https://youtu.be/gUA52vZyL84
There are 7 total shootables to unlock the hidden ending and avoid doing all the parkour twice.
1. above the first juggernog machine
2. in the grate in the lockdown room
3. in a window in the trap room
4. on the ceiling in the pack a punch room
5. in the axe room in the maze in a corner somewhere
6. in the second maze on the right side on a ceiling in a dead end
7. on the parkour in the final area up high
Holding on to the m1911 or the m14 is recommended for pack a punch.