The El Tirano sniper rifle is one of the better sniper rifles you can find in Far Cry 6. It comes pre-equipped with armor-piercing rounds, the Fresh clip mod that gives bonus damage to the first bullet fired after reloading, and extended mags. It also has a pretty nice paint job to boot.
The game will tell you that the rifle is found by searching Escila Fort but gives you no direction to locate the area. Take a look at the map below. Escila Fort is located almost at the top of the map on the eastern side of Yara. It is quite a trek to reach the area, but worth it.
Once you reach the fort, you will probably get notified that there is an Ambush nearby. If you would like, you can take out the enemies inside the fort before and grab the supply drop before proceeding to the weapon. It is recommended to clear them since they can be a problem if you are under-leveled for the area.
After dealing with the enemies, climb to the top of the fort and locate the lighthouse. Head over to where you will be in range of the grappling point on the lighthouse railing. Use your grappling hook to ascend to the top of the lighthouse.
Now that you are at the top of the lighthouse, search around for an opening. Inside the opening is the chest that contains the El Tirano sniper rifle.