I've had this happen a couple of times now; instead of getting 4 rockets I get only 2 when grabbing the heavy ammo in the crucible. For instance, without changing any gear, during one match you might get 2 rockets the first time heavy spawns and 4 on the 2nd spawn. I haven't noticed any pattern in this, but it has happened multiple times now and it's quite annoying. I always wear boots that give you more heavy ammo and the launcher can hold 7 rockets.