Combat Medic here with "Muzzle Fluting" Talent, and yes, I can confirm that when using SCC, Grav Round cost is zero.
The problem I see from the DPS perspective, is that in order to get the max out of it, you need to almost run out of ammo, and in that amount of time, the 5 stack debuff may fell off, plus you will get no procs for full auto and so on.