I did not want to open a new thread on a bull-pup machine gun. Here is a modification of the PKM (Pecheneg) in bull-pup configuration, being held by Dmitry Rogozin.
I would rather go for a tavor rifle with 75 or 100 drum magazine and extended barrel. It can fulfill role or LMG and MG .
Ukrainians have been using Tavors chambered in 5.45 x 39. There are tonnes of aftermarket kits available for tavors (similar to AR-15) so it is possible.