Bone attachments
I'm looking at a model in the Model Viewer, and I see that it has all sorts of bones but only 3 predefined attachments. Doing some research I see that if i wanted to add such attachments to the model I'd have to modify the .qc file and recompile the model -- that would of course involve first decompiling the model, editing the current .qc file and recompiling -- which really turns out to be a head ache when there are few (if any) functioning decompilers available.
Is there any other way to parent an entity to a model at a particular bone that doesn't have a predefined attachment? Or am I s.o.l?
Alright, so let me get more specific and see if my particular problem can be solved differently.
I'm looking to use a Chell model holding a portal gun, so the gun would have to attach to the weapon_bone of the Chell model.
In Hammer's model viewer there seem to be 2 models for Chell. 1 "proper" model (playerchellplayer.mdl) with textures and working animations, but the only attachments defined are for parts on the head, none involving the weapon_bone. The other model (player.mdl -- not in a subdirectory) is not textured and most of the animations don't appear to work, but it does have a defined weapon_att attachment for the weapon_bone. In that same directory there is also a player_animations.mdl which I would figure is involved in giving that model proper animations, but I don't know how that works.
I would try to edit the .qc file from the first Chell model and hope that nothing breaks between de- and re- compiling, but the only MDLDecompiler program I've found seems dated and keeps crashing on me.
Yeah, nothing with animations will ever decompile right. Sorry.
Pessimistic much?
Actually, with the right program, you can get it to decompile correctly. The only problem is Valve is not about to give you THEIR compiler. Without it, yes, the decompile will fail to work correctly.
I've been hacking for weeks trying to decompile and recompile all the shared human animations from HL2 into the Portal 2 animation format. So far I've managed to get one glitchy gesture SMD file that launches the right arm off.
Sounds stupid. Valve doesn't need any decompiler! They have ALL SMD files.
And yes, decompiling Portal 2 models is a bit tricky. There is something called "MDLDecompiler 0.5 by Cannonfodder # Fixed & Patched by Hooch". It works better than the last version Cannonfodder released, at least for Left 4 Dead 1 and 2 models.
For Portal 2 it works better, too. But you still have to copy some of the VTX files and give them names with .90, because this MDL decompiler still needs "the old" files that are now missing in the new MDL version.
You do know that the gesture of launching your right arm off is perceived as highly offensive in some cultures, right?
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
That wasn't what it was supposed to do! 
I didn't even know that bones could detach in animations, or that gesture_turn_right_180 had detaching bones.