The following Arnie's Airsoft review has all the information you'll need. If you're not yet a member over there, it's worth joining the forum just for that thread (and a few others pertaining to the A&K 249.)
http://www.arniesairsoft.co.uk/forum...amp;amp;K+m249
It lays out most of the problems and their solutions in a nice concise manner with lots of pics and some videos.