The alien event card "Ork mekaniac" talks about an ork using an experimental weapon that blows up once fired. I was thinking it would be cool if the Space Marines had a high power weapon whose high power were of-set by the chance it would go ka-boom!
I propose a blue "experimental weapon" dice that has: 4, 3, 2, 1, 0, -3.
On a roll of a negative number, the damage is applied to the mini with the weapon.
Now 1 in 6 chance of dying is a bit high so i reckon combine it with a red dice, this reduces the chance to die to 1 in 12 or about 8%. (since -3 on the blue die plus say a 1 on the red die = -2 which is not enough to kill a marine)
max damage of 7 but a chance to kill your own guy!
I haven't play tested this, just sharing to see what you guys think.