I'm sorry to say all the references I have are secondary source of information put on various websites - which I admit, some may object to as being unreliable. However, I do seem to notice in most cases the Soviet UBS machine guns are considered pretty much one of (if not) the best heavy machine guns ever to be used in WWII.
From what I've read:
The UB(Universal'nyj Berezina) machine guns used 12.7mm rounds and were divided into three groups:
*UBS: fixed, synchronized installation(800rpm)
*UBK: fixed, wing-mounted installation(1050rpm)
*UBT: flexible mounting (1050rpm)
UB
RPM: 800rpm (sync), 1050rpm (unsync.)
Muzzle velocity: 860m/s
Ammunition weight: 48g
M2
RPM:750rpm
Muzzle velocity: 870m/s
Ammunition weight: 48.5g
I'm not sure if the weight of the ammunition means the whole shell. I keep hearing that the UBS fired heavier bullets.. maybe the warhead itself is heavier? Sure wish Tony Williams was here to clarify things..
R.A.M website (follow the "WEAPONS" link on the UB)
Russian Ammunitions Table of guns