YMMV with MGs against infantry because infantry don't actually get suppressed until given a new move command.
So if you or your opponent tells an infantry squad to throw a grenade before an MG starts firing, they will waltz up to range, even once under fire and being suppressed, and throw the grenade.
If you keep giving orders to the unit while under fire, they will drop to the ground once that suppression its. The only thing that breaks this is being pinned, as it disables the abilities.
It's not 100%, as the suppression triggers when the entities have a new command, not necessarily the squad as a whole, but it's usually more than enough to get that grenade off.
MGs in CoH are supposed to be about map control and denying infantry movement.
