Load the 14 g. projectile into the gun, hold at floor level, and shoot upwards observing the height obtained.  Then load the 30 g. projectile at floor level and also shot upwards.  Observe that this height is only half of the 14 g. projectile height. 

An interesting variation is to use two guns.  Load one with a regular dart and the other with a massive dart.  Point them both at the ground from the same height.  Which dart will strike the ground first if both are fired at the same time. 

Craig Kletzing's variation is to do this horizontally on the air track.  Use the apparatus in the picture as this will give you the most accurate measurements.  Note that the photogate is at 75 centimeters on the air track and that the 600 gram mass is one with a large magnet while the 300 gram mass is one with a small magnet.