https://en.wikipedia.org/wiki/The_Thin_Red_Line_(Battle_of_Balaclava)
The musketeers shot in 3 volleys. Why not let individual soldiers to shoot any time they wish?
Would the rate of death be bigger if they shot in volleys?
If anything, the number of 2 bullets shooting the same guy or horse will be less.
This answer goes into why archers would synchronize fire, and the same applies to line infantry. Essentially, you don't have to kill the enemy to win a battle, you only need to make them run away. The point of firing as a volley is to break the opposing line's morale.
If you were watching an enemy line of musketmen shoot at you as fast as they could, you'd see little puffs of smoke and hear sharp cracks from their guns constantly, and may occasionally see the person next to you or nearby cry out and fall down due to a bullet or two hitting them.
If you were watching an enemy line of musketmen shoot a volley at you, the entire enemy line would be shrouded in smoke and a cacophony of noise, and several people to your left and right suddenly fall down, riddled with bullet holes, leaving you standing alone for a second before the people behind you take their place.
Seeing several of your fellow soldiers fall at once makes your position seem untenable. A soldier is much more likely to realize how much danger they are in when under volleys of fire, and much more likely to panic. They may misload and not be part of the next volley, or even try to run away to safety, potentially causing the whole unit to follow suit and rout.
Volley firing is a procedure. It involves multiple lines of troops, firing and reloading in succession. While one line fires, the other is reloading, preparing to deliver the next volley. This organization allows a commander to control the actions of his troops, and keep them ordered and calm. Military action is about discipline, not 'each guy does what he wants, when he wants'.
The volley also creates a more drastic result on opponents. This case was about a cavalry charge, and a single shot might have toppled a horse, but a volley could bring down many, disturbing the charge itself. Which apparently happened in this case:
The 93rd discharged three volleys: at 600, 350 and 150 yards respectively, however they did not get a chance to discharge one at point blank range as the Russians turned away.
(Update adding a little more specifics concerning the weapons and loading drill actually used in the referenced Battle of Balaclava.)
A little more research brought up an interesting article:On the Thin Red Line: Loading and Firing British Muskets during the Crimean War, 1854-1856. by Robert Henderson. This goes into some detail on the weapons the 93rd Highlanders would have had access to during this battle:
The British Army was in the midst of a significant weapons transformation from smoothbore muskets to rifled muskets. While a number of regiments had been supplied with the pattern 1851 Minie Rifled musket, the majority of the army still carried the 1842 pattern smooth bore musket. By the end of 1853, the Enfield Rifled musket as approved by the War Department for the army and was put into production. ... Suffering from a chronic shortage of rifled muskets, regiments not going to the Crimea were stripped of their Minie Rifles which were given to the troops going. By the time the British force left in September they were armed with a mixture of the three different muskets.
This same article also goes into detail concerning the actual loading drill followed, which had been recently changed.
By the time the Regiments embarked they would have been well versed in the new loading and firing drill. This new drill was published in The Infantry Manual containing Directions for the Drill and Instruction of Recruits, the Manual Exercise, the Revised Platoon Exercise, an Abstract of the Field Exercises and Evolutions of the Army &c. &c. &c. (Whitehall, 1854).
The article provides a long detail of the loading drill itself (to extensive to repeat here) but a list of the steps involved (the articles shows details, with each of these steps often requiring several individual sub-steps):
The article also discusses the ranges involved, for those concerned with the accuracy of such weapons at these ranges (emphasis mine):
Sights varied on each model of musket. The 1842 musket had a block sight good for about 150 yards. The Pattern 1851 Minié Rifle Musket had a graduated backsight to 1000 yards; and the pattern 1853 had graduated backsight to 900 yards.
For those interested a little more concerning these weapons can be found here:
There was more shock value to in having, say, 20-30 men in the other guys' line of infantry hit simultaneously (and graphically maimed, falling, and screaming - if not dead), than them seeing individual soldiers taking potshots at their line and missing half the time (or, if hitting, probably hitting someone a few yards from them, which their stress-induced tunnel vision might allow them to not notice anyway).
Also, it was to the benefit of a unit to have all of their soldiers follow the same prescribed 30-second-long series of steps under a drilled-in set of orders (reload, cock, aim, fire), to keep them focused on task and not have some of your soldiers looking up from their ad hoc reloading to gape at the approaching Frenchies and turning around and running in panic.
There are a few reasons.
Drill. Troops are drilled in time to reload, aim, fire. A group of troops doing this to the same beat work quicker than all individuals racing at their own speed. Reloading of muskets can also get in the way of others aim, volley fire allows for rows to fire one after another and limit how much they get in each others way. Feeling part of a group/unit that is acting as one in this manner greatly improves morale, group discipline supercedes individual discipline
Aim. Muskets fired shots (round balls) that tend to have really poor aim (wind especially effects them). Odds are a musketeer wont hit what they are aiming at anyway, but 50 muskets firing will give the appearance of hitting their target.
Deadliness. One bullet hitting may not be a kill, multiple bullets are more likely to have atleast one kill shot.
Enemy morale. A volley drops a noticable amount of soldiers at once...firing one at a time does not give the same image of effectiveness. Most musket battles did not end im one side obliterating the other...it usually ended in one side fleeing from the other.
Own morale. Firing in a group produces results and the unit feels effective...even if your individual shot missed, odds are a few in your group did hit. Firing and missing (like you most likely will) repeatedly decreases how effective you feel and increases your likely hood to run.
Morale is important but bullets are bullets.
Originally hand-held firearms were not very safe. If you are loading and thus your gunpowder pouch is open, and your neighbor is firing, a spark from his shot or priming may ignite your pouch and it will explode, killing your and possibly causing a chain reaction or some other people are loading too.
Thus arquebusiers had to fire and load all at the same time, which led to volley firing.
As muskets became safer, the importance of volley firing for safety decreased, but by that time the drill was so advanced that even without volleys the rate of fire would not be much higher.
However, by that time the morale effects of volleys was discovered, so the trade off between the morale effect and a slight increase in rate of fire was an easy choice.
Two reasons: accuracy, and discipline.
While rifles did exist at that time, they were slow to load due to the need for a tight fitting bullet to engage the rifling, and also required a heavy mallet to drive the slightly oversized bullet down the barrel. (except for those sneaky colonials who used a greased patch of cloth to complete the seal, that could be quickly reloaded without need for a heavy mallet) For that reason, muskets of that day were smoothbore with a slightly undersized bullet for rapid reload. With no rifling and a loose fitting bullet, aimed fire wasn't useful beyond maybe 30-50 yards.
Firing in volleys with everyone aiming in the same general direction turned a musket platoon into a giant shotgun. An individual firing a musket at your unit at 80 yards isn't going to do much harm. 15 or 20 musketeers firing at your unit at 80 yards, some of you are going to get hit. This could also be demoralizing to an opponent - armies breaking and running away was not uncommon.
It wasn't until the development of the Minie Ball, a bullet with a cone in the base that could be dropped down a barrel quickly and would expand and engage the rifling when fired, that militaries began to adopt rifles... first major use was the US civil war, with horrendous casualties as soldiers marched into aimed fire from rifles accurate beyond 200 yards.
There is also discipline to consider. Combat is a hectic, confusing, and occasionally terrifying experience. At that time, a lot of soldiers were involuntary conscripts with not a great deal of training, thrown into the confusion of a battle. It's important to remember this as we tend to think of soldiers today as the very highly trained volunteers in the major military organizations.
Soldiers, especially fresh recruits pressed into service, perform better when given explicit orders for every action in the chaos of battle, and are less likely to break and run. Musketeers were typically organized in rows of three - the front row firing on command, then stepping to the back and starting the reload process while the other two rows took a step forward, all on command. By the time the other two rows had fired and stepped back, that first row had completed the reload.
By doing this, a steady and heavy rate of fire could be kept up, with semi-aimed volleys.
Ironically, the British Army had developed a rifle that could be rapidly reloaded and was accurate beyond 100 yards, in the 1770's... the Ferguson Rifle, the first operational military breechloading rifle. 100 of these saw use in only one battle, the Battle of Brandywine in the US revolutionary war. And then, most of them disappeared when the unit commander (and inventor) Patrick Ferguson was wounded, and the unit was disbanded. The British didn't expand the use of this weapon, partially because it was expensive, and partially because they hadn't developed the tactics to take advantage of it's high accuracy and rapid reload... still using pole arm formations and firing in volleys. Had the Ferguson Rifle been in the possession of the colonials, who had already made good use of the woodsman's Kentucky Rifle and unconventional tactics, it might have been a different story.
It came down to accuracy. Or rather: the lack of it.
Until rifled guns became technically and economically feasible around 1840 - 1860 the best way to fight was to shoot with a large body of men simultaneous at an approaching large body of men. You wouldn't aim your gun, but point it towards the enemy. Muskets usually didn't have sights on them. There was no need for it. Effective range was up to about 100-150 meters. Beyond that was not realistic. A waste of powder and shot.
There were (a few) rifled guns, for example the Baker rifle. Those guns were accurate, for the time even very accurate. The British 95th regiment was an elite unit. You could only join it if you were an accurate shot already. Problem was that reloading was very slow. You had to literally ram your bullet down the barrel with a mallet. This, of course, resulted in a very low rate of fire. Very accurate, mind you. The men of the regiment were instructed to aim at important targets, preferably officers. Just like marines or SEALS today, men like these were in short supply. The British had only 2 rifle regiments.
The Austrians had a few units equipped with air rifles. Those rifles weren't bb guns but deadly, highly accurate and (for the time) rapid fire weapons. The French often shot soldiers armed with air rifles on the spot. They hated them as much as we do modern snipers. Those rifles needed air cannisters for refills, and the system was later abandoned. Too fragile in battle, and too complicated. Manufacture of the air cannisters was very difficult.
But that kind of precision was not available for the rest of the army. The British had them, there were some German and Austrian jäger units. That's about it. The French didn't use rifled guns a lot. Their light infantry carried the standard musket of the army.
So, the first thing you need to understand is the lack of accuracy. One could at best hope to hit something in a battalion sized target. Of course this was tried out many times in exercises by every army.
Second: Firepower. You could fire 3 to 4 shots per minute with a Brown Bess rifle. (Rifles had at best half that firing rate) That is the average. One could fire more rapidly, but not in battle. All you could do is try to hit the approaching column and hope to hit something in it.
Third: combat stress. 3-4 shots per minute doesn't sound much, but try to do it, in battle. Lots of times soldiers forgot to pull out the ramrod and fired that as well. Or the musket misfired, and they added another charge on top of it. Misfired. Reload. Misfired. Another reload on top of that, and ... BOOM. That 3-4 shots per minute dropped considerably during a battle due to fatigue.
Fourth: smoke. Black powder weapons create enormous clouds of smoke. You have to see it to believe it. Imagine +500.000 men shooting at each other with black powder artillery support. Smoke really obstructed the view.
The only way an army could effectively fight with muskets at that time was to mass fire in the direction of the enemy.
Another reason is that if a group of musketeers are firing 'at will' then most soldiers will find their target obscured by cloud of smoke coming from the muskets firing every couple of seconds either side of them.
Consider a dozen men firing at will with a weapon that takes 30 seconds to re-load. That works out (overall) to one shot every 2-3 seconds. Each shot produces a large cloud of smoke that will linger for long enough to obscure the target for the next man to fire.
Now consider the same men firing as 3 ranks of 4 muskets. With the same 30 seconds to re-load, that works out to one volley of 4 shots every 10 seconds - given even a light breeze, that 10 seconds will be long enough for the smoke to disperse giving a clear target for the next rank.
Contrary to what some of the other answerers have said, volleys were not really about killing efficiency; firing individually was arguably more effective for that, hence why most armies had some variant of "commence fire" or "fire at will" in its drill. Rather, it had to do with the relative importance of morale.
Remember: muzzle-loaders have a significant reload time. Thus, if you command a company to commence fire, the result is not a continuous rain of fire; rather, it is more like a series of exceptionally ragged and unimpressive volleys. In effect, it is a tradeoff: improved accuracy and slightly better rate of fire, but you lose the morale impact of a massive, crisp volley.
Something people tend to forget is that the point of war is not generally to kill the enemy; it is more an unfortunately necessary consequence of achieving your tactical and strategic ends. If you can make the enemy soldiers break and run, it is often a superior outcome to outright killing them, since they are less likely to kill yours in return. That is why the American Civil War and WWI were such utter bloodbaths; the combatants' operational doctrines were based around routing the enemy, but that rarely happened—instead, they slowly ground each other to bits.
TL;DR: Individual shots give up the extremely important morale factor for a relatively small gain in killing efficiency. This was necessary in some cases, but generally not worth it.
