Continuity mistake: While the Squad is patrolling the city, the ends of Harley's bunches keep changing in style; they are straight when she's changing at the prison and on the helicopter with the Squad, really curled after Rick kills someone, then jump between straight and slightly curled for the rest of the film, with the exception of the end scene where she hasn't got her hair in bunches anymore, but a different hairstyle.
Continuity mistake: When Captain Boomerang kills his accomplice, he falls to the floor, and his money bag is in his left hand. Then it is shown on his right side.
Continuity mistake: When Harley is in the elevator when the doors open you can see that her blue hair piece is coming off, but when they cut to the next shot it's secure and tight to her head.
Continuity mistake: In the extended cut of the movie, during the final fight with the Enchantress, Harley Quinn hits her in the head with her baseball bat. A couple of shots after that are inverted, you can see it because she should be wearing the blue part of her pants and ponytail to her left side, but in these shots, the blue side is the right one.
Continuity mistake: After Harley hits her head against the bars of the cell, her body and face change position between shots.
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