This is a list of some machine gun locations...

As soon as you exit the warehouse door to paradise plaza go up the stairs to the right to find that you can jump up to the blue overhang on your right, and walk down to VERY end to find a sub machine gun.

In the fountian at Al feresca plaza.

On top of the block house in wonder land plaza. To obtain this either jump off of zombie backs or time your jump from the space rider ride.

In the maintenance tunnels, where Carlito locks Brad out of the room in their epic battle.