Ah yes, my mistake. Small only works on turrets.
As for swarm... =o
I'm going to have to check that out, I couldn't get it to work for player primaries last time I tried.
As for muzzel flash, I don't think it can work on anything except flak cannons yet. The SCP mentioned it somewhere, they may or may not have implemented it, I'm not sure on that one.
The Field of Fire line basically sets a cone that the gun will shoot within. So rather than being dead accurate, or reliant on AI being crappy aim, it makes the gun spray. So it makes thing a little more realistic as if there was recoil and shots arn't always going to be dead on target. Most effectively used on Rapid fire weapons.
There is more stuff in the wiki.
Generally they say that $FoF: 0.35 is the highest you should go for player used weapons, but I think you can get away with much higher if your careful in your testing.

Just chuck the line at the bottom of a weapon entry.