How could I best create a machine gun effect with particles? How about a slow artillery fire effect?
Help is appreciated!
Ok… um… what do you mean by machine gun effect? The muzzle flash? The shells shooting out the side? What?
The previous post alludes to an important point: a “machine gun effect” is the product of many distinct visual effects. Bullets, muzzle flash, ejecting rounds, and so-on.
“What you see in the movies” was never produced all-at-once in some single grand-poohbah render! Rather, it was broken down into component parts, and rendered separately, then composited.
Furthermore, out of the pantheon of things which could have been added to the compositing-mix, a deliberate choice was made as to what would be sufficient to “carry the shot.”
It will help you, will help you very considerably, to think early-on about what your particular shot actually [u]needs.[/i] (Not “could have” … “needs!”) When all the dust settles, what you want is a sufficiently-convincing notion that here we have a machine-gun going off. But only a CG-nerd 
will actually be studying the gun or admiring the cool 3D effects. The audience will be worrying either about the guy who’s doing the shooting or the guy he’s shooting at. They will consider “the whole gun thing” only en passant. They will accept it without noticing it at all, if you’ve done your job well. Your job is, first of all, to make sure that they don’t. And, second of all, to get a convincing(-enough) shot “in the can” quickly so the project can move on to the hundreds-more shots just like it.
As they said in the show A Chorus Line, “don’t draw my eye!”