits is still definitely true, Im going to have a wee look and see if I cant find out how the HARP gun was able to achieve such high muzzle velocities. Maybe Im off on the range of where the upper limit lies, but I didnt think I could be that far off. 

It did use eye-opening quantities of propellant (650-850lbs), but then the projectile weighed 400lbs. 

I might come back on this.

PS: Since Im very cynical, and since the HARP project was the brainchild of one Gerald Bull, I cant dismiss the possibility that the results were falsified. Part of me is protecting my ego, I know, but there you are.

https://en.wikipedia.org/wiki/Gerald_Bull

Though I think the most likely explanation is that the rules can be broken if you use a large excess of energy, or perhaps the pressure was so unusually high in this device, that the local speed of sound was much higher than in conventional firearms.

***********

 

51 minutes ago, Bill Phil said:

The 20 mm could be depleted uranium, making it much heavier than a convential 20mm shell. Gun could be a light gas gun with a large charge. Depends on how it's portrayed, really.

None of these suggestion require a 50-70m long barrel, and sniper-like accuracy at 11km is probably impossible regardless, without active guidance.

Edited March 16, 2018 by p1t1o

[magnemoe]

8 hours ago, YNM said:

Well, they had the V-3.

Yes, but this is an fixed installation, same with the Babylon gun. 
On the other hand APFSDS  anti tank shells is hypersonic , https://en.wikipedia.org/wiki/Armour-piercing_discarding_sabot 
Scaling this up to an battleship gun would let you use an guided projectile. 
Yes this is an weapon, one who would be able to do long range precision bombardment and perhaps been used for AAA or ABM, for an orbital launcher you needed to go way larger.

 

[DDE]

12 hours ago, magnemoe said:

hypersonic

Hypervelocity? Yes. Hypersonic? No. Need a railgun for hypersonic.

Which in turn brings us US military looking for guided railgun slugs.

[PB666]

1 hour ago, DDE said:

Hypervelocity? Yes. Hypersonic? No. Need a railgun for hypersonic.

Which in turn brings us US military looking for guided railgun slugs.

I thing electromagnetic railgun would only be used as an augmenting device. Chemical propellants typically have much higher energy densities available. There is one reason I could think of for a railgun and that is your powder magazine is essentially a self-destruct button your enemies can push.

[ARS]

Or, a chemical propellant could be used as a backup system for propelling the projectile, so when the electrical system got knocked out, at least the railgun can still be used to fire like regular cannon

[DDE]

6 hours ago, ARS said:

Or, a chemical propellant could be used as a backup system for propelling the projectile, so when the electrical system got knocked out, at least the railgun can still be used to fire like regular cannon

The performance would be degraded so horribly, you’d be better off legging it to the maintenance depot. It’s like trying to use a bayonet once out of ammo for your assault rifle.

[kerbiloid]

Chemical propellants are limited in energy, So, electromagnetic acceleration is the only way.
Kinetic projectiles are limited in heat resistance. Move them a little faster - and they evapourate. So, a cloud of plasma is the only way.
But plasma likes to expand. It should be held compact by self-electromagnetism (like self-gravitation, but electromagnetism). So, an electromagnetically self-confining plasma cloud is the only way.
The most compact shape is sphere.

Ergo.
Relativistic fireballs are the weapon of future.

[DDE]

4 hours ago, kerbiloid said:

Relativistic fireballs are the weapon of future.

https://en.wikipedia.org/wiki/MARAUDER

[KerikBalm]

On 3/16/2018 at 6:07 AM, razark said:

Doesn't the longer barrel allow for more complete combustion, thus greater speed, and therefore greater accuracy?

Its not about combustion, but expansion of the gas. Greater speed produces less bullet drop, but that's not necessarily more accurate.

If you point the gun at a certain point 100 yards away, and all bullets fall within a 1 inch circle, its said to have an accuracy of 1 MOA (approximately, this is a coincidence that 1 inch at 100 yards is approximately 1 Minute of Angle, or 1/60th of a degree). If another rifle is producing a group that is 3 inches across, it has an accuracy of 3 MOA.

If the 2nd rifle has a higher velocity, and the bullets drop less, do you care? If you zero the sights for 100 meters, which one will his the bulls-eye more?

Higher velocity reduces bullet drop, and minimizes the effect of errors in range estimation, but that doesn't mean that its inherently more accurate.

 

On 3/16/2018 at 6:19 AM, ARS said:

Assuming the shorter barrel has enough length for complete combustion, and has grooves designed to provide the same number of bullet rotation compared with longer barrel, and the difference is only in length, does this produce a different ballistic trajectory compared with using longer barrel?

Yes, in general, up to a point (as discussed), longer barrel= higher velocity. The force accelerating the projectile will drop as the gas expands inside the barrel, and there are friction losses, so there's a point that it doesn't make much sense to extend the barrel.

For example, most .308 rifles have a barrel length of 22-26 inches. There are some specialty rifles that are 32 inches in length, but the velocity increase is negligible. The weight increase is nearly linear, but the muzzle energy increase is far from linear.

Obviously, a higher velocity produces a different trajectory.

On 3/16/2018 at 1:56 PM, ARS said:

I read a story about guns and at one point, there's a sniper that used a heavily modified anti-material rifle with absurdly long barrel (around 50 to 70 meter in length) and used ammunition with a caliber of at least 20mm, if not more. It's said that he can snipe the target (humans or armored vehicles) using that sniper rifle from more than 11 kilometer away. Assuming we create a weapon like this, is this claim possible? (Obviously it's impractical)

No. Such a long barrel basically won't work with gunpowder propellants. Maybe with a light gas gun.

https://en.wikipedia.org/wiki/Light-gas_gun#Design_physics

Quote

The limiting factor on the speed of an airgun, firearm, or light-gas gun is the speed of sound in the working fluid—the air, burning gunpowder, or a light gas. This is essentially because the projectile is accelerated by the pressure difference between its ends, and such a pressure wave cannot propagate any faster than the speed of sound in the medium. The speed of sound in helium is about three times that in air, and in hydrogen 3.8 times that in air.

so with no "combustion", and a hydrogen gas inside (and typically a piston to compress the hydrogen in a big tube, with a very small barrel diameter). Then you get into problems with velocity of small projectiles. Its not going to be fast enough to shoot "flat"... at that range, you're looking at mid-range artillery guns.

Even ignoring that... suppose its a super accurate 0.1 MOA rifle.. 11 km = 36,100 feet - 12,030 yards. The spread of the bullets at 0.1 MOA (which is already 10x better than most rifles), is still going to be: 0.1 *12030/(100) = 12 inches/1 foot... so actually, not as bad as I thought... but with a 1 MOA rifle, that would be a 120 inch spread/10 feet/3 meters... and you're not going to be sniping anything with that spread.

[ARS]

A bit unrelated, but I've seen a scene where the antagonist lay waste to the office room with a handheld minigun. The main character managed to evade it and get close using office tables as a cover and slash the minigun with a katana that cuts it's barrel assembly. The antagonist respond it with ditching the minigun and draw the knife for a melee combat. The minigun is only cut (cleanly) at barrel assembly, it's basically halves the barrel length (all 6 barrels cut). In theory, this still allows the minigun to be used right? Does this affect accuracy?