From reading the books and putting two and two together, the best opinion and theory I have as to what a "bolt" actually is; is that its essientally a miniture rocket-propelled gernade.
If you look at the discription given by many of the authors -- the bolt essientally is a mass-reactive shell. Meaning the tip has some sort of programming (doubtful) or rather some primitive sensor that registers that its hitting something with mass of some sort, then it simply explodes.
We actually have technology similar to that today.
Actually, in a ironic display that can only mean Tzeentch himself has ordained this thread with his complexity of blessings -- I found this on Youtube.
How accurate is this? Not 100% sure, but their explanation and presentation are excellent.