I'm working out some bits and pieces around weapons and armour for a Sci-fi setting and it has occurred to me that when using modern firearms there is a small (as a percentage) but significant (in absolute terms) amount of "back flash", EM radiation (mostly Infrared and Visible Light) that falls on the user and those nearby. Not the physical recoil of the body of the weapon but the muzzle flash and waste heat of a shot that impacts the shooter rather than going in the direct of the target. What I want to know is whether there is a quick and simple way to determine how much energy a weapon's user is exposed to, for example as a percentage of the kinetic energy imparted to the projectile?

I'll be using this as a measuring stick for weapons to give me a rough guide on whether a user can get away with no armour, a little armour, or needs to have a full sealed suit when using a particular gun. I'd like to use similar energy efficiency limitations as those in current firearms if possible but I'm open to suggestions concerning possible mitigation or improvements as well. The weapons in question shouldn't be an issue given the question but slightly resemble Eldar Shuriken Guns from Warhammer 40,000 in operation.

  • $\begingroup$ If you scale up the weapon power by orders of magnitude, there is no reason to assume the same percentage of energy would be directed back to the user. For modern handguns, the absolute value of the backflash is small enough that there is no real design reason to worry about more than is already part of the weapon design. If the weapon were 1000 times more powerful, it would clearly become a more significant design feature. $\endgroup$ – Gary Walker Oct 17 '17 at 13:57

This varies a lot from weapon to weapon, even if you are shooting rounds from the same box. And last time I checked I got less flash using carbine than using Margolin pistol - when kinetic energy of bullet coming from carbine was higher. And of course, both were designed not to give much flash.

Weapon only allows some gases and light and gunpowder to go in the direction of its user, because it is cost prohibitive to stop all of it. Thus, guns are designed to stop enough. More powerful guns have more sturdy mechanism that protects user more. I believe that in your world it will be the same - percentage would vary wildly, but total output will be kept at safe levels anyway.

Still, I highly advise visiting a shooting range and shooting various weapons. Anyone who wish to design new weapons for his world, for his book or game, would benefit from getting hands-on experience with what is already available.

  • $\begingroup$ Cool thanks for that, I may well do that, though we don't really have public ranges in NZ, I'll have to find a gun club or similar. $\endgroup$ – Ash Oct 17 '17 at 14:05
  • $\begingroup$ @Ash I'm shooting at club's range - I'm not a member but they are quite happy to take my money ;) $\endgroup$ – Mołot Oct 17 '17 at 14:08
  • $\begingroup$ Yeah in New Zealand handguns and civilian ownership are not things that go together very often, less than an eighth of our population have firearms licenses and those are by default restricted to lever- or bolt-action long-arms with a 7 round maximum capacity. To even own something like a 9mm pistol you need a Grade B license and there is no such thing as a carry permit. $\endgroup$ – Ash Oct 17 '17 at 14:23
  • $\begingroup$ @Ash 1/8? In Poland, there are about 5 gun permits per 1000 citizens (often two or three permits are owned by one person as you need separate permit for sport, hunting, self-defense), and about 1 gun per 100 citizens. NZ seems liberal from where I live. $\endgroup$ – Mołot Oct 17 '17 at 14:28
  • $\begingroup$ Be that as it may, there are maybe 30 places in the country where a civilian can access firearms or ammunition, including all our gun clubs and licensed hunting/camping supply stores. Maybe five of those have anything other than sporting rifles and shotguns. The nearest one to me is an hour-and-a-half away at highway speeds. $\endgroup$ – Ash Oct 17 '17 at 14:49

The amount of energy expressed as light is a small percentage of the muzzle energy.

Consider a .357 Magnum round, the muzzle energy I looked up was 580 foot pounds. This is 7.86e9 ergs. The most powerful laser you are allowed in the US without a license is 5 milliwatts. You would have to shine such a laser for 200 seconds to produce 1 watt-second or 1e7 ergs of light energy. Clearly the light produced energy produced by the gun flash striker the shooter is a small fraction of 1 watt-second. Perhaps somewhere on the order of 1 millionth of the muzzle energy.

  • $\begingroup$ Is that last number based on something I can use or purely a guess as to the magnitude? $\endgroup$ – Ash Oct 17 '17 at 14:25
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    $\begingroup$ @Ash It is a guess of magnitude, but I think it is accurate, probably conservative. You just don't see much light from the flash when you operate a gun, because it is near instantaneous and weak enough that you really don't notice it in daylight. At night, it can temporarily blind your night vision, but the total light energy is still quite small. Staring down the barrel, the muzzle flash is much brighter (and decidedly unsafe). Most of the light that does not go down the barrel is emitted to the side and again misses the shooter. $\endgroup$ – Gary Walker Oct 17 '17 at 20:55
  • $\begingroup$ Cool so probably a useful number to bear in mind. $\endgroup$ – Ash Oct 18 '17 at 10:36

Are you referring to recoil? The Eldar shuriken gun uses some sort of gravity pulse to launch extremely light projectiles at a very high rate of speed. There is no significant radiation produced. Presumably there is some recoil but the gravity pulse uses probably blunts it to a significant degree.

A more real world analogue would be a rail gun, using electromagnetisim to propel a bullet. This would produce a small effect on the wielder, but the EM fields generated are extremely short ranged and cycle very quickly as the bullet passes by each magnet. The recoil would be significant though, just as if the bullet was fired by a chemical source.

Lasers DO produce EM, but by their very nature it is DIRECTED radiation, so there isn't much "backsplash" on the wielder unless they are shooting into some sort of reflective substance. But even this wouldn't reflect much since the laser would heat up the target and turn it into plasma. A pulsed laser, firing in microsecond pulses, would have even less reflection since it would burn through the reflective medium in staggered bursts. I suppose if the laser was being used in a really humid atmosphere that could absorb and radiate heat there could be some IR radiation transmitted back to the wielder, but in general this isn't an issue because if the laser is heating up the air the weapon then ther are other more serious cooling issues to deal with, see below.

Unless the gun is shooting something like gamma rays that could penetrate through the side of the weapon, or is powered by a nuclear charge, it doesn't seem like the wielder needs to worry about backsplash.

For kinetic weapons using a chemical charge, there will be a significant blast, which can kick up a lot of surface material and produce a shockwave if there is an atmosphere to conduct it. There shouldn't be a significant fireball unless there is excess chemical residue that continues to burn once the bullet leaves the barrel. This is common with battleships/artillery since they stack extra powder charges into the chamber to manipulate the shell velocity for a given range. In a military rifle with specific ammunition, powder reside and "flash" is minimal since the exact correct powder amount can be used and what is used is low flash anyway (since a large Hollywood style fireball is a great return fire indicator :)

If firing a significantly large kinetic weapon beyond what a person can hold easily, recoil compensation can include shooting something out the back of the weapon, aka a "recoilless rifle" or bazooka. In this case, the wielder has to be aware of what is behind him since there is essentially a rear rocket blast that counteracts the recoil of firing a large or very fast projectile. Scale this up and you would need substantial hearing and eye protection, as well as some body armor since a lot of debris can be kicked up by the blast.

Heat from weapons is an issue. This is usually dealt with by ejecting a heat sink (i.e. the brass cartridge), firing in short bursts to allow for some air cooling, and eventually replacing a hot barrel. But you can water cool a machine gun, or use spinning barrels like a gatling gun. There is very little heat transferred to the wielder since he can be easily insulated from hot weapon parts via a stock and grips of low conductive material. A laser can't dump heat like a conventional machine gun, so it will need some sort of cooling system. Radiation fins could generate enough heat to affect the wielder (basically like standing next to 1000 degree heating coils).

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    $\begingroup$ No sorry not recoil, more about the percentage of muzzle flash that gets back to the shooter. $\endgroup$ – Ash Oct 17 '17 at 17:09
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    $\begingroup$ "EM radiation (mostly Infrared and Visible Light)" seemed pretty clear and this answer totally ignores it. $\endgroup$ – Mołot Oct 17 '17 at 17:16
  • $\begingroup$ @Mołot I do address it, heck read paragraph 3, but it is an odd way to express the concern and the listed weapon type (Eldar shuriken) doesn't seem to use EM at all, so it is a very confusing way to ask the question. A meltagun would seem more appropriate. $\endgroup$ – Jason K Oct 18 '17 at 12:38

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