According to Wikipedia, the Reffye mitrailleuse shot a 13 mm 50 gram bullet at 480 meters per second. That's a kinetic energy of 5,760 Joules.
My simplified version of the gun damage formula is:
( (a^(1/2)) / (b)^(1/3) ) × c = d
Where
a is kinetic energy in Joules,
b is bullet diameter in mm,
c is the arbitrary constant 0.23, and
d is damage in dice.
That says it should do 7d+1 pi+. Using the more complicated equation from
Interior and Terminal Ballistics for GURPS, I think it is slightly higher, around 7d+3 pi+. Either way, it's a lot of damage for the time.