I'm no expert on bullet cartridges. The 30/30 round has been around a long time. It was a carbine style rifle with a internal magazine which ejected and injected a fresh round with an underhand cocking motion. It saw action in the Old West as being a repeater and not having to manually chamber a round. Cowboys loved it and Indians to back in the day because it was considered rapid fire. It was very accurate for a 100 yards but accuracy quickly dropped off after that.
It was largely replaced by the 30/06 which held much more gunpowder in a larger casing resulting in a longer range and accuracy.
Many casings are stamped with a date and in your case, I'm guessing you have found a 30/30 casing from 1918. Theses guns were really popular back at this time.
I'm not a collector but I'm guessing it's not worth much other then being a piece of history which is still very cool. Was someone hunting game or fighting when that bullet was fired? We will never know I guess.