Shoot the Monkey Physics Project -- need a little help

[Quad]

in case you've never heard of "shoot the monkey", basically what i hafta do is fire a projectile at a monkey suspended in the air. the instant the projectile is fired, the monkey must begin to fall, and the projectile must hit the monkey.
by using an electromagnet, u can make the monkey fall at the same instant the projectile leaves the gun

now my question is: how do i trigger the electromagnet the instant the projectile leaves the barrel? or should i trigger it once i fire the gun? will it make a difference?

thx in advance

 

[amnesiac]

have the projectile hit a switch or complete a circuit by leaving the barrel

 

[bizmark]

it depends. I would set the end of the barrel of the gun at the origin and trigger the electromagnet once it leaves the barrel. However, it doesn't make any difference if you set the origin at the chamber of the gun and do it from the trigger pull. (Assuming you know the length of the gun.. if you don't, do the barrel thing.)