Starting a little series discussing probabilities in Space Hulk. Now in space hulk since each piece gets to do several things, tactics and probabilities collide into dependent chains of events. It is not as simple as 40K where calculating the number of dead marines after being doomed then attacked by fortuned harlies. Each piece can have several actions and what they do later depends on what happens first since you do not have to declare more than 1 action at a time.
We will look at the odds of the individual events in space hulk and some common chains of events. We will start with the simple space marine storm bolter shooting at a genestealer.
For basic shooting of a storm bolter at a stealer you have a 30.6% (11/36) of killing it. With sustained fire that goes up to 55.6% (20/36) for that second shot. Now there are several ways for Terminators to shoot at the genestealers, you can walk toward them and shot with each step or you can move the first step and then stand still the second. The first one gets you toward the destination faster but does not benefit from the sustained fire rule. Assuming the genestealer lives through the first shot, the second shot with sustained fire has a 81.5% better chance of killing the stealer but you did not get to move. This is where the tactics come in, move and shot or just shot. Do I need that extra square or do I need this stealer dead.
Now stealers going after a model on overwatch is another place where probabilities and tactics run into each other. First shot to kill 30.5%, successive shots 55.6% with the 16.7% chance of jamming every time. Here is a chart for a stealer running into the storm bolter on overwatch were the space marine player does not spend any command points to clear a jam. The plot is whether the stealer is alive or dead and whether the space marine has an active overwatch or a jammed storm bolter.
As you can see, you get that first square pretty easy but after that you are pretty much only going to make it far if he jams early. You are essentially not making it past 2 squares unless he jams so what to do if you have a group of stealers and he has not jammed after moving your 2 squares. Do you push you luck and move the one more(55.6% chance of being dead and no longer blocking LOS) or do you leave the front stealer where he is then move up one behinds using the first to block line of sight?
Or maybe he has jammed? Does he have a command point to clear it? Well since he cannot spend the command point to clear it until you do something in his LOS again you can move up the square to see if he clears it. If he does you are back at a non sustained fire level on your next move so you have a good chance of getting the next square(69.5%) before moving the rest of the pack up behind.