Zug's Guides for Warhammer 40K: Darktide, Veteran Build Guide. Patch #15 Keystone ideas. 

I hope they do but it's looking like they will leave it as is with the internal 3/4 second delay on each "free" ammo consumption

Agreed, that is the problem with RNG on abilities you use 5 to 10 times per match, sure you'll get extra grenades a few times but not when you REALLY need it. A 3 minute cool down would be fine if when secondary firing a grenade(underhand) it threw 2 when off cool down. That way there would be some control.