Some critical hits ignites the ammunition of the poor sod. Last gaming session due to my seneschal lighting up some poor sods w/ a stub revovlers and nearly killing the party due to the "if the target is carrying any ammunition it explodes dealing 1d10+5 Impact Damage to anyone within 1d10 metres". Literally the guys were on top of us and this happened three times (Did I mention my seneschal can be rather deadly with his bolter at times) .

The NPCs with us died due to the explosions.. most of us were down 10 or so wounds (leaving 3 to 5 wounds left). Due to stub revolver ammunition cooking off.

New rule for ammunition cook off is.. to do 1d10+5 it has to be mass ammunition (read bolter), melta, flamer or plasma. Or you could look at this way and say .. SP ammunition does not cook off and cause 1d10+5 damage.