Trivia: The actor Ted Whittall, who took a small part in the film, portrayed Rick Flag in Smallville season 10.
Trivia: The first words that David Ayer said to Jai Courtney on set were "Embrace your inner shitbag."
Trivia: Amanda Waller operates out of the "John F. Ostrander Federal Building." John Ostrander is the comic book writer who reinvented the Suicide Squad using the concept of a supervillain team forced to do the government's bidding. The Suicide Squad title was originally a World War II comic, akin to the Dirty Dozen.
Trivia: Stay after the credits for another scene with Viola Davis.
Trivia: The footage of Harley and the Joker dancing is similar to Alex's Ross' art work from Harley Quinn #1.
Trivia: Fleetingly visible on screen among Harley Quinn's bio-data is the phrase "accomplice to the murder of Robin." In the comics, Jason Todd, the second Robin, was killed by the Joker - this took place before the character of Harley Quinn was created.
Trivia: While he's only referred to as "Rick Flag", the character is actually Rick Flag Jr. - in the original Suicide Squad comics, Rick Flag Sr. Was the leader of the first Suicide Squad, a Dirty Dozen-style commando team operating during World War II.
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