Once you fire you must press R, and unless you have a pistol, you must stand still. After you have reloaded which you can tell by the ability to turn with the mouse, you can fire again. You have unlimited ammo in your pockets, so don't worry about running out.
Gun reload speed is on their wiki page. This page is for strategies when reloading and what to do.
The rifle has the longest reload time of any gun in the game making you an easy target for other riflemen if your positioned poorly. As a rifleman hide well when reloading is essential. If you hide in a bush or a low wall, you're probably going to get shot.
Reloading rifles are easy targets for cavalry as you stand still for so long. It is advisable to hide in a building when reloading or at least behind a sapper's fortress wall.
Yes I know it is a sin to put these two in the same section, but their reload speed are only one second apart.
Both muskets should be reloaded with caution, but you don't have to worry about as much things as a rifleman does.
When you reload with the musket you should find the nearest tree or wall to hide behind. Crouching and using the '/e dance2' emote will help you avoid getting shot when fired at.
If your line infantry you can lure cavalry to you by reloading. When they're getting pretty close, stop reloading and brace the bayonet to kill the horse.
The pistol has the ability to move while reloading (and aiming). Use this to your advantage, move side to side when someone is aiming at you, run if you see someone coming, or go back through a door when you come in on some enemies trying to kill a dude that's stuck in the corner.
Pistols should be use in close quarters so retreat when reloading so you allies can attack while you reload.