The two previous responses are accurate. I just want to add a couple of things generally restating what has been said.
1. The terms 'suppressor' and 'silencer' are used interchangably. Suppressor is the correct term. Also called a 'can'.
2. The sound a firearm generates comes from 3 areas: 1) the cycling of the action, 2) the rapid, uncontrolled expansion, of hot gases from the round being fired, 3) the sonic boom of the slug if it exceeds about 1100 fps. To suppress the noise from a firearm you try to have as much of an effect as possible on these 3 areas. For action noise you can use a single shot or bolt action. Some semi-auto pistols have a slide locking device so the pistol becomes a single shot and the pistol must then be cycled manually. The sonic boom is easy to fixed by using sub-sonic ammo or, judging by your handle, as the MP5 does by bleeding off enough gas so super sonic ammo becomes sub-sonic. To quiet the 'explosion' requires slowing down, controlling direction, and cooling, the expanding gases. This is why when you look into a suppressor you will find, baffles, heat sinks, and expansion chambers. It is also why many suppressors become less effective when hot and less effective when used dry if they were designed to use wet. Also, the lower volume of gas a cartridge creates (ie. less gun powder) the easier it is to suppress. So a standard velocity .22 rimfire is easier to suppress than a .380 ACP, which is easier than a 147 gr 9mm, which is easier than a .45 ACP, which is easier than a .223, which is easier than a .308. Less rapidly expanding hot gases also mean that a suppressor can be made smaller.