Lots of those old revolvers have free spinning cylinders. Those with a spur usually have a half cock, which would be how it should be carried. If you were to shoot a round, the firing pin will stay in the dent it made in the primer, holding the cylinder still until one shoots the next round. At least, that's how mine work.
If you don't like the cylinder to spin freely, you can lower the hammer between rounds and the shells' base will keep it from spinning.
No matter where or how the cylinder is resting, the ratchet action will engage the cylinder and will position it correctly for the shot when you pull the trigger.
What's an H&R Model 1918 look like? I'm not familiar. Post some pictures!
Note: The above is how my guns work. I don't know if your piece is functioning correctly so proceed with prudence.
