After retrieving the shotgun ammo and unlocking the "Alone In The Dark" achievement/trophy, as shown in the previous section, Lara should be standing on the ledge above the carved pillar overlooking the cavern. Walk to the edge, hop back...
...then take a running jump to grab the high ledge directly ahead. Pull up.
Vault onto the angled block directly ahead and look off to the left, where you'll notice some shotgun shells (22) on another high ledge. That's where you're headed.
To get there and unlock the "Schmitty's Leap" achievement/trophy in the process, first turn around so Lara's back is to the open area. Stand near the middle of the block. Sidestep to the right as far as you can. Then flip to the right to land on the sloping center-right block facing uphill.
Slide back, grab the edge, and traverse to the right until you can pull up on the darker colored flat ledge.
Turn around again to face the ammo. Walk to the edge. Hop back once. Then save the game just in case. Take a running swan dive (i.e., run Forward then quickly press Jump + Walk simultaneously...
...to dive onto the ledge with the ammo.
If you do this correctly, the achievement/trophy should pop just as the chime sounds for SECRET #3. Pick up the shotgun shells (22)—or the SHOTGUN itself if you missed it earlier.
NOTE: I'm not sure of the specific lore around this achievement, but I assume it's a nod to Mike Schmitt, Core Design's producer for the classic TR series.
To get down, safety drop to the tan rocks below. Then carefully drop/slide to the ground.
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