You know, there are hard-to-find Easter Eggs, impossible-to-find and then there's Respawn's category because this one gave the community quite a few headaches. Nevertheless, they came through and managed to find a way to make the Firing Range dummies come alive.
To be fair though, it was the devs who alerted the community to the existence of a yet-to-be-found Easter Egg in Apex Legends, and they provided a few hints too, which were sorely needed. I mean, ask yourself how long would it take for you to think of doing this? Thankfully, the community did the work for us, so bless them for their efforts.
Apex Legends players need to fire up the Firing Range and if you've unbound the Crouch Toggle key, bind it back because you'll need it. Drop your weapon and head on over to the rightmost cave - that's right if you're turning your back to the training dummies.
This is where you'll need Pathfinder, so that you can grapple up to the catwalk above. When you do, stand in the absolute middle of the catwalk, press Crouch Toggle, look straight down and change the legend. If you've done this right, you'll hear a characteristic sound after the selection is done.
When you Apex-slide to the bottom of the hill, you'll notice that the dummies have come alive and their bullets are quite real, as you can see. Heck, it seems that the ones on the sides really like Peacekeepers too, and while it's not exactly an advanced AI - moving targets are still better than stationary ones.
Your new Apex Legends enemy will keep up with their lively behaviour even after you're dead - simply drop your weapon on the ground when you respawn, and your little Frankensteins will start running around again. We've heard this works even when the rest of your team is in there, so you can actually freak them out a little.