If I am grinding though rounds that fast I am probably using my 10/22, but even for that what I usually do is keep a full mag in the ammo pouch as I am shooting I refill the one in the gun in every lull unless I get a collection of gophers wanting to get shot and I get caught with an empty mag then I switch filling both in the next lull.
With the HMR I top up every 3-4 shots or if there is a big gap I'll top up after one.