Some is reloadable, some isn't. Pull apart one round and inspect the inside of the brass for a single flash hole and see if there is any corrosion on the inside of the brass.
Nitrocellulose offgasses nitric acid, so old ammo can be a tad unsafe to fire. However, if the ammo is in good shape, I don't see why a Steyr M98 (I'm assuming it is an M98, please correct me if I'm wrong) wouldn't handle the ammo just fine.
But, I recommend just getting some modern brass and loading your own. Leave the old nazi marked shells for the cartridge collectors.
Machine guns are awesome until you have to carry one.