Hi Arralen
Size 0 is a compromise, If you have them size 1 then they -cant- be "well dug-in and camoflaged"
Tripod mounted MGs are a defensive weapon so giving them size 0 models them as a defensive weapon. In game terms if you do give them size 1 they are quickly shot up, this wasnt the case historically.
As for size, you picked an interesting example, 50 cal. HMG (Actually not really a support MG like those in the 7.62/7.92 calibres. It had no quick change barrel and so a very low practical ROF (40 rpm). Much better at punching holes in walls, planes and APCs........ anyway)
From
http://www.korean-war.com/Archives/2.../msg00112.html
"But that 50 cal. MG was a mother to handle. One carrying the receiver, one carrying the barrel and the third man to carry the alum. tri- pod. Man carrying the barrel had to switch off with the receiver man, as it was just too heavy
for one person to carry constantly."
Obviously this is a good argument to give the 50 cal a movement speed of 1.
Unfortuneately they are lumped in with the 30 cals in a hybrid MG formation so getting, as you rightly point out an unrealistically high movement allowance.
check this thread.
12.7mm does it deserve the range?