The adage for the past couple of decades among Joes (skip this if you are sensitive as it may be NSFW) is that the 5056 NATO-caliber M249 SAW is like a high-maintenance first wife: you have to pamper and court her and maybe, just maybe, she will work out. The 7.62 NATO-chambered M240 on the other hand, is just a dirty whore: no matter what you do to her, she’ll keep on working through the night, rain or shine.
Thus endith the addage.
There may be some smoke to that, as, in my experience, I have never seen any but a factory fresh and over-lubed SAW run a full 200-round belt without a stoppage under field conditions whereas I have also seen some downright grungy and funky M240s chew through belt after belt. This may be why the Marines have largely dumped the SAW for the M27 IAR and the Army is looking to move on to the NGSW-AR to put the M249 in the rearview.