Think of an object you throw to the ground, that causes a seismic blast, and by that I mean an earthquake.
How it works, You could lay a transmitter on the ground (perhaps only getting so many a round) and when activated, any enemy walking in close distance get's a shakey screen for 5 seconds, causing aiming to be almost impossible (think of how your screen gets after someone close by going kamikaze) giving an easy kill to anyone who's using it.
Drawbacks - Simply jumping could negate the effects of an activated transmitter, since only the initial activation can affect an enemy. Jumping then immediately landing would safeguard the target.
#2 Person using transmitter could also possibly be permitted not to move for the 5 seconds that the enemy's aim is messed up, this punishes poor use of the weapon, wielding an easy target for n00b weapon users, this may possibly unbalance the weapon and make it no fun, but being overpowered is worse, any thoughts?