MUNDAUN

Unlock the door on the top floor of Grandpa's house and get his rifle.

Day 3

In the bunker, after the flashback where the old man causes an avalanche to wipe out the enemy, you'll pick up the soldier key.

Day 4

At the start of day 4, go all the way back to the first area. Go to the top floor of grandpa's house and use the key to unlock the door. The rifle is by the soldier mannequin.