I tested the MG42, and it is simply twice the time.
Normally it takes approx. 3 seconds to overheat.
At L2 Heavy Weapons it becomes about 6 seconds.
Weasel, what you are saying is far more than twice cooling.
Let me try real quick to show how your logic would cool the MG42.
MG42 fires 50 bullets per second, so 20ms between bullets. Firing a bullet causes it to heat up to 4/100 of the bar. By defualt, it cools at 2/100 of the bar per 20ms. So you fire a bullet, bar goes to 4/100, in 20 ms between bullets it cools to 2/100. Fire next bullet, up to 6/100 and back down to 4/100. So after 50 bullets, 100/100 and it overheats.
Now you have L2 heavy weapons. Twice the cooling rate. Instead of 2/100 per 20ms, it’s 2/100 per 10ms, or 4/100 per 20ms. You fire a bullet, bar goes to 4/100. In 20ms between bullets, it cools all the way back down to 0. You fire another, up to 4/100, down to 0.
This shows that your math is not how it works, as it would mean at L2 HW MG42 would go forever too.
note: I scaled the firing rate to approximately the same percentage of inaccuracy he used, both guns actually fire much slower (approx. 3 times).
Also, as Vengeance said, The no cross over stuff is in the manual.
It says, at the beginning of each class, something close to, “This class-specific group of skills is rewarded by doing blah blah blah”.