Re: Need info on a Hembrug 1918
No, both the Dutch Mannlicher and the Italian Mannlicher-Carcano are turnbolt actions. Both used the Mannlicher type magazine with en-bloc clips. Hembrug and Gardone are the cities where the respective arsenals were located and where the guns were made.
I assume your grandfather was in the Dutch military or the resistance if he used that Hembrug rifle. Talk about fighting against long odds!
The caliber is (if not altered) 6.5x53R.
As to ammo, the Germans made a batch because they used the Dutch light machnegun, but it is scarce. I think someone is making cases but you can probably Google "6.5 Dutch ammo" and see what comes up.