MP5 Cylinder?
 

What is the use of this hole in the side of the cylinder?
What would happen if it was covered up?

I heard that the sniper rifles don't have them.

Sorry if this is a stupid question but please could someone answer me!!!

It's a port and causes a rapid compression of air, required for shorter barrels (like sub-M4 length barrels). The longer barrels dont' require it due to higher internal volume. If you cover it up, your fps will drop among other things.

That's the nutshell, was MadMax or Illusion at one point that wrote a large explanation about ports on barrels.

The position of the hole is required depending on the length of your inner barrel.
Sniper rifles don't have the port because they tend to have longer inner barrels.

By introducing a port on the cylinder, the effective volume of air that gets pushed out of the cylinder is reduced to just what's in front of the port. This also increases the rate of fire.

The reason why the port is put towards the rear of the cylinder is so that the most effective range of the spring's power is applied to the BB, which is when the spring reaches full extension, rather than at the beginning of it's extension.

Wow, you know your stuff ILLusion.