For starters I don't know much about early firearm warfare or how much was conducted aside from the basic premise of one musket line fires while the other reloads etc, so forgive me for a dumb question cuz the answer isnt obvious to me. So for instance you have one army lets say British all full of musket soldiers etc. All of which who have shitty one shot muskets and maybe a bayonet attached. So what stopped the opposing army from simply arming all their men with swords and attempting to rush or ambush the enemy betwern volleys. Once youre upclose the musket soldiers with only a bayonet to defend themselves suddenly become much more vulnerable after breaking rank and have to fight one on one, if anything wouldn't this lead to an easier route as the musket soldiers arent used to or trained for such fighting. So was this ever done aside from the brittish fighting some african or native tribe with no firearms? And if not why not? I'm not very Knowledgeable of this time period as most of my history knowledge is of late antiquity and so on.
Well of course they had melee infantry, it was the musketeers themselves!
Line infantry is often misunderstood because of the focus on the muskets that they used, but they were not light infantry like longbowmen or 16th-17th century musketeers, that is, infantry tasked primarily with shooting at the enemy from a distance, they were heavy infantry, which fought in tight, dense formations to establish an actual physical presence in the battlefield to resist enemy charges and charge themselves. In this light the musket was really a support weapon to the bayonet, and musket volleys only a prelude to the bayonet charge, this is because any charge against an enemy that remained disciplined and cohesive would most likely fail, so musketry served to disrupt the enemy formation and weaken their resolve. Thus both weapons complemented each other, and the line infantry was both a musketeer and a spearman at once, their tactics and formations reflecting this.
Now the balance between volley firing and charging with bayonets depended on a lot of factors, and it varied from army to army, from battle to battle, from general to general; the prussians under Frederick the great for example were greatly renowned for their superior firepower over their contemporaries, as were the British during the Napoleonic wars. On the other hand, the Russians considered the bayonet to be the superior weapon and had a saying: "Пуля - дура, штык - молодец!", "The bullet is dumb, the bayonet is brave".
Sustained fire in this period, contrary to the modern interpretation of being lackluster, was actually far more voluminous than it had ever been in previous centuries, because of the universal adoption of the much faster flintlock mechanism, the shift to shallower formations that allowed all musketeers to be able to present fire at any time, the adoption of the bayonet which led to the pike being dropped and all infantry units being equipped with muskets, and the introduction of platoon firing, which is rather distinct from the better known rank firing in that instead of ranks taking turns to fire, different sections of the line would take turns to fire, so that the more cumbersome maneuvers of rank firing could be avoided, although this was a very difficult tactic to maintain in the heat of battle and things would often devolve into firing at will. So infantry firefights were a very finely tuned art that had decades of tactical and technological innovation behind it, and the understanding of the 18th century musketeer as heavy infantry does much to properly contextualize them, as the musket wasn't really meant to be a weapon accurate over long distances since infantry combat was meant to happen at close distances, before the bayonet charge, and units relied on overall volume of fire to get over the individually slow reloading procedure.
However, the musket also supported the bayonet charge in a more direct way, through the shock volley, in which soldiers instead of shooting various synchronized volleys would just straight up walk towards the enemy formation, shoot a single volley at extremely close ranges, and then charge with their bayonets. During the Great Northern war for example the Swedish devised the Gå På tactic, in which infantry would quickly close in with the enemy, shooting only two volleys at 80 and 20 paces (later shortened to 20 and 10) before charging. The Swedish army as a matter of fact kept using pikes longer than their contemporaries because it better suited this approach. The English armies of the civil war would often also shoot a single volley before closing in with pikes and musket butts. Sometimes the musket fire was completely skipped and soldiers would literally just charge. Of course, such a decidedly offensive tactic only makes sense from the understanding of line infantry as heavy infantry. But this is why musketry was important nonetheless, to push back enemy charges or wear down the enemy from afar before one's own charge.
A funny thing with the bayonet charge was that, if successful, it actually often didn't lead to a melee, it was actually far more of a psychological weapon, as armies would often melt away in the face of a decided infantry charge which marched on despite cannon and musket fire. In the military mythos of the time the musket was considered to be the coward's weapon, killing from a distance, while the bayonet was glorified as the weapon of truly brave men with enough resolve to close in with the enemy line. Melee fights were more common in close quarter situations when fighting in chateaus, villages, towns, redoubts, defiles, gardens, and woods.
While the ordinary line infantry was definitely the most represented type of unit in the battlefield due to their relative cheapness as well as versatility, being both able to provide short range firepower as well as engage in close combat with bayonets, other military arms also played into this balance between firepower and the melee. Most armies of the day had Grenadier units, which were elite units formed of taller, stronger men with superior training and morale, and were used sort of like assault troops specialized in melee combat. On the other hand you had light infantry, who were the ones actually employed for dedicated long range firing, and as such armed with rifles and relying on open order tactics and individual initiative and marksmanship. Then you had cavalry, which was naturally even better at infantry at charging, being able to close in with the enemy faster, as well as being far more frightening to the wavering soldier. In fact cavalry was so problematic that there was an infantry formation specifically used to defend against cavalry, the square formations, in which soldiers would form up in a hollow square to avoid being hit on the vulnerable flanks, presenting nothing but bayonets against the enemy. A well timed cavalry charge against the enemy, such as exploiting an opening in their formation or surprising the flanks or rear, could absolutely wreck havoc. Lastly you have artillery, the ultimate expression of long range fire, able to bomb enemy positions from tremendously far away and opening huge bloody gaps in dense formations, or using canister shot at closer ranges to kill as many as possible.
So in general melee combat was alive and well during the 18th century, but it was a tool that had to be used very carefully, charging against withering cannon and musket fire against an enemy formation that didn't break ranks could be disastrous. Armies would generally want to position their men in favorable defensive positions, as for example in the reverse slope of a hill, or in a trench or field fortifications, to maximize the effects of firepower and make the proposition of a charge difficult and bloody for the enemy. So firepower was necessary to break the enemy formations and maximize the effect of an infantry or cavalry charge, and this far into the age of gunpowder armies had lots of firepower to spare.
Sources:
There's always more that can be said, but you might like some of these older threads on why people fought that way with muskets.
/u/dandan_noodles wrote about Firing Lines here.
Were battles during the time of the american revolution truly fought with two lines of armies standing across from each other with each army taking turns firing at the other? from a sadly deleted user.
/u/eighthgear did Why did armies in the 18th and 19th centuries face each other through lined combat?