A couple of months ago I played against a very good player from our club, Gav, and his Tyranids. I got a bit downhearted with my inability to wound hit Trygon with my Twin-Linked Assault can and in the end stopped shooting it. However, this meant that is pretty much rampaged through my lines. I figured that I had a better chance of wounding some Tyranid Warriors and hence taking out a scoring unit. That didn't quite work either as the Trygon ripped the Razorback to pieces. In retrospect I should have kept shooting the Trygon and I should have consoled myself with how difficult it would be to take it down with bolters. Although you could still kill it with Bolters...
P(Wound Trygon) = P(Hit) x P(Wound) x P(Fail Save
= 2/3 x 1/6 x 1/3
So I'd only need 27 shots on average to take a wound off. So that's 162 shots to kill it. Hey, that's only 81 guys if they're rapid firing. Ah well, could be worse, could be using Imperial Guard.