'You have to merge the two models in med.'

not particularly neccisary

for 1st person mode you would define it as a view entity
( see the script example for the shotgun on screen in the manual)
for 3rd person mode you would either have to attach the model by vertex attachment ( is a demo of this on acknex site)

or possition and animate the gun to match the player entity ( see the handgun model that comes with cbabe)