DJSunhammer said:

last I checked recoil is caused by the rapid expansion of gas when the powder in a current day bullet explodes, rail & gauss weapons is about using magnetism, directing all the force forward along the barrel (unlike a bullet, where the gas is leaked both front and back, the later the cause of recoil)

Not quite:

First, you have to remember that this is total energy, and not all of it goes directly back into the person firing it. Some is instead lost as heat, light, vibration, some is even reflected back and partially granted to the outgoing projectile. In addition, the amount of energy it takes to accelerate, say, a one pound slug at those velocities is small compared to what would be needed to accelerate the weapon firing it [if the weapon is even just ten pounds, it would gain a fraction of the momentum imparted to the projectile].

The 'equal and opposite reaction' is not a direct "the gun must move backwards" effect unless you're a hollywood movie writer. Even in regular guns, only a small portion of the total energy produced goes into producing recoil.

A hand-held rail weapon is likely several kilograms, yet is accelerating just a few grams of metal at a time. As it uses a directed force to eject the projectile instead of a contained explosion, it is also easier to predict and compensate for atop all that. Given its propelled by effects of the Lorentz Force, you could even argue that the weapon is specifically designed to transform electricity into recoil; said recoil being what fires your little metal dart.

The tthree main problems with rail guns is that, due to Newton's First Law they would have monstruous recoil that would require extremely powerful compensation, that the energy required to accelerate any object larger than a subatomic particle is colossal, and that the magnetic forces that accelerate the slug put the rails under steel-bending stress. How the Tau figured out how to solve these problems is anybody's guess.

remmus said:

Recoil is caused because, in essence, you are moving something at very high speeds. In order to move even a small piece of metal like a bullet, you need something to push off of. This is true for both conventional firearms and for magnetic weapons like rail and coilguns. If you push a projectile in one direction there will be an exact copy of that force pushing in the opposite direction. Any heat created by a railgun is going to be caused by two things. Friction and electrical resistance.

@ Kiton:

Yes, quite.

To put it simply. For every action there is an equal and opposite reaction. Always. There are no exceptions. All of the force used to push even a small chunk of metal is also going to be pushed into the weapon that fired it, and the person holding that weapon.

If, even in normal guns, only a small portion of the energy produced goes into producing recoil the opposite is also true. Only a small portion of the energy produced goes into pushing the bullet out of the barrel.

That last part of what you said makes no sense at all. Recoil is reactionary force caused by firing a gun. It perfectly balanced in relation to the forward momentum of the projectile and anything used to fire it. You cannot fire something with recoil. Nor does trying to rewriting the definition of recoil make that last statement anything less than true.

@JuankiMan: You mean Newton's Third Law, not his first.

JuankiMan said:

You know we have rail guns now right? It's just they are bulky as heck because we do not have access to 40k batteries. Also capacitors.

In any event if you want to hurt someone with a given amount of energy, you will get much more bang for your buck if you use that energy to propel a slug (or arrow, or shiruken) than you will by turning it into radiant energy and giving them a tan. Sure, you can hurt someone with a kilowatt laser, but unless they are unconsious or tied up it's only going to be painful but not incapacitating surface burns. To hurt someone quickly you need a LOT of energy in that beam.

Plus with a slug thrower you can take advanatge of useful material properties in your ammo types. For example titanium cores or depleted uranium (who are we kidding? The Imperium would use enriched uranium for better damage, they weren't planning on paying those guardsmen retirement anyway) to help you punch through the armour on a tank.

You know, when I wrote that post, I considered if I should exclude the part about "28 megathule" due to the threat posed by a physics debate derail, but I ultimately decided to copy over the entire piece for context.

I am unsurprised that it did take this turn.

I wouldn't worry about it. Yours wasn't the comment that caused the physics debate.

Andor said:

JuankiMan said:

While that would indeed pith a baneblade it's also a Naval Cannon! That's your prototype for macrobatteries, not an infantry weapon. They can be

If you're over stressing your rails, it means you did a poor job of stress analysis on the design. It's not like newtons 2nd law of motion is news.

On topic the Backpack Ammo supply is clearly a terrible, terrible option. It's NEVER better that simply carrying a sack of ammo around and therefore pointless. It should at very least hold as much ammo as the equivalent weight in charge packs/magazines/shot pouches/what-have-you. It should probably also give you some bonus (like 10% extra ammo) to pay for the inconvienience of not being easily broken down into smaller and more sensible loads. Or it chould provide some other benefit like an improved rate of fire. Technically it does I suppose, since you don't have to stop and reload, but frankly I've never seen a 40k combat go on long enough for a gun to need reloading, not even when we fought the Bloodthirster. Maybe my group just doesn't use full auto enough? Heck I designed my characters custom gun and gave it 5 shots!

(Imagine the Tarus Judge carbine blown up into a sniper rifle and designed to shoot bolts. And then given a lever action because it's 40k and everything is a little bit stupid. Barneto M-8 Boltrifle, Basic, 1d10+5X Pen 4 S/-/- 200m Range Reload 3 full, Accurate, Tearing, 15kg.)

Maybe the Imperium did have efficient, man-portable rail guns that skirted the issues discussed here. I mean considering what else has been done it's not completely unfeasible (Also, actual legitimate magic), but like many things Imperial we just forgot how to build/operate such device.