Bring back the MG81!

How often did you have to change the barrel on the MG81? After every belt?

I'm more curious about how that MG42 next to it shot.

I meant because of heat, not wear.

The biggest drawback to the high rate of fire was that the barrels would heat up very quickly, but the engineers thought of that. The MG81 (and the MG42 shown next to it) had incredibly simple and fast methods for changing barrels; the standard kit included a second barrel as well as an asbestos lined mitt.

There's a scene in Saving Private Ryan where they're attacking an MG42 emplacement and Tom Hanks' character instructs his men to not make the final rush against the position until the MG crew has to change their barrel. This leads me to guess that standard practice was to rotate barrels after maybe 150 rds of continous fire (since I'm guessing that's about 3 belts of 50 each).

Granted, my knowledge comes from reading books and watching documentaries.

Like I said, I'm not a historian, but some of the key stats to look at is rate of fire and weight:
MG81 ~ 15 lbs, M2 ~ 80 lbs
MG81 ~ 25 rds per sec, M2 ~ 8 rds per sec

Granted, the MGs used the smaller 7.62 cartridge, but the M2 has more than 5 times the mass to absorb and dispate that heat.