To get the Heritage Gun in Dawn of Reason in Play Edward Scheme in Super Shifty in Karl’s Bay in Deathloop, you need to find all the gifts around Karl’s Bay and open them based on the map near the Heritage Gun.
Their location is randomised on each loop, although one in Hangar 2 (click here for a guide) and Dawn of Reason (click here for a guide) always appears. Be careful not to damage each gift and always run away from the gift because some explode and damage you.
After you have opened all of the gifts, you will need to go deeper into the building. Destroy the boards on the wall and use the hackamajig to disable the field nullifier.
Keep going deeper into the building and pull the lever.
This sets of a timer within which you will need to hit another lever based on which one is ticking and beeping. The levers are located deeper into the building and protected by lasers. If you run out of time or hit one of the lasers, you will need to start again. Be sure to use your powers to avoid the lasers. There are levers across 2 levels in the area and you have a decent amount of time between each lever.
After you have hit all the levers, go back to the front of the building and take the Heritage Gun from the safe.