As a gunshot requires an explosion and there is a vacuum in space, what would happen when a gun is fired in space?
It will fire as normal, although the bullet will travel at a higher velocity due to the absence of air-resistance and will not slow down either.
The cartridge contains all the propellant and oxididiser to create the gases that needed to release the bullet - it does not need any outside oxygen to complete the process.
The obturance or 'fit' of the bullet is such that that it is tight enough to pass through the barrel without jamming but not so small that the propellant gases escape around it, thus reducing its overall velocity.
Whilst therefore you could fire a rifle in space it would be inadvisable to do so. Newton's 3rd Law states that " .. for every action there is an equal and opposite reaction " so unless you are securely fixed to your space vehicle, when you pull the trigger your bullet will speed off in one direction and you will 'shoot off' in the other ( albeit at a slightly lower velocity due to your larger mass ). Even if you are firmly strapped to the nose of your space shuttle, firing the weapon will cause your orbiter to move out of position.
Another consideration is that without any other external influences other than the gravitaional effect of the sun, the bullet that you fired will eventually return to the spot from where it started albeit many many years later
As mentioned earlier, a gun can be fired under water, although it is extremely inadvisable to do so - remember those cartoons where a certain rodent inserts a carrot into the muzzle of the hunter's rifle ? - liquids are incompressible and to fire a weapon with a barrel full of water would result in serious injury or even death.
Essentially, the water is not able to flow around the bullet quick enough and is forced out sideways. This results in an overpressure of the barrel walls which at the very least will cause a ring-shaped bulge and in the extreme will cause the barrel to explode. Ouch !! This is why rifle barrels should be pulled through before firing, especially if they have been cleaned and oiled.
0.22in service rifles, such as the Lee Enfield No8, used by many of the UK cadet forces, have brass plates on the underside of the butt showing the number of bulges the barrel has. After 3 bulges, the barrel is considered to be overstressed and is destroyed and with luck the RCO who allowed the weapon to be bulged will be stripped of their range ticket !!!
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