Here's how:
Go to events and put this:
Onactivate > Self > Firerelay
Onrelay > Self > Firerelay
Onrelay > VCEIf_Value > <var:pl:firing> > 1
Onvariabletrue > Player > Spawnprojectile > Grenade
Hope this helped.
P.S. : Change the projectile to shoot anything you want.