I have solved it by using this in the fire loop:

if(proc_status(tracer_right) < 5){gun1=ent_create(NULL,vector(-100,-170,-80),tracer_right);}
if(proc_status(tracer_left) < 5){gun2=ent_create(NULL,vector(-100,170,-80),tracer_left);}

This way it is not causing the error so often(only rarely). But do I still need to kill the entities created or particle entities die by themselves?

I added these lines instead of the commented ones but do not seem to make much difference:

if(proc_status(tracer_right) == 0){ptr_remove(gun1);}
if(proc_status(tracer_left) == 0){ptr_remove(gun2);}