An example is on the challenger tank e.g. see http://www.army-technology.com/projects/challenger2/challenger29.html either side of the main gun. My wags include ventilation, mortars, smoke generators? I tried googling without success
I think these are used to generate smoke. Not sure if they generate smoke, or if they contain smoke grenades.
Smoke grenade launchers. Used to provide cover for an attack or a retreat. Some modern tanks can also fire anti-personnel grenades or infrared decoys from them.
Just to be really boring, the Challenger 2 uses two five-round L8 grenade dischargers, manufactured by the Helio Mirror Company.
Bet your glad you know that now, aren’t you?
“your”? Argh.
/me notes company name down and starts thinking of were to attach those Challenger 2 babies to the car (In the battle for parking spaces EVERYTHING is allowed 
)
were=where
Right Zumpel?
thanks people, another one of lifes mysteries solved
Most tanks used to have persiscopes, too.
Most tanks used to have periscopes, too.