Revealing mistake: We see Deadshot talking to his daughter with snow falling. A layer is digital (at least the one is in focus, between the characters), and the snow, first goes up, it glitches, and then goes down on the exact same trajectory. (00:07:30)
Revealing mistake: The subway station is identified by its internal signs as "Central" station, but as the Suicide Squad enters its exterior entrance, the sign above declares that it is "Bay St." station. Bay Street Station in Toronto is frequently used for movie shoots, as it has a complete lower platform which has been out of service for decades (https://en.wikipedia.org/wiki/Bay_ (TTC) #Lower_Bay).
Answer: Exactly as you said, to show off. He wants them to see he can fire just as accurately at an angle.
lionhead
I am not a gun expert and attempts to search this to verify my point have failed. However, it seems to me that he rotates the rifle to use the secondary sight. While this may be showing off, I assumed this secondary sight was intended to be used for a different range of target, or made a specific type of target easier. When he rotates the gun, it seems he is switching targets. Research has shown that the gun does have 2 sights (DDM4 MK18).
The DDM4 MK18 Deadshot is using has a single red dot sight, nothing "secondary." A gun has only 1 sight, perhaps also a scope, but nothing on the side that requires the gun to be used at an angle. Firing a gun at an angle seriously reduces the accuracy. If there is something attached on the side it's either a flashlight or a laser.
lionhead
If you re-watch the scene, there are very obvious iron sights that he is using that are accessible by tilting the gun sideways. Holding it normal gets you the scope, sideways gets you the iron sights. Not sure what the benefits would be but there are absolutely 2 types of sights on the gun he is using.
oldbaldyone