I can’t decompile any so I’m asking you.
Counter-Strike/Left 4 Dead/Day of Defeat, TF2/Portal, or original HL2?
HL2: ValveBiped.Bip01_R_Hand or ValveBiped.Anim_attachment_RH
…Then it’s because the original model lacks the bone used by default for weapon attachment.
I tried on default playermodels, still doesn’t work.
Provide a link to the model for the gun so we can take a look at it. I assume the origin for the w_model is in the wrong place, assuming the pistol is what you want the player to hold in that picture.