Fixing the Dropship gun
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Introduction
This article explains the process of fixing the Dropship gun's broken rotation in Source.
Fixing the Gun's Rotation
Copy the contents of this page, (courtesy of GRAI+) and use it to replace 
src/game/server/hl2/npc_combinedropship.cpp — You're done!
Alternatively, if you wish to edit the code, the main change it does is add these two lines at the top of CNPC_CombineDropship::UpdateContainerGunFacing, immediately following the assertion statement:
m_poseWeapon_Pitch = m_hContainer->LookupPoseParameter("weapon_pitch");
m_poseWeapon_Yaw = m_hContainer->LookupPoseParameter("weapon_yaw");
Tip:If you're developing a Source 2013 mod, you could opt to use a community repo, and have bugs like these fixed by default.