Here is a quick tutorial on how to make a usable machinegun in multiplayer.

1. Open up your map and right click in the 2d-view. Go to misc>turret



2. Open up the entitypanel by pressing 'n'. Fill in the following values: (press enter everytime you've filled in one)

Key: angles
Value: -0 2.50448e-006 0

Key: bottomarc
Value: 30

Key: export *
Value: 2

Key: model
Value: xmodel/weapon_saw_MG_setup

Key: spawnflags *
Value: 1

Key: weaponinfo
Value: saw_bipod_crouch_mp **

* I'm not sure if these are necessary. But they can't harm

** You can also use 'stand' and 'prone' instead of 'crouch'

EDIT: paulb39 points out that you should also have "xmodel,weapon_saw_mg_setup" in the zone file updater

It should look like this (apart from the origin)



3. Shoot!