I soloed him down to half using just my sword earlier today. My fingers got tired after 15 minutes -.-, but hes really easy compared to some of the other quest bosses.
Your machine gun has 120 rounds per clip dealing 18 per hit. Thats 2,160 per clip, meaning you'd have to reload at least 6 times to defeat him, minus 3 times because you can use the other clips on your other machine gun. From experience, I can tell you that the goblins mass up real nice after 240 shots, and after 480 shots, its no different.
You know what, never mind, I believe you now. The numbers just didn't seem to add up, but they kinda do.