Okay, fair enough, but a 75% damage reduction still seems too steep. With the current mechanics a player using two rifle squads to assault an MG42 along the edges of its cone of fire won't do enough damage for 30 munitions to be worth the cost.
Who said it's 75% reduction?
All grenades have 50% reduction vs heavy cover and NO reduction vs garrison.
This is why riflenades/etc can instakill some garrisoned MG-team members instantly.
If you want to have NO reduction vs heavy cover, there would be no reason to use cover again; ever. The squad formation already punishes you for using cover by making all team-members bunch up (and thus make them a prime target for explosives).
Relic already trialed this between TBF release and the Leigfest patch, and it was a failure for the reasons I outlined.
PS: If you want to insta-gib an MG team that uses cover, you have to flank them first and THEN throw the grenade.