# Hammer of Math: Necromunda Ammo Checks

It’s been just a little bit of time since Hammer of Math went into the Underhive, but we’re back this week with a look at the math behind Ammo checks.

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Nearly every ganger fears that sound. Those that don’t are usually praying for it, assuming they’re not too busy praying to Khorne, the Four-Armed Emperor, the regular two-armed Emperor, or some other (likely heretical) cosmic entity to protect them as they charge the enemy. In the Underhive ammunition is a scarce, unreliable quantity and even the common lasgun may run out of charge at the worst possible time. Understanding the mechanics of the Ammo check, and the consequences of failing that roll, are critical for keeping your gangers alive and (ideally) shooting.

### The Rules

Every time a ranged attack is made (and maybe every time a las cutter is swung?), a player is obligated to roll at least one Firepower die. If the weapon has a Rapid Fire (X) quality then they roll X Firepower dice. One side of the Firepower die has a little bullet symbol on it; if that symbol appears then the player is required to make an Ammo check. The Ammo check is a d6 roll; if the result is less than Ammo value of the weapon being fired then the weapon is Out of Ammo until the weapon is reloaded by performing a Reload (Simple) action and passing the subsequent Ammo check. ForÂ Rapid Fire (X) weapons if two or more Ammo checks fail then the gun has jammed and can’t be used for the rest of the battle.

Mathematically the resulting probability of failure is the product of the probability of requiring an Ammo check (1 in 6 for most weapons) and the probability of failing the Ammo check itself (which ranges from a 2+ to a 6+ depending on the gun). The probability of running Out of Ammo over time can be calculated using binomial probability. For the given number of attacks and probability of each trial, we can calculate the probability that at least one trial will force an Ammo check that fails. The results are pretty compelling; even a weapon with a 6+ Ammo check won’t run out of ammo the majority of the time until it’s fired at least 5 times. Of course that doesn’t help when it fails on the first shot, but at least you know on average that the math will be more forgiving. The 4+ column also shows the probability that firing anÂ Unstable weapon like a max power plasma weapon will put the firer Out of Action, as the probability of rolling 1-3 after the Ammo symbol is rolled is the same.

ForÂ Rapid Fire (2) weapons things get a bit more interesting. Not only do you need to consider the possibility of the weapon running Out of Ammo, but if two Ammo checks are forced and failed then the weapon will jam and be inoperable for the rest of the game. Of particular note is the probability of such a failure for the heavy bolter, which combined RF2 with a 6+ ammo check. Chances are pretty good that you’ll get a failed check after a few attacks, so having a backup gun (or Gunk Ammo) is a wise idea.

Rapid Fire (3) gets even more interesting. To do the math here you actually need two sets of binomial probability calculations. The first being that at least 2 out of the 3 dice will result in a failed Ammo check, and the second being how likely such an outcome will happen over subsequent attacks. It also reinforces exactly how terrible the Slave Ogryn Storm-Welder really is, as theÂ Unstable quality means that it will injure the firer 23% of the time it shoots.