I think the Gigante shooting was all an act.

I have heard the wound as being superficial yet, Costello made it a point to wear a ridiculously large bandage that looks like he took a 12 gauge in the middle of war.

He also made it a point to have blood drips shown on his suit. They looked like someone artistically put them there.

Where was the scar on his head? Has anyone ever seen a photo showing any signs of damage?

Gigante was smart and knew he probably was in a very bad situation....Probably could be killed for not following orders and in retaliation had he killed Costello.

I think possibly, him and Costello came to an agreement that would satisfy everyone. A staged murder attempt.