It turned out to be more difficult than I thought, because I had to add some features that would let guns fire from on top of other collidable objects without damaging them (as in the gun on top of the station in the above picture). Now when each bullet fires, the sprite object that represents the particle carries with it a) the name of its weapon b) an array of sprite names that are "safe" from it. This way the collision method never even gets executed if the particle is being checked against a "safe" object.
Currently the gun platforms aren't very smart, if you fly fast enough they can't catch up. Fortunately if you have a bunch of them fairly near to the object they are meant to protect, it actually is fairly difficult to avoid them.
This officially concludes the 4 weeks of my final project, and I'd say that what I have is pretty good for the time I had. I plan to continue working on it, but I may be more slow over the coming weeks because next week is the last week of classes, then the following weekend/week is final exams, and then I have to drive back to Massachusetts and get settled in my apartment in Boston.