I have & still carry (at certain times) a Colt XM-177 select fire in 5.56mm. It is terrible load.
When I practice with it, I use plus
AND good muffs.
Mods:
(1) I installed a home-built compensator (Cutts-type) that makes it even louder, but absolutely
STOPS any climb.
The sound & concussion to the operator is hard to describe when on full-auto.
It has a resoundin' pulse that actually shakes the windows of a neighbor 1/2 mile away.
(2) It used to be terrible fast cyclin'. It was like a MAC-10. You could not tap off less'n 3 or 4 shots on auto mode.
I also messed with buffer spring tension & bolt carrier weight & gas port size. It now shoots around 800 RPM. Much more manageable.
I understand the one you depicted above has been designed to shoot slower on auto.
Have fun!!!
MG (which might also stand for Machine Gun)