The most basic thing we can do is to use GetWeapon command. It makes player wield a weapon, given to it as a parameter (only if there's ammo for it). With that you can assign certain weapons to the keyboard keys. To do so you need to find [Engine.Input] section in User.ini. It contains lines like these:
General form of each line is <key>=<command>. So if you want to set a certain command to a certain key you must find key's name and enter command after equals sign '=' to it's right.
For example,
will assign pipe bombs to the keyboard key '6'. You can view list of all in-game weapon' names in 'KillingFloor\System\KFMod.ucl' or in tables I prepared at the end of this guide

Described method allows us to bind any weapon to any key, but, for obvious reasons, you can assign hotkeys only to a very limited amount of weapons that way. We can improve on that through command chaining.
You can assign to a hotkey a series of commands that will be executed one after another. If you do it like that: <key>=<command1> | <command2> | ... | <commandN>, then <command1> will be executed first, <command2> next and then process will continue in order until <commandN>, that will be executed at the very end.
How can we use this? Suppose that when you play as a support you want to have AA12 on the button '3' and when you play as a demo you'd like to have there a harpoon. Then you do the following:
If you happen to play as demo with harpoon, then last executed command GetWeapon SealSquealHarpoonBomber will make your character wield harpoon. But, if you're playing as a support, you probably won't have a harpoon in your loadout and last command will do nothing. Therefore only first command will take effect and your character will wield AA12.
Basically, pressing hotkey will choose the latest weapon in the chain, that you also have in your inventory and that still got ammo. Clever usage of that may allow you to assign different weapon to a key for each loadout. These are chains I use for support, firebug, demo and sharpshooter:
For demo it moves harpoon to key '3', making it much more convenient to use with M32 and pipes (pipes bound to '6'). For sharpshooter it allows to have LAR and musket on different keys, should I choose to use both of them. For support it allows to always have key '4' reserved for hunting shotgun and '3' for any another shotgun. And if you change '6' binding to
you can also easily switch to a grenade launcher and avoid situations where you choose it instead of a shotgun.

You can just assign these chain on keys like '6', '7', '8', etc. But, once again, we can do better. I, personally, want them on standard keys '1', '2', '3', '4'. And for that we will now learn how to change what chain bound to what key and how to return default behavior during the game.
For that we will need to use aliases. Aliases allow us to give names to our chains of commands and invoke them by using that name. Syntax to define one is
To use alias you must assign it to one of the slots in User.ini file. There's a bunch of them that look like that:
And that's where the problems begin. You can't just add a new slot with new number, game only seems to notice first 40 of them, starting from number 0. And they're all used for something. So we must free them. How do you do that? Have a look at the very first alias definition:
After that, whenever command Fire is used, a chain "Button bFire | Fire" is invoked (looks like Fire in the chain doesn't invoke alias, but some other command). So, if we want to change this alias, we must replace word Fire with "Button bFire | Fire" everywhere in the User.ini.
If you don't want to mess with game too much, there's a bunch of aliases you probably won't need:
Now, if you wanna mess with it all the way, you can use this file[pastebin.com] I prepared, where I wiped every alias from 0 to 39, so that you can easily use them.
To put aliases to use we first need to assign them to our chains:
Aliases with indices 0 and 1 are just chains from previous section. Alias with index 2 restores default behavior of the modified buttons. Now, to allow in-game layout switching we just need to bind a couple of hotkeys. I like them bound on the numpad:
And that's it. I usually don't need anything you can't get with methods described here. But we still can go deeper.

No additional explanation should be necessary about how to put mouse wheel to use, but I still would like to describe it on an example as it'll simplify next section and will allow me to introduce another idea. For that we'll create layout for some of my medic loadouts. I assume that player carries two medic guns and something else (like Blower Thrower Bile Launcher, Lever Action Rifle, Single Piston Longmusket or one of the zed guns).
First, we need to add following aliases:
Notice that chains for aliases with indices 4 and 5 are inverses of each other.
Also we will need to modify defaultLayout:
to allow it to restore default behavior of every button and the mouse wheel. You may, of course, change that behavior if you want.
Hotkeys now would look like this (our new medic layout is bind to 'NumPad6'):
Addition of defaultLayout at the beginning of every command chain here and it's modification allows us to assume that everything behaves as it should by default before we begin to change it into a desired layout.
Now let's see how our new layout works. Buttons '3' and '4' will select additional weapons of our choice. Rotating mouse wheel up will execute chain medicwheelup and choose some weapon from it. Rotating wheel down will execute medicwheeldown and, since chain medicwheeldown is an inverse of medicwheelup, will always choose a different weapon (provided that you have more than one weapon from the chain). Why? Assume you have two weapons A and B. If in first chain weapon A is placed after B, -- A will be selected. But then in the inverse chain weapon B will be placed after A, and thus B will be selected. So if you want ability to choose not one, but two different weapons from a given list then reversed chain is a way to go.

Methods described so far allowed us to bind key to one weapon per loadout. For different loadouts different weapons might be selected with the same key, but, unlike default behavior, you can't cycle through weapons by pressing the same button. And there's no good way to create a custom weapon cycle. But there is a bad way that you shouldn't have to use. If your needs weren't yet satisfied, have a look at this section.
We will setup mouse wheel to be able to scroll not through 2, but through 3 (one can go up to 40) weapons! As an example we will use gunslinger loadout. It is simple and consists of dual handcannons, dual 44 magnums and dual MK23.
We will need these aliases:
And that hotkey to choose our layout:
What does all this do? After selecting this weapon layout rotating wheel up will choose dual handcannons and rotating it down will choose dual MK23. Let's assume you rotated it up once. Now, while rotating wheel up would still select dual handcannons, rotating it down will select dual 44 magnums. Or, if you first rotated wheel down, then rotating it up will choose dual 44 magnums, while rotating it down will still choose dual MK23. Basically, you now have 3 states

• Dual Handcannons
  
• Dual 44 Magnums
  
• Dual MK23

You can be in one of them at a time and are initially in the middle one Dual 44 Magnums. Rotating mouse wheel up forces you to go up the list of states and rotating it down moves you down the list. Each rotation also causes player to choose corresponding weapon.
This concept of states is what allows us to cycle through 3 weapons with mouse wheel and also what would allow us to cycle through several weapons by pressing a single key.
Each state is defined by what is bound to mouse rotations:

• Dual Handcannons: Up - do dualheavy, Down - do dualmed
  
• Dual 44 Magnums: Up - do dualheavy, Down - do duallight
  
• Dual MK23: Up - do dualmed, Down - do duallight

And to change state we just need to rebind MouseWheelUp and MouseWheelDown to new values. Chains dualheavy, dualmed, duallight are designed to do that. For example, if we are in lower state "Dual MK23", then rotating mouse wheel up would invoke:
which would select dual 44 magnums as a weapon and rebind keys so that new state is "Dual 44 Magnums".
One can also make it cycle from lowest state to the top when mouse wheel rotates down and from the top to the lowest when wheel rotates up.

Finally we'll do what's promised in the previous section. I didn't have to use this yet, so example will be a bit stretched.
Imagine that you're berserker who wants to roll with katana for small stuff and claymore sword for something bigger. The problem is, you need katana more often than claymore, but claymore got the priority over katana, so it takes more time to get to it. Therefore we want to change that priority by creating a new loadout for berserker in which katana would came out before claymore. For simplicity I will assume that the only melee weapons that player will carry will be katana, claymore and a knife.
States will be like this:

• Katana: 1 is bind to zerkClaymore
  
• Claymore: 1 is bind to zerkKnife
  
• Knife: 1 is bind to zerkKatana

where aliases are like this:
As should be clear after reading previous section pressing 1 repeatedly cycles through states like this: Katana -> Claymore -> Knife -> Katana -> ...
To make use of this layout, as usual, make a hotkey:
By changing and adding more states you can create any weapon cycle you want. But I have found little use for that as methods described before seem to fix every problem with default weapon selection system in a much easier way to both implement and use. Also, compared to how regular in-game system works, such cycling has certain flaws:

• Amount of weapons you can put in such cycles is limited by amount of available aliases.
  
• There's no way to change behavior depending on whether player has or hasn't certain weapons. Therefore you must have all the weapons in the cycle or it will work incorrectly.
  
• It remembers which weapon player used last and always selects next in cycle, not first. In our example, when you press 1 you get a katana, but if you later switch to Syringe and then press 1 again it will select a claymore sword.

Last issue breaks our solution. This problem can be mitigated if you add
to every other key that changes weapon, but such changes will create a mess and still won't help in cases when game changes your weapon automatically. So I advise to avoid using method described in this section. Unless you're fine with all three issues.

• Medic
  
  Name

  In-game name

  MP7P Medic Gun

  MP7MMedicGun

  Blower Thrower Bile Launcher

  BlowerThrower

  MP5M Medic Gun

  MP5MMedicGun

  Camo MP5M Medic Gun

  CamoMP5MMedicGun

  M7A3 Medic Gun

  M7A3MMedicGun

  Schneidzekk Medic Gun

  KrissMMedicGun

  Neon Schneidzekk Medic Gun

  NeonKrissMMedicGun
• Support
  
  Name

  In-game name

  Shotgun

  Shotgun

  Camo shotgun

  CamoShotgun

  Hunting shotgun

  BoomStick

  HSG-1 Shotgun

  KSGShotgun

  Neon HSG-1 Shotgun

  NeonKSGShotgun

  Vlad the Impaler

  NailGun

  Multichamber ZED Thrower

  SPAutoShotgun

  Combat Shotgun

  BenelliShotgun

  Golden Combat Shotgun

  GoldenBenelliShotgun

  AA12 Shotgun

  AA12AutoShotgun

  Golden AA12 Shotgun

  GoldenAA12AutoShotgun
• Sharpshooter
  
  Name

  In-game name

  9mm Pistol

  Single

  Dual 9mms

  Dualies

  Lever Action Rifle

  Winchester

  44 Magnum

  Magnum44Pistol

  Handcannon

  Deagle

  Golden Handcannon

  GoldenDeagle

  MK23

  MK23Pistol

  Crossbow

  Crossbow

  Dual 44 Magnums

  Dual44Magnum

  Dual MK23

  DualMK23Pistol

  Dual Handcannons

  DualDeagle

  Dual Golden Handcannons

  GoldenDualDeagle

  Single Piston Longmusket

  SPSniperRifle

  M14EBR

  M14EBRBattleRifle

  M99 AMR

  M99SniperRifle
• Commando
  
  Name

  In-game name

  Bullpup

  Bullpup

  Tommy Gun

  ThompsonSMG

  Dr. T's Lead Delivery System

  SPThompsonSMG

  Rising Storm Tommy Gun

  ThompsonDrumSMG

  AK47

  AK47AssaultRifle

  Golden AK47

  GoldenAK47AssaultRifle

  Neon AK47

  NeonAK47AssaultRifle

  M4

  M4AssaultRifle

  Camo M4

  CamoM4AssaultRifle

  MKb42

  MKb42AssaultRifle

  SCARMK17

  SCARMK17AssaultRifle

  Neon SCARMK17

  NeonSCARMK17AssaultRifle

  FNFAL ACOG

  FNFAL_ACOG_AssaultRifle
• Berserker
  
  Name

  In-game name

  Knife

  knife

  Machete

  Machete

  Axe

  Axe

  Katana

  Katana

  Golden Katana

  GoldenKatana

  Scythe

  Scythe

  Chainsaw

  Chainsaw

  Golden Chainsaw

  GoldenChainsaw

  Dwarfs!? Axe

  DwarfAxe

  Claymore Sword

  ClaymoreSword

  Buzzsaw Bow

  Crossbuzzsaw
• Firebug
  
  Name

  In-game name

  MAC-10

  MAC10MP

  Flare Revolver

  FlareRevolver

  FlameThrower

  FlameThrower

  Golden FlameThrower

  GoldenFlamethrower

  Dual Flare Revolvers

  DualFlareRevolver

  Trenchgun

  TrenchGun

  Husk Fireball Launcher

  Huskgun
• Demolitions
  
  Name

  In-game name

  M79 Grenade Launcher

  M79GrenadeLauncher

  Golden M79 Grenade Launcher

  GoldenM79GrenadeLauncher

  The Orca Bomb Propeller

  SPGrenadeLauncher

  Pipe Bomb

  PipeBombExplosive

  SealSqueal Harpoon Bomber

  SealSquealHarpoonBomber

  SeekerSix Rocket Launcher

  SeekerSixRocketLauncher

  M4 203

  M4203AssaultRifle

  L.A.W

  LAW

  M32 Grenade Launcher

  M32GrenadeLauncher

  Camo M32 Grenade Launcher

  CamoM32GrenadeLauncher
• Non-perk
  
  Name

  In-game name

  Zed Eradication Device

  ZEDGun

  Zed Eradication Device MK2

  ZEDMKIIWeapon

  Welder

  Welder

  Syringe

  Syringe