Alrighty, time for installment #3 of this small series. Today we'll be looking at defining the stuff that shoots out of guns (and all other things too). MOPixels, MOSParticles, and MOSRotatings (Attachments and AEmitters we will get to in a later tutorial).
These three particles alone make up a good 3/4 of cortex modding, and knowing how to define them properly will make everything very much easier.

Let us begin with the humble MOPixel. Here is a very basic definition of one.

AddAmmo = MOPixel
	PresetName = Basic MOPixel
	Mass = 0.01
	LifeTime = 1
	Sharpness = 0
	HitsMOs = 0
	GetsHitByMOs = 0
	Color = Color
  R = 255
  G = 0
  B = 255
	Atom = Atom
  Material = Material
  	CopyOf = Air
  TrailColor = Color
  	R = 255
  	G = 0
  	B = 255
  TrailLength = 0

You may be wondering, "But Duh, that's hardly anything at all! How could it be so useful?". But, in actuality, MOPixels are quite useful. If you notice, MOPixels define two colors, simple Color, and TrailColor.
MOPixels are single pixels and collide as such (HitsMOs and GetsHitByMOs still apply), but you can add a non-colliding trail to them to make them more visible or to apply an affect (Lasers work particularly well with these).
Color affects the color of the colliding pixel, TrailColor affects the color of the noncolliding trail.
mopixelvisualxk4.png
The red outline is the colliding pixel, the green outline the trail.

A quick but very important note, LifeTime controls how long the MOPixel will exist after it is spawned. When an object reaches it's LifeTime, it will simply disappear. No gibbing or anything, just plain disappearing. Things fired out of guns benefit from this, as you don't want your bullet to fly clear around the map and hit you in the back of the head.

Sharpness is not a value that did us much good for the HDFirearm, but here it is very important. Sharpness is a factor in both what kind of materials your MOPixel can penetrate, and also how much terrain damage it does. Setting Sharpness to 0 makes it so that your MOPixel will never damage the terrain, no matter how fast and/or heavy your MOPixel is. Setting it to a value greater than 0 increases the factor of how far into terrain your pixel will penetrate. Increasing the Sharpness, Mass, or Velocity (however you spawn them, usually FireVelocity in a Round) will increase the depth it will penetrate. Setting it too low, however, will leave you frustrated trying to do some damage to a door or other armored target. A good value for an MOPixel of weight 0.08 and velocity of about 50 is about 100 sharpness. There is no perfect formula to determine how far it will penetrate, however, so you must test, fiddle, and retest until you have it perfect.

A couple small things:
You can add in the line "GlobalAccScalar = X" to change the effect of gravity on the particle. GlobalAccScalar = 0.5 will halve gravity for the particle. GAS = 2 will double it. GAS = -1 will reverse gravity.
You can add in the line "AirResistance = X" to chance the effect of air on your particle. AirResistance = 0.5 will halve the velocity of the particle every frame. AR = 1 will totally stop the particle from moving, as in, subtracting 100% of the particle's intended velocity every frame. I haven't tried it, but I think AR = -0.01 will increase the particle's velocity by 1% previous every frame.

Now, you likely did not look into Materials for HDFirearms, so let us take a quick look at those. The stock materials (which you rarely need to deviate from) are defined in "Base.rte/Materials.ini". At some point you should open that up and take a look at it. The Material of your MOPixel (and any MO, like HDFirearms) determines several important factors. Bounce and Friction are the most important for MOPixels, though there are several more for when you begin to make BunkerModules.
You can also apply a ScreenEffect, which is an alpha-blended image centered on the pixel.

	ScreenEffect = ContentFile
  FilePath = Base.rte/Effects/Glows/YellowTiny.bmp
	EffectAlwaysShows = 1//Set to 1, I don't know how you would make it NOT show
	EffectStartTime = 0//When in MS should the effect begin showing after spawning
	EffectStopTime = 2500//When in MS should the effect stop
	EffectStartStrength = 0.50//What to multiply the alpha effect by when at start time
	EffectStopStrength = 0.0//What to linearly scale the EffectStrength down to by the StopTime

ScreenEffects, or Glows as you will more likely consider them, are simple .bmp or .pngs with a black background.
yellowbignf5.png
Lighter pixels will have a higher alpha value (show less through), darker pixels will have a lower alpha value (show more through).

Now, on to MOSParticles. There is almost no change from MOPixel to MOSParticle, except for one important point. They are pretty much MOPixel++'s

AddAmmo = MOSParticle
	PresetName = Bare MOSParticle
	Mass = 0.01
	Sharpness = 0
	HitsMOs = 0
	GetsHitByMOs = 0
	SpriteFile = ContentFile
  FilePath = Base.rte/Null.bmp
	FrameCount = 1
	SpriteOffset = Vector
  X = 0
  Y = 0
	AngularVel = 0
	Atom = Atom
  Material = Material
  	CopyOf = Air
  TrailLength = 0
	Framerate = 0

MOSParticles collide at one point like MOPixels, but they display a sprite! You can animate this sprite like you can animate the HDFirearm, sprites named the same way (if more than one frame, name the sprite and add "000", "001", etc, to the end).

And that's pretty much it for MOSParticles. On to MOSRotatings. You will notice a lot in common with HDFirearms and MOSRotatings, mainly because HDFirearms are MOSRotatings with extra functionality.

AddAmmo = MOSRotating
	PresetName = Basic MOSRotating
	Mass = 1
	Sharpness = 0
	HitsMOs = 0
	GetsHitByMOs = 0
	SpriteFile = ContentFile
  FilePath = Base.rte/Null.bmp
	FrameCount = 1
	SpriteOffset = Vector
  X = 0
  Y = 0
	AtomGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Air
  Resolution = 1
  Depth = 0
	DeepGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Air
  Resolution = 1
  Depth = 1
	DeepCheck = 0
	AddGib = Gib
  GibParticle = MOSParticle
  	CopyOf = Fire Puff Small
  Count = 3
  Spread = 3.14
  MaxVelocity = 10
  MinVelocity = 4
  InheritsVel = 1

Does it look very similar? It should. There is in fact less things to mess with in a MOSRotating than in a HDFirearm. You can apply AngularVel to make them rotate around their SpriteOffset, add wounds to allow for GibWoundLimits, add gibs to throw out when the GibWoundLimit or GibImpulseLimit is reached, and GlobalAccScalar and AirResistance still apply. You can also animate and apply a ScreenEffect, though you have to set your SpriteOffset outside your actual sprite if you want the ScreenEffect to show.
One more thing that people who want to make missiles will like. OrientToVel = 1 will make your MOSRotating face the direction it is going, with the front of your MOSRotating being the right side of the sprite. You can also use any decimal value from 0 to 1 to make it face more or less quickly (0.1 will make it slowly rotate to face the new orientation, instead of immediately following any changes).

And that's pretty much it for a MOSRotating. Here, let's make a fun gun out of contact explosive MOSRotatings.
Define first the little bits that the MOSRotating will explode into.
Quote:

AddAmmo = MOPixel
PresetName = Mod Grenade Launcher Grenade Bit
Mass = 0.05
LifeTime = 50
Sharpness = 40
HitsMOs = 1
GetsHitByMOs = 0
Color = Color
R = 200
G = 200
B = 200
Atom = Atom
Material = Material
CopyOf = Mega Metal
TrailColor = Color
R = 180
G = 180
B = 180
TrailLength = 10

The somewhat long PresetName is important so that we do not accidentally override any other MOPixels in Base.rte or any of the basegame .rtes. Next, let's make that grenade.
AddAmmo = MOSRotating
	PresetName = Mod Grenade Launcher Grenade
	Mass = 1
	Sharpness = 0
	HitsMOs = 1
	GetsHitByMOs = 0
	OrientToVel = 1
	SpriteFile = ContentFile
  FilePath = Coalition.rte/Devices/Pistols/MagazinePistolShortA.bmp
	FrameCount = 1
	SpriteOffset = Vector
  X = -2.5
  Y = -3
	AtomGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Metal
  Resolution = 2
  Depth = 0
	DeepGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Metal
  Resolution = 2
  Depth = 1
	DeepCheck = 0
	AddGib = Gib
  GibParticle = MOPixel
  	CopyOf = Mod Grenade Launcher Grenade Bit
  Count = 20
  Spread = 3.14
  MaxVelocity = 50
  MinVelocity = 25
  InheritsVel = 0
	GibImpulseLimit = 0.01//To make it explode at the slightest touch
	GibSound = Sound//Give it a little oomph
  AddSample = ContentFile
  	FilePath = Base.rte/Sounds/Explode1.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Sounds/Explode2.wav

I used a sprite from Coalition.rte this time, a little box that looks somewhat like a grenade. Next, the Round and Magazine.
AddAmmo = Round
	PresetName = Round Mod Grenade Launcher
	ParticleCount = 1
	Particle = MOSRotating
  CopyOf = Mod Grenade Launcher Grenade
	Shell = MOSParticle
  CopyOf = Shell
	FireVelocity = 20
	ShellVelocity = 8
	Separation = 0

AddAmmo = Magazine
	PresetName = Magazine Mod Grenade Launcher
	Mass = 1 // kg
	HitsMOs = 0
	GetsHitByMOs = 0
	SpriteFile = ContentFile
  FilePath = Base.rte/Null.bmp
	FrameCount = 1
	SpriteOffset = Vector
  X = 0
  Y = 0
	ParentOffset = Vector
  X = 0
  Y = 0
	EntryWound = AEmitter
  CopyOf = Dent Metal
	ExitWound = AEmitter
  CopyOf = Dent Metal
	AtomGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Military Stuff
  Resolution = 2
  Depth = 0
	DeepGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Military Stuff
  Resolution = 3
  Depth = 1
	DeepCheck = 1
	JointStrength = 200
	JointStiffness = 1
	JointOffset = Vector
  X = 0
  Y = 0
	DrawAfterParent = 0
	RoundCount = 5
	RTTRatio = 0
	RegularRound = Round
  CopyOf = Round Mod Grenade Launcher
	TracerRound = None

I made the magazine invisible, just because. You can set a SpriteOffset and JointOffset and ParentOffset for a null magazine, but I don't see the point.
Last but not least, the gun itself. I'll be copying the SMG again, because it's easy.

AddDevice = HDFirearm
	PresetName = Mod Grenade Launcher
	AddToGroup = Weapons
	Mass = 10
	HitsMOs = 0
	GetsHitByMOs = 1
	SpriteFile = ContentFile
  FilePath = Base.rte/Devices/SMGs/SMG.bmp
	FrameCount = 2
	SpriteOffset = Vector
  X = -7
  Y = -4
	EntryWound = AEmitter
  CopyOf = Dent Metal
	ExitWound = AEmitter
  CopyOf = Dent Metal
	GoldValue = 50
	AtomGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Military Stuff
  Resolution = 4
  Depth = 0
	DeepGroup = AtomGroup
  AutoGenerate = 1
  Material = Material
  	CopyOf = Military Stuff
  Resolution = 4
  Depth = 10
	DeepCheck = 1
	JointStrength = 75
	JointStiffness = 0.5
	JointOffset = Vector
  X = -3
  Y = 3
	DrawAfterParent = 0
	StanceOffset = Vector
  X = 6
  Y = 5
	SharpStanceOffset = Vector
  X = 7
  Y = -2
	SupportOffset = Vector
  X = 5
  Y = 4
	SharpLength = 165
	Magazine = Magazine
  CopyOf = Magazine Mod Grenade Launcher
  ParentOffset = Vector
  	X = 3
  	Y = 1
	Flash = Attachable
  CopyOf = Muzzle Flash SMG
	FireSound = Sound
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1601.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1602.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1603.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1604.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1605.wav
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/SMGs/M1606.wav
	EmptySound = Sound
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/EmptyClick3.wav
	ReloadStartSound = Sound
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/ReloadStart.wav
	ReloadEndSound = Sound
  AddSample = ContentFile
  	FilePath = Base.rte/Devices/ReloadEnd.wav
	RateOfFire = 200
	ReloadTime = 1500
	FullAuto = 0
	FireIgnoresThis = 1
	ShakeRange = 10
	SharpShakeRange = 2
	NoSupportFactor = 3
	ParticleSpreadRange = 3
	ShellSpreadRange = 8
	ShellAngVelRange = 2
	MuzzleOffset = Vector
  X = 10
  Y = 0
	EjectionOffset = Vector
  X = 1
  Y = -1
	AddGib = Gib
  GibParticle = MOPixel
  	CopyOf = Spark Yellow 1
  Count = 6
  Spread = 2.25
  MaxVelocity = 20
  MinVelocity = 8
	AddGib = Gib
  GibParticle = MOPixel
  	CopyOf = Spark Yellow 2
  Count = 5
  Spread = 2.25
  MaxVelocity = 20
  MinVelocity = 8
	AddGib = Gib
  GibParticle = MOPixel
  	CopyOf = Drop Oil
  Count = 4
  Spread = 2.25
  MaxVelocity = 10
  MinVelocity = 1
	AddGib = Gib
  GibParticle = MOSParticle
  	CopyOf = Gib Metal Rust Micro A
  Count = 6
  Spread = 2.25
  MaxVelocity = 15
  MinVelocity = 5
	AddGib = Gib
  GibParticle = MOSParticle
  	CopyOf = Gib Metal Grey Micro A
  Count = 4
  Spread = 2.25
  MaxVelocity = 15
  MinVelocity = 5
	AddGib = Gib
  GibParticle = MOSRotating
  	CopyOf = Gib Device Small K
  Count = 1
  Spread = 2.25
  MaxVelocity = 10
  MinVelocity = 1
	AddGib = Gib
  GibParticle = MOSRotating
  	CopyOf = Gib Device Small I
  Count = 1
  Spread = 2.25
  MaxVelocity = 10
  MinVelocity = 1
	GibWoundLimit = 3

Now, stick that code into a .ini file, load it in the index.ini, and try it out.

Here is the modified Mod.rte with the new gun inside. Check it out if you're getting errors.