William remined me that after mentioning the plastic gearboxes that Star plan to use in their L85 series. Star have already made a plastic earbox for the M249 series. I’ve not seen or used it (and not heard much feedback from those that have one) so it’s hard to tell how good they will be. (I also forgot to add their URL, which is http://starairsoft.com/ if you need it)
Whilst metal is always preferable to me, I can see that having a transparent gearbox mechanism can be useful for diagnostics, although you’ve got to get the entire box out of the frame of the rifle firts before you can see into it. Either way the copies of the L85 look good in the pictures available so far, and they can always have their internals replaced by users that don’t like plastic mechs.