Firearms are a staple feature of the thriller genre and those authors who own and regularly use them (particularly North American writers) tend to know what they are writing about, although this is not necessarily true of film and TV producers. Many of the errors relate to basic physics, concerning forces, momentum, sound and heat (as well as common sense). Here I present a number of issues to bear in mind when including firearms in a story and point to a number of improbabilities and impossibilities I have spotted. Necessarily, this is from a UK perspective but some of the arguments apply everywhere. The internet has many sites demonstrating weapons being fired and their owners' delight at the results.
The sound of silence(rs)
There is no such thing as a silent firearm but measures can be taken to reduce the noise when one is fired. The soft "plop" favoured by many screen and novel writers is largely mythical, especially when applied to automatic weapons. The noise from a gunshot comes from several sources including the expansion of gases on firing, the movement of metal on metal as the parts operate and the crack as the bullet breaks the sound barrier. Methods for reducing the noise include:
• Using low-calibre ammunition modified to contain less propellant (powder) so that the bullet does not break the sound barrier. This, however, reduces the range and the damage that the bullet can do
• Fitting a suppressor (the proper term for a silencer) which will reduce the noise somewhat - some gun websites include demonstrations
• Using a single-shot weapon which has fewer moving parts
One possibility is the Welrod, a very quiet, single-shot, pistol developed for British special forces during WW2 which is only useful at short range. A Kate Atkinson character used this in One Good Turn.
Air canes - effectively souped-up airguns which look like walking sticks - are also quiet and are legal to own in England if they are collected as curios and not used. Macdonald Hastings' book on a famous gun expert, The Other Mr Churchill, describes a probable murder by air cane.
Police officers practising on the range wear ear protectors for a reason: gunshots can be very loud. As with explosions, a prolonged exchange of fire in an enclosed space is likely to leave surviving participants with tinnitus. The actor Linda Hamilton suffered permanent hearing damage during shooting the film Terminator 2 when she forgot to put in earplugs for a scene when shotguns were discharged in a lift - and the cartridges used probably had much less powder in them than regular ammunition so were considerably quieter.
We've all seen the films where someone fires a shotgun or other heavy-duty weapon at someone else and the victim is blown across the room or through a window while the shooter remains virtually still. This contravenes the laws of physics. Given that the shot/bullet does not magically acquire energy on its journey from barrel to target, the momentum of the victim plus the shot would be the same as the momentum of the shooter (law of conservation of momentum) albeit in the opposite direction. Therefore, if the victim is blown backwards, so would the shooter be. Of course, being shot with a shotgun would be quite likely to make you fall over, and you may well convulse, but you wouldn't fly backwards through the door or through a sheet of glass. (For a detailed analysis see Tom Rogers' excellent book Insultingly stupid movie physics.)
Lead versus steel
Trying to shoot out a lock with a pistol or rifle is a bad idea. If you fire at a steel lock, e.g. on a barred cell door, the bullet would bounce off and, quite likely, curtail your escape plan if it hit you. Steel is much harder than the lead or copper surface of the bullet. You could shoot around a lock in a wooden door so that it falls out, if you have enough ammunition, but that would take a considerable amount of time and effort. Firing into the mechanism of a lock would not unlock it but would wreck the insides, possibly leaving the tongue still protruding into the lock-keep and jamming the door shut.
Supply and demand
In the UK it is illegal for a private citizen to possess a handgun unless it is a low-powered air pistol. Pistols can be owned provided that they are fitted with extensions so that the barrel is not less than twelve inches long. Automatic weapons and self-loading rifles are also prohibited. Members of the armed forces are not allowed to bring back firearms as souvenirs from war zones in which they have served, although some undoubtedly do. Also, many illegal weapons appear to be smuggled into the UK from Eastern Europe and elsewhere.
Weapons that have been deactivated can be owned legally but underworld armourers have established a cottage industry reactivating them - not always successfully as shoddy engineering can lead to the weapons blowing up when fired. Similarly, starting pistols are sometimes converted to fire live ammunition with variable results.
Owning a shotgun for clay pigeon shooting or killing animals is relatively straightforward although the police will take account of your location and standing before issuing a shotgun certificate. Target and hunting rifles can also be owned legally by people who have good reasons to do so and, again, there are police checks. Self-defence is not a valid reason for owning a weapon in the UK.
Air rifles up to a certain power can be owned in the UK without a firearms certificate but this does not mean that they are not dangerous - in 2016 a child in Bristol was shot in the head with one and suffered serious injuries - see https://www.theguardian.com/uk-news/2017/feb/24/all. The velocity of an airgun pellet, and hence its potential for causing damage, can be increased by "dieseling" - adding a drop of diesel fuel to the back of the pellet. As the weapon is fired the diesel explodes, as it would in an engine. This is, of course, illegal.
Many stories feature "Dad's/Grandad's old service revolver" or a Luger pistol captured from a German soldier in the First World War. Perhaps surprisingly, most of these old weapons will still work if they have been kept dry and looked after. Even old ammunition will still fire although corroded cartridges may not and a rusted weapon could either fail to work or disintegrate dangerously if fired - see Paul Finch's The Killing Club for a graphic example. Many war souvenirs, however, were handed in under the firearms amnesty which preceded the tightening-up of firearms legislation in Britain following the 1996 Dunblane tragedy, in which a deranged gun owner used legally held pistols to kill children and teachers at a Scottish school.
Range and accuracy
Pistol rounds can be lethal over many tens of metres, depending on the type of weapon and ammunition. This does not mean that they are accurate at anything like this distance. Lee Child, in Personal, quotes FBI statistics to the effect that 11 feet is the average distance for "a successful handgun engagement" but this could be extended for some weapons, shooters and circumstances. In general, the longer the barrel is, the more accurate is the weapon. The Sten gun, as developed in WWII, had a short barrel and was made cheaply hence it was highly inaccurate. It has been said, perhaps cynically, that its main role was as a morale-booster for the French resistance rather than as a useful automatic weapon.
The scenario where someone who has never fired a gun before picks up a revolver with a two inch barrel and manages to shoot a villain fifteen feet away in the head, without hitting the person they are using as a shield, (seen in a TV adaptation of a gangster novel) is frankly ludicrous.
Firing through glass will inevitably affect the path of the bullet so a protagonist aiming to shoot someone on the other side of a window, with a rifle or pistol, would normally have to fire twice: once to break the window and once to hit the target (who might not wait around to be shot). An automatic weapon would fire quickly enough but it may be difficult to maintain accuracy. The scene in a popular television series, where the hero shot someone 20 or 30 feet away, through two windows, with a handgun was beyond silly.
Shooting through a toughened vehicle windscreen is also difficult as the trajectory of the bullet - if it penetrated it - would be unpredictable. Following the terrorist attack in Westminster in 2017, when a vehicle mowed down numerous pedestrians, the police have been trained to shoot the driver in such circumstances and have been given specialist, highly penetrating, ammunition with which to do so.
Back at you
Remember the recoil when weapons are fired - an example of action and reaction. This would unnerve someone not used to firearms and reduce accuracy. Weapons fired on TV and in films normally use blank ammunition so there is little recoil and actors can blaze away without harm. Firing the .44 Magnum revolver used by "Dirty Harry" without due care could easily lead to a broken wrist.
All you can shoot
Like the mobile phone which never runs out of charge, the weapon with an infinite magazine is a regular occurrence in films and on TV. Automatic rifles, for instance, will hold around 30 rounds and fire at about 600 rounds per minute. This means the magazine is empty in around three seconds yet in many films they keep on firing for much longer than that. In the film Lock, Stock and Two Smoking Barrels, for instance, one character seems to get several hundred rounds out of a 28-round Bren gun magazine without reloading. Furthermore, the barrels of such weapons never seem to get too hot to touch after prolonged firing, unlike in real life. The magic of cinema!
Authors of crime novels, it must be said, do not normally make these mistakes and the magazine capacities of various weapons are readily found on the internet.
Gunshot wounds tend to result in death through loss of blood, either within the body cavity or externally. A bullet through a major artery, e.g. the femoral, in the thigh, is likely to cause death within minutes unless effective measures are taken to stop the bleeding. If the bullet hits the brain, the higher parts of the spinal column or tears through the heart, stopping it in the process, immobilisation and death will be rapid. Otherwise it takes time.
Bones and nerves
Bullets can smash up other organs, bones, and nerves, and the scenario where someone shot in the arm or leg has it bandaged for a couple of weeks and is then back to normal is wildly improbable - rehabilitation can take months and recovery may be severely limited, especially if the spine or a major nerve complex is involved. A bullet in the shoulder, for instance, could easily lead to irreparable damage to the nerves serving the arm as well as wrecking the bones. Even if the bones healed the chances of you using the arm for anything useful again (let alone shooting, climbing ladders or arresting villains) would be very slim. Nerves heal very slowly, if at all, and nerve damage from a knife wound would also be difficult to overcome.
For helpful guidance on writing a shoot-out try "6 rules for writing realistic, meaningful gunfights" on the CrimeReads site https://crimereads.com/6-rules-for-writing-realistic-meaningful-gunfights/