Attachments are a very important part of gun mods.
If you don't have them set up properly then the shell ejections may come out of the barrel and the flashlight and laser will appear from thin air.

So make sure your mods have them set up properly.

Around 20th of August 2021 Authoring tools got and update in which they just deleted the "QC String" box which we need.
Good job Update team. useless as ever :)

So here's the "old" version that actually works.
https://cdn.discordapp.com/attachments/689810102013788265/878937459927441438/hlmv.7z
(Unpack the 7z into steamapps\common\Left 4 Dead 2\bin )

If you cannot use that link, Join the Dead4Mods Discord server[discord.gg] and head to the "Misc_tools" channel. it is a pinned comment.

In order to even load your gun in HLMV you must compile it with a different name or location.
HLMV is set up to always load content from the pak01_dir.vpk files.
If you compile your model with the exact model name it will replace ingame, hlmv will ignore it.
Just rename the $modelname line to whatever you want.
Easiest thing is to just comment a your modelname line so it's ignored, and adding a second one just containing $Modelname !/temp.mdl.

Then load !/temp.mdl in HLMV, make the attachments as shown below, save them in the qc.
Do not forget to swap the $modelname lines back before shipping!

By the way, Since you are making the gun mods, you can add bones that are in the correct location for attachments. that way you only have to worry about the attachment rotation. But that is up to you.

Once you have opened your model in HLMV, head to the "Attachments" tab.
The picture below is that you will see.
1. The attachment points you have compiled into your model
2. The bones you have available for attaching things to. You could attach the flashlight to the pump shotgun pump if you so desire.
3. input field for location and rotation
4. QC string field. Holds the finished QC line that you can copy into your QC file.

If you do not have any attachments visible, simply add the following qc line to your qc file with an attachment name listed below, this will give you an attachment point that you can manipulate in HLMV:
$attachment "NAME" "ValveBiped.flash" 0.00 0.00 0.00 rotate 0 0 0

WARNING
If you edit the location and rotation of an attachment and accidentally select another attachment, then all the settings you have made will be reset to the compiled settings. Be careful when editing the attachment values.

The Attachments you can have for worldmodels are:

The Attachments you can have for viewmodels are
Attach_muzzle (or forwardnode), Attach_shell_eject and Muzzle_flash should be in same spot with same rotation. Together, they build the complete muzzleflash.

To move the chosen attachment around, use the Translation text box.
To rotate it, use the Rotation text box


Now that you know how the system works, you need to know how the attachment has to be orientated to get the desired result. The pictures and descriptions below will show you how.

Muzzle_Flash - red line in firing direction.
Attach_muzzle - red line in firing direction.
Attach_shell_eject - red line in firing direction.
muzzlesmoke - red line in firing direction.

Flashlight (worldmodel) - blue line in firing direction.
Flashlight (viewmodel) - red line in firing direction.

Shell (right side, worldmodel) - blue line up, red line right.
Shell (left side, worldmodel) - blue line up, red line left.

Shell (right side, viewmodel) - red line up, blue line right.
Shell (left side, viewmodel) - red line up, blue line left.

The viewmodel also has an optional camera attachment. Technically it has two, but only one is actually used.
The one that is used it called "attach_camera"
The only purpose it serves is to animate camera rotation onto the screen. only rotation. movement is impossible

The correct orientation for the camera is as follows:
"attach_camera" - Red line in looking direction, Blue line pointing up.
In a nutshell, Red points towards where you will be looking while blue shows where "up" is.

Sometimes when you build a viewmodel the screen will rotate 90° clockwise. you can fix that by either:

• Rotating the "ValveBiped.attach_camera" bone in the other direction (usually 90°CCW) to make it straight. You can do that in all poses or with the bone directly.
  
• Deleting the "attach_camera" attachment. This is the easiest fix if you do not want to have any camera rotation in the first place.
  
• Rotating the "attach_camera" attachment itself so that it fits.