Pack a Punch is located on top of the watchtower. You will need the help of a ladder to get up there. You must find three scrap metal pieces to craft the ladder. The following pieces can be found in any order.

Part One
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In between Deadshot Daiquiri and the Double Tap Root Beer Building there is a destroyed car. Inside the back seat you will find a piece of scrap metal

Part Two
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At the bomb site near the trip mine wallbuy in the middle of the room leaning up against the multiplayer bomb from old Firing Range.

Part Three
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In the garage on the bottom floor next to the car in the corner of the room.

Once you have all pieces go to the table near Speed Cola and place down the pieces. Once done it'll ask for a Blacksmith Hammer to weld the pieces.

Blacksmith Hammer
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Located in the middle of the map next to the watchtower leaning on sandbags next to the car in the middle

Go back to the table and put the ladder together. Pick it up and place it on the watchtower and you will be able to access Pack a Punch. Once you go up there you will have 45 seconds to upgrade. After the time is up you will be kicked out and will have to wait 5 minutes to access the ladder again.