Greets all, new poster here.

I've been toying around with an idea for my first SR4 character and came up with a mage using fire based spells like Ignite. In the descriptions of these spells and of fire based damage the book lists ammo or explosives possibly detonating due to the heat. However, I can't seem to find any actual rules to determine whether ammo cooks or not, and how much damage that would do. Anyone have any insight into this issue ?

generally, your caseless round works best in a light beef broth, especially if you splash it with sherry for flavor. for case rounds, you want to fry them lightly, then add fresh garlic and cloves.

Uhhh.... adding to what mfb said, the rules for cooking off ammo in SR3, though not thorougly developped, are as follows:

make Object Resistance test with OR-1 vs power of fire
If test fails, victim rolls vs damage code of the weapon. No armour, but Combat Pool.

So, you now have to convert that to SR4

This is silly. Why not discuss the real matter at hand?

I'm actually partial to some deep-fried 172gr 7.62x51mm FMJ-BTs with 44gr of Vihtavuori N550.

I might do something like have the burning player/NPC make a resistance roll against an attack of half the power of the weapon his carried ammo is for.

As far as whether it ever cooked off or not, I'd probably just base that on how burny the character was. Jacket caught fire and he spent an action doffing it? Nah. Jacket caught fire and he spent his next two actions running and firing? Roll to resist damage.

It'd depend on the situation. Oh, and this may or may not be SR4 accurate at all.

here is what i do when that happens. I look at them and go how much ammo are you carring. I know the answer to this before he even says so half the time, and its always going to be less than what they really brought in.. all of a sudden.. the 100 rounds of extra ammo becomes 10 when they get lite on fire. Funny thing is, they just gave me of the number of rounds they had left when the ammo cooks. So they say 10, and earlier they said 100, 90 rouns went of. And i'll usally just role dice = the number of rounds cooked... every hit 5+ is a round fired at them, keep track of that and then roll agian and on 5+ it hits them agian..... but if i roll really bad, i'll give them atleast one box of extra damge form the fire, and say they got lucky. I know i'm kind of a douche, but thats the best way, i hate players that do the ole so famous "stat swap" I have 100 kils of C-4, oh im on fire i have only 1 kilo, don't you remember i left the pack back in that other room.

Hehe. Reminds me of a Nightbane game I was playing in. The partial reconstruction combat borg Russian packed not only tons of rounds for his mini-grenade launcher, but also carried several grenades and not a few fusion-blocks (mega-damage C4 type stuff).

He got hugged by a living fireball spell.

Umm... correct me if I'm wrong, but my understanding of C-4 (C-12 in SR) is that it's a stable plastique - requiring a detonator. No detonator cap, no boom.

well, it requires both heat and pressure--eg, an explosion--to detonate. a blasting cap will do that, as will lighting it on fire and then trying to stomp it out (hee hee!). incidentally, that's also a good recipe for chunky-style salsa.

So there's the lesson, folks! If you're turned into a human bonfire and you're carrying large amounts of high explosive, don't start hitting yourself with a hammer!

Using fire to "ignite people's ammo" is a movie myth. Just do some google searches. Here's a quote from a site for people who collect ammo as a hobby.

I would say that ammo would cook off only in glitch of the damage resistance of the person being burned.

Though I'm very inclined to believe dcpirhana's post about how useless cookoff really is. It's not like bullets would become less safe in 60 years.

Now, heavier rounds are an issue... next to the entry for ammo cook-off in the encyclopedia should be pictures of M-4s and T-72s.
If somebody drops an ignite spell on a box of grenades... I might worry.

Yes. All of my stuff only applies to small arms ammo.

If someone's carrying around a mortar/assaultcannon/missle round, all bets are off.