rhubarbraspberrypie answered your post:
I thought it was to emphasise that Dean's finger wasn't on the trigger and therefore telling Cole there was no risk of being shot at
That's a point. Could be could be. And the reverse, when Cole gave Dean the gun, was to emphasise that COLE'S finger wasn't on the trigger maybe.
Idk though, because I don't think the touching was strictly necessary to convey that.
Also, when Cole gives Dean the gun, Dean proper REACHES a finger across it in order to touch him.
So I think there was def a nod to an emotional connection/understanding between them with the touch as well (not necessarily, or likely, sexual, like I jumped to in my excitement though :p)