1. hold a rifle or shotgun that doesn't have a mag, like the SKS or mosin or the BK, and have the ammo for it in your hotbar.
2. hold down the key for the ammo to load it, you'd think that i would keep loading ammo until it's full, but it doesn't

You can load more rounds if you load one round in and while your character is loading that round, you hold the ammo key again and wait for the animation to be done, then you hold left click.