Confusion about how the forge works on heavy ammo

The forge is said to restore 5% of magazine per second. This does apply to heavy ammo, but the restore speed is really slow. For example, my launcher has 5 magazine size, I shoot it once, and it took 35 seconds to restore that one ammo. Does anyone know why this is happening?

I have noticed the same long time between regenerations, though it does seem to depend on magazine size for heavies as well. If I equip my Atlas with magazine size 3 it takes 1m 5s to regenerate one rocket. If I however equip a CoV launcher so I have my 27 as magazine size, it takes 20s to regenerate one rocket. Now if the skill text was correct it should be regenerating over 1 rocket per second with the CoV launcher but clearly it does not. Nor is it using the old 1.5% speed, since that would be around 3s per rocket.
Oddly though is that these two rates do not add up to a single rate. The CoV launcher regens at 0.185% While the Atlas one goes at 0.51%, so I can’t really make heads or tails of it.