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.

Just to note, SP ammunition may not be as individually dangerous when cooking off but SP weapons tend to pack a lot of ammunition. A lot of small explosions instead of fewer larger ones.

Id still allow the blast area effect ( by giving the damage the "x" trait ( how far would depend on how much ammo...and what type )

However..some things to note..ANY regular ballistic ammo ( IE anything with gunpowder in it ) WILL automatically detonate when hit with a flamer type weapon ( within following round of hit ) due to the intense heat. Pesonally id reccomend ( havin seen a box of ammo cook off literally and witnessed the area of effect of a box of 7.62mm ) at blast trait X (2) or X (3) and all targets that fail a difficult AG test take full damage from ONE round worth of the ammo that are caught in the blast radius ( that will give a sense of holy crap dive for cover NOW!..but still allow a fair chance at survival even if they roll horribly )

As for simulating the effect of a character havin their weapons ammo detonate...thats simple..ONLY when the weapon has a called shot made on it that inflicts critcal damage on the weapon ( or a accidental hit to weapon inflicts a critical ) as the weapon will in that event instantly detonate with whatever is left in the clip/mag for that weapon...id say simple rule of thumb...if its halt to fully loaded..then the poor SOB holding it when it goes off takes same damage as a full auto burst from it...if its half or less...then the SOB takes semi-auto burst damage...if its a bolt action or single shot type..then they take the damage of ONE round.( depending on how they are/arent aiming it could be automatically damage to the face/head area as well as hands/arms ( especially if using any aim actions )...

Just some food for thought..those are the basics my group and i have been using and it gives them some real options to using called shots on opponents gear to give maximum effect + morale loss on enemy side as well as scattering their forces thereby blunting any massed enemy assaults as a few well placed shots to the gear for that effect will have them diving for cover instead of charging ..But i also will use the same tactic on the PCs/NPCs attached to party for fairness and the "feel" of it being a dire situation and not to have so many cookie cut-out enemy thugs...making my PCs think and plan instead of simple hack and slash

I really don't think ammunition going of due to enemy fire makes sense from anything less than a somwhat continous burn from a flame weapon, and even then bullets going of like that have awfull ballistic performance and should be stopped dead by any half-decent armour. Neither will a pistol or rifle ever realistically explode and hurt the guy holding it from any attack that wouldn't have taken of his hand anyway.

Tanks used by flamers and meltas are obviously a different matter and should indeed make a big boom*, but gunpowder firearms and ammo more advanced than blunderbusses don't really explode in battle due to enemy fire, at least not in anything but one-in-a-million freak accidents.

*Well, realistically it should probably require incendiary ammo to cause a fuel-canister for a flamethrower to explode but 40k is pretty cinematic.

Mythbusters:

http://www.youtube.com/watch?v=nfoJAwlUopI

Basically ammo going of due to heat outside of a barrel doesn't hit hard enough to do any serious damage. Which makes sense given that a bullet doesn't really pack more powder than a big firecracker.

I love Mythbusters

Also: diving under water will protect you from bullets (just not from las blasts)

It'll probably work better on the bullets than the lasblasts, I seem to recall a 40k book with shots from a laspistol having enough force to break a large fish in half through two meters of water.

Anyway, I wouldn't start changing parts of the crit tables without thinking of changing them all. Something about all of the beheading and flying 1D10 meters from bolt rounds might clash a bit with more accurate rules for ammo cooking off. By all means make the crit tables less pulpy though, they can be silly at times.