I used to be a SAW gunner, I can try to help. How much do you know about 'em? You have a FM and TM for it? If not, you can sign up to Scribd for a month for free and download all the manuals you can find. I have 'em for M4's and M2's but not M249's.
I'm trying to understand what exactly you're talking about --is it not lining up when you install the barrel or are you talking about the regulator? This is full auto or semi?
If it's not installed properly (the gas block) I'd send it back to FN if that's an option.