It does not appear to matter how many rounds you load, the animation and sound of rapid reload (like machine gun fire) is persistent after selecting to Load ammo.

Tested from a stack of 5, 10, 15, 20 into an unloaded Mosin.
Tested loading 2, 3, 4, 5 rounds at once from stacks of 2, 3, 4, 5.

Mosin had long range scope and bipod deployed attached, if it matters.

A full inventory or unloading the box on the ground (not in inventory) seems to destroy the rounds.