An easy way to do this would be to trigger it on the reload.
If they press the 'R' key then take the health off by adding a negative number.
:state=1,scancodekeypressed=19:plraddhealth=-50
Of course, that does nothing if they let it automatically reload.
Another way would be to start a timer when they equip the weapon, and take health off until it stops or they die.
Stop the timer when they equip another weapon or hit '0'.
Just give them another weapon and this one to start with.
That way they have to hit '2' to equip it, and that triggers the timer.
Hitting '1' or '0' would stop it.
The timer delays the loss of health so that it is not instant death.
An activated state turns it on and off.