Instead of using sound samples ...

[PissyChrissy]

I like this idea. You can always tell when a sound in a game is just a timed snippet of a recording. You can hear the sound cut in and out, and change in an unnatural manner.

I'd also suggest doing the same with the guns. You can tell that it's not a series of individual shots, rather just one long recording of a burst that starts and stops as you fire the guns.

I'd rather see each individual shot (or firing of a cylinder) represented as it's own event, and played repeatedly at a frequency matching the frequency of the engine/guns.