96 Meters + 117 Meters
http://www.youtube.com/watch?v=zfr_3hUFNA0
The goal is to shoot Dante Moro and Silvio Barbarigo from as far away as possible. This mission relies on the unusual mechanics of the hidden gun. You need to be within 15 m to lock onto a target. Once locked on, you can maintain that lock if you remain within 30 m. If you start to aim the gun at your target while you are locked on, you will be able to maintain your aim. The shot will hit its mark as long as nothing obstructs the path of the bullet, regardless of how far the target travels from you.
Dante Moro
The mission starts with Dante and Silvio fleeing towards the ship. Lock onto Dante and position Ezio at the exact location shown. I look at the edge of the hay cart near the bottom of the screen to help position Ezio appropriately. Start aiming before you lose your lock on Dante. The plan is for the bullet to pass between the rack and the scaffold to hit Dante just before he reaches the ship's gangplank... 96 meters away in this case. What limits this shot to 96 meters is that Dante and Silvio will "wait" for Ezio as they flee unless Ezio is within a certain distance.
After the shot, confirm that Dante is dead by checking that "Targets killed: 1/2" is displayed on screen. If not, kill 3 civilians to restart the mission. Beware that you should not backtrack to the entrace of l'Arsenale where you started. If you do, you will likely inadvertently kill Dante and Silvio with "There is no spoon":
http://www.thehiddenblade.com/dante-moro-and-silvio-barbarigo-assassinat...
Silvio Barbarigo
Now it's time to shoot Silvio, who should be inspecting Dante's dead body. I kill 3 archers in this video, but the second archer is the only one that is required. Position yourself as close to the ladder as possible. Lock on to Silvio, and throw down a smoke bomb. The smoke bomb scares Silvio into fleeing without him seeing you. This is important because, if Silvio engages with you at all, you will be held to a maximum distance of 80 meters from him as he flees, which is insufficient to make this long-distance shot.
You won't be able to see Silvio through the smoke (unless you use Eagle Vision), but it doesn't matter. Switch to the hidden gun and start aiming before you lose your lock. Silvio will run all the way back to where you started - at l'Arsenale entrance... 117 meters away in this case. Wait for him to stop moving before you shoot, and look carefully to avoid shooting someone who just happens to pass between you and Silvio at the wrong time.
Other Notes
It is possible to shoot Dante from a distance 1 meter (maybe 2 meters) farther than what is shown. In this case, position Ezio by the tree next to the hay cart. There is a very small spot that will give you a clear shot to Dante while still meeting the distance requirements of the mission. I've managed to kill Dante from this position only once (distance of 97 meters), and I haven't been able to reproduce it.
Sometimes, the smoke bomb attracts guards who will then interrupt your aim on Silvio. If this commonly happens on multiple attempts, consider quitting the memory and replaying it with less Notoriety.
96 meters + 117 meters = 213 meters, the title of the video
Great job!
The only thing I would edit is the title of the video. I understand the math, but it's a little misleading since I think most of us expected a 213 meter shot. Regardless, excellent work yet again.