To figure out the chances of your killing at least one Space Marine with a pair of twin-linked rail guns:
BS 3 is equal to 1/2, twin-linked giving you a 3/4 chance to hit. 3/4 to hit x 5/6 wound = 15/24 or 5/8. However, to find the chance of it failing to kill a Space Marine both times is 1-(1-chance of killing)# of guns, or in this case 1-(1-5/8)2 = 1-(3/8)2 = 1-9/64 = 54/64 chance you will kill AT LEAST one Space Marine.
As the number of guns, attacks, and other stuff increases, things get more complicated. Suppose that you wanted to kill at least 3 Space Marines with 6 twin-linked railguns in one Shooting Phase on your last turn in a Clease battle so you can get them out of your quarter. This means that you could kill 3, 4, 5, or 6 Space Marines and succeed. As the chance of a Space Marine being killed by a Railgun shot is 5/8, and the chances of survival is 3/8, the total number of possiblites is 86, or 262,144. The chances of killing 6 Space Marines (every shot succeeding) out of 262,144 is 56, as that is the number of combinations that will produce 6 kills: 15,625. The number of combinations that will produce 5 kills is equal to 55 (5 kills) x 31 (one failure) x 6 (the number of ways the kills and failure can be arranged in order). This comes out to 56,250.
Recall the method for finding numbers of combinations, for x numbers taken y at a time:
! means factorial; for example, 4! is equal to 4x3x2x1. If you end up with 0 over 0, the result is 1. In this application, x is the number of guns you're firing, and y is the number of failures (or sucesses, it doesn't matter) you're trying to find.
For 4 kills, 54 x 32 x (6 x 5) /(2x1)= 84,375.
And 3 kills, 53 x 33 x (6 x 5 x 4) / (3 x 2 x 1) = 67,500.
Adding them together: 15,625 + 56,250 + 84,375 + 67,500 = 223,750. The probability of killing at least 3 Space Marines is 223,750 out of 262,144, or about 85.35%.
There might be an easier way to do this, but if there is, I haven't found it yet... obviously this is best done with a good calculator and a lot of time...