Hi Paul
I've been waiting for years for some decent british helo guns in 1/72. The only GPMG I've sourced are from a company called Aroclub. They are will need modifying for a modern version. As Jabba said the Chinook has an M60 on the ramp so look at some of the 1/72 Humvee modles for something, but again it will probably need modifying.
Now for the good news! For Chinook door guns you can't go wrong with these beauties, http://www.armycast.cz/products4.html 1/72 mini guns. I got some of these and the quality is awesome. I highly recommend Armycast.
Good luck
Matt.