Battletech is a board game +. What i mean by that is that it is played on a board like a traditional board game, but it has many extra components that you'll need to fully engage with the game. The base mechanic is simple, as it takes a base skill value for the action attempted, applies modifiers based on a variety of factors, and then roll the dice and see what happens.
The Skill value of the unit is determined while you're setting up for the game. If you're playing a one off or a pick up game, these are usually randomly determined. However, a longer term campaign game gives you a variety of options for determining initial skill values, and as the campaign progresses, improving that base skill, or picking up special abilities that will modify specific actions.
There are four primary modifiers for the most common action, the "fire a weapon at the target" action. These are, Range, Attacker Movement modifier, Target Movement Modifier, and Cover. In addition, there are two other situational modifiers, Heat, and Damaged Components. There are other modifiers, but they are extremely situational, and have special rules that cover them.
Range is the easiest modifier to calculate, but is the one with greatest variability. Each weapon in the game (and there are a lot of potential ways to hurt or maim another unit) has its own specific range bands, expressed in hexes. A standard medium laser has a range band of 3/6/9. What that means, is that any target up to three hexes away is at the short range increment, and has no modifier to hit based on range. Any target between 4 and 6 hexes away is in the medium rage band and has a +2 modifier to hit, based on range to target. Any target between 7 and 9 hexes away is in the long range band, and has a +4 modifier to hit, based on range to target.
Some weapons are built for close up work, like the aforementioned medium laser, whereas other weapons are designed for longer range combat, like the Autocannon 10 (AC/10), which has a standard range profile of 5/10/15. Getting familiar with the differences in the weapon systems can give you a head start in figuring out what the expected role each unit is supposed to play.
The next modifier to keep track of is the attacker movement modifier (AMM). In game, this is a representation of the difficulties in hitting a target while moving. The more active you are as an attacker, the less stable the shot is, making it harder to hit the target. A target that doesn't move, has no AMM. A target that uses walking movement has a +1 AMM, while a target that uses running movement has a +2 AMM. Some units have the ability to use jumping movement (rocket jumping from point A to point B and ignoring intervening terrain and the modifiers that slow movement). A unit that uses jump movement has a +3 AMM.
The third modifier in the group is the Target Movement Modifier (TMM). It's a lot harder to hit a moving target than a stationery one, and that's what this represents. Based on the number of hexes moved, the target's Modifier to be hit increases. Lighter units that move faster are harder to hit than heavier units that don't have the speed to necessarily keep up with the lighter cousins. The Battletech Introductory box in that wonderful pack of charts has this table in an easy to find place so you don't have to flip back and forth.
The fourth modifier is Cover. Cover represents a variety of things (trees, hills, scrub, buildings, hydrogen storage tanks) that get in the way of lasers, missiles and bullets hitting a target. These modifiers will vary from a +1 modifier to hit, up to the target can't be fired on (this is generally because the target is occluded by a building, lots of trees, or they are submerged underwater).
The two situational modifiers that I'll cover are heat, and damage.
Heat is one of the interesting mechanics of the Battletech game. Based on movement, weapons fire, and other environmental effects, a battlemech keeps track of its heat buildup from turn to turn. Some Battlemechs run cooler than others, and some weapons generate more heat than others. Units that are running hot have several effects, ranging from reduced movement, to having the possibility of shutting down from the heat. At certain heat thresholds, it applies a +1 to hit modifier (these are cumulative, so a unit that is running incredibly hot will have a +3 or a +4 to hit modifier).
As units start to take internal damage (Units have an armor value and an internal structure for each location) components can suffer critical hits that impair a units combat ability. The one that comes to mind immediately would be a unit that has suffered a sensors critical hit has a +2 to hit modifier to all its ranged attacks. Other critical hits can affect specific actions, but those are specific to that component.
So, those are the basics of shooting combat for Battletech, and I'll cover movement next time on By The Numbers.