When using the M203 you must take into the accounts of gravity more than pretty well any other weapon. You must aim above your target to compensate for the arc the grenade will take - this takes practise - it is recommended that you practise this locally as it takes some getting used to and you WILL damage your team mates if you haven't got the hang of it, something that should be avoided at all costs.

Another point to consider is that the grenades don't activate the second they leave the launcher - they will activate after around 10 metres of flight - this is as much to protect the firer as anything else. You should take this into account as it becomes ineffective if you are too close to your intended target - if you are firing this weapon and your grenades are not exploding then this is the reason. A last ditch tactic - if the enemy is very close to you fire an M203 grenade at them - it will do damage even though it doesn't explode - it takes a few to take them down but if they are low on health it might just do the trick.

Used correctly this is a highly effective anti-personnel weapon - you will be lobbing grenades through windows a hundred metres away with practise - just be extremely careful using this around your team mates as it will hammer your ROE.