# Evolution of gun under 10 atm

I am currently building an Earth-like planet with 1g gravity and 10 atm(due to 10 times the atmosphere's molecules compare to Earth). The atmosphere does not contain explosive gases. The exact composition of the air is up to me, but I want it to be as earth-like as possible except for the 10 atm. The surface temperature is 20 degrees Celsius. Through millions of years of evolution, human beings live on this planet, and they differ from Earth humans only in their respiratory mechanisms to adapt to the large partial pressure of oxygen and nitrogen, and so on.

When using the formula from https://en.wikipedia.org/wiki/Drag_coefficient, I found I don’t know how to calculate the drag coefficient.

Intuitively speaking the effective range of the gun is at least ten times lower because the air density has increased tenfold.

What will be different in terms of warfare? For example, the 10% effective range may bring some special tactical evolution, shotguns may become mainstream.

• A denser atmosphere will certainly reduce the maximum range, defined as the distance at which a bullet still has enough energy to be lethal. But the effective range is usually much less than that, as that's limited by accuracy of the weapon more than speed of the bullet, and accuracy won't be affected so much by air density. I don't think you'll see a 10x reduction in effective range, since drag is not generally the limiting factor in a gun's effective range - a bullet past the effective range likely still has enough energy to kill, the gun just can't deliver it accurately enough. Commented May 30, 2023 at 15:36
• I have no idea if it's the case or not, but in an environment with 10 times the partial pressure of oxygen, I worry about old-fashioned gunpowder spontaneously combusting. You might not get guns at all until after you get to late 19th century chemistry.
– g s
Commented May 30, 2023 at 19:54

I haven't done the maths, but IIRC Drag is a squared function (although the square might be velocity, not pressure) - so I'm not sure your tenfold reduction is correct.

Anyways - to answer your question - firstly: No Shotguns would not be more prevalent.

Firstly, we need to consider the premise of the question (this is why I didn't both with the maths) - We assume that both Rifle, Pistol and Shotgun rounds all suffer the same penalty for the increase in pressure (this isn't strictly true, I'll cover that in a second) - So, let's run some numbers:

Max Effective range for Double-Ought Buckshot is about 50-60 yards.

Max Effective range being a combination of accuracy, terminal ballistics etc - it's not the maximum lethal range.

Max Effective range for 5.56 is around 500-600 yards

So, let's assume that everything is reduce in effective range by 10:

Shotguns now are lethal out to 5 Yards, whereas 5.56 is lethal out to the same range as Shotguns.

You'd get more utility out of a Claymore sword than you would a Shotgun.

But wait! There's more! Remember how I said I was being lazy and assuming everything suffers the same penalty? Well, Shotgun pellets are balls. They are less aerodynamic than a Rifle Projectile.

Using our same comparison - a Shotgun Pellet has a drag coefficient (as a sphere) of about 0.4, whereas a 5.56 round has a drag coefficient of about 0.28 - therefore shotgun pellets would suffer more than Rifle rounds.

To give an answer to the second part - how would Firearms develop differently?

My answer is that you would see longer, thinner projectiles with a greater mass than current rounds. This is because it is a more aerodynamic shape. Possible you'd even see sub-calibre munitions, like an APFS-DS rounds from Tanks (Armor piercing, Fin Stabilized, Discarding Sabot) - essentially giant tungsten darts.

This would be so that there is minimal drag and in order to maximize terminal ballistics, you have a greater mass (therefore greater momentum) to offset the higher rate of deceleration once the round leaves the muzzle.

This would significantly increase recoil - so a move to heavier firearms or perhaps a reliance on fixed emplacements, rather than man-portable firearms might be common.

Other options would be an earlier switch to man-portable cannons, where the terminal ballistics isn't reliant on velocity, but is due to it being a cannon shell with a warhead (so it goes boom when it impacts on target).

However - there is another possibility, and I'm raising it because I'm both a WH40K nerd (and the Bolter is cool) but also it would be better in this scenario - which is the Gyrojet family of Firearms.

Now, granted in reality they never took off because it was new, the bugs weren't ironed out and it didn't do anything better than existing technology.

In your context though - having a low-velocity (and therefore low recoil) heavyweight projectile that has it's own method of propulsion (so it maintains it's velocity over a greater distance) would have such a clear advantage over 'conventional' firearms in terms of range that this innovation would have been developed further. Before anyone points out - yes, I know the Gyrojet had an effective range of only 50-60 yards - but that was limited by the amount of fuel in the rocket - more fuel = more range, and the possibility of having a larger projectile that was powered for significant portion of it's flight time to hit out to targets 4-500 metres away would be game changing.

In short:

Shotguns, being relatively short range, would be even shorter range to the point that they are essentially a Melee weapon. Rifles would still perform better due to ballistics. We would use different styles of Bullets for better performance and most likely Gyrojet technology would be adopted and used.

I think i know what you are trying to ask. bear with me in case i dont.

The 10atm would make difference on the propulsion of the bullet because although pressure doesn't (for example) prevent a grenade from exploding it does make the shrapnel travel less.

When it comes to your point of will it have any impact in tactics.Yes. It will be nearly impossible to shoot from more then 300meters because the atmosphere will be super unstable under that pressure and the bullet will be very difficult to predict because the slightest changes of pressure would make some 10x denser air to move your bullet (sniper shots)

So yes shotguns would be the go to weapon since precision would be so difficult it maybe wouldn't even be profitable to make such weapons and train such specialized people to shoot it.

As for nuclear missiles, the fallout would be catastrophic (thanks to L.Dutch for correcting me) since it would take a lot more time for all the radiation to fall, it would spread and most certainly end up in the ocean leaving a trail of midly to higly radioactive particles that later will fall and cause havoc

• I think you are following a wrong reasoning. A denser medium provides more buoyancy and thus will let particles float longer in the air. As a result fall out would be bigger. I also don't see why a denser atmosphere would more unstable.
– L.Dutch
Commented May 30, 2023 at 9:29
• I said that a nuclear bomb from our universe would spread way less because it would be a much smaller explosion, and it depends on what the air is made of (i assumed something needed to be different from the real one). Denser atmosphere would be more unstable due to the difference between height pressure; most sniper shots would be impossible because any weapon made to shoot in 10 atm, would blow up when shot from say the top of a mountain because maybe up theres not even 2atm Commented May 30, 2023 at 9:42
• Too many variables to even make an educated guess as to what could happen, but the main point is, the difference from pressure between high and low altitude would be too massive to ignore i think, in our world it works because theres a slight deviation, with 10atm i think there would be more deviation then a couple of 0.1atm Commented May 30, 2023 at 9:44
• and about the atmospheric unstability, it would be like shooting through molasses Commented May 30, 2023 at 9:45
• I don't understand why a denser atmosphere would be "lumpy", taking on different characteristics at different points between the shooter and target. You might have to adjust for wind a bit more, and shooting over a big elevation change may be tricky, but I don't see why the atmosphere is suddenly so locally inhomogeneous that the bullet doesn't even travel in a straight line. Commented May 30, 2023 at 19:57