MO: Moving Object.

MOPixel: MOPixels are the smallest type of effect. They are one pixel, with one color, and some properties. Their singular and manually defined atom defines the material that the MOPixel is made of, as well as a trail effect. They fill a lot of roles, they can be used as sparks, blood, shrapnel, exhaust, bullets, the list goes on.
MOSParticle: "MOSParticles are quite simply MOPixels with an animated sprite. They can be gibs, but they cannot be gibbed."
MOSRotating: MOSRotatings are your basic gibs. They can be gibs and can be gibbed.

Atoms; no, see... Sharpness and Mass are just two characteristics of interaction. They each define one aspect of how things interact, not everything. Mass is mass. Sharpness is sharpness. Neither of these things define what might be called a "hitbox" or "collision mesh", which are the important bits. In most cases AtomGroups and DeepGroups are automatically calculated and you don't really need to worry about them at all.

Tracers; not every round in a machinegun is a tracer. What this mostly does is allow you to properly simulate the use of tracers, but you can be a bit more creative than that... for example, you could make a gun alternate between two different projectiles that have entirely different properties. The RTTRatio controls how often the TracerRound is fired instead of the RegularRound.

I forget who made this but the comments explain a lot about how bullets, rounds, magazines and guns work;

In other words, AtomGroups collide with each other (and terrain), providing the collision detection of CC that we all know and love. They can also destroy terrain, though more realistically than DeepGroups, which DON'T collide with each other or AtomGroups, but rather - you guessed it - destroy terrain on contact.

You generally want DeepGroups to have a greater Depth than AtomGroups, because the former overrides the latter. This means that if your DeepGroup has equal or greater Depth (how many pixels of "outline" to ignore when determining collisions), then you won't get collisions with other objects, and your object will eat terrain for breakfast.

Is the tabbing done correctly?

You should use the code display [code][/code] to show it better, or directly upload the file.