The standard Humvee is NOT an armoured vehicle... it is the replacement for the jeep, which wasn't armoured either.
People just looked at the enormous weight of the Humvee and assumed it must be armoured.
Armoured Humvees on the other hand are even heavier and slower and will likely stop most assault rifle calibre rounds... I rather doubt they would stop 12.7mm rounds.
Most MRAPs have better armour and will likely stop 12.7 as well... that is why the Russians have shown portable 30mm cannon rifles for shooting MRAPs.
Firing at a high rate of fire does not improve penetration except against ceramic armour, where the armour is weakened by repeated impacts.
Proper metal armour is not effected by multiple hits.
Hitting a metal plate with 100 rounds per second just means 100 dents where it didn't penetrate instead of 10 dents where it didn't penetrate.