Sorry if this has been covered in this thread already. Mendes has said that he did intend to put the gun barrel at the beginning, but when he looked at the finished film he thought that it would be too odd to have Bond turn and shoot in the gunbarrel, then open the barrel up to see him coming toward the viewer with gun in hand. So apparently he saw the similarities as a liability rather than an asset.
Yep, we've been discussing that.
I don't like his decition. FRWL also starts with Bond walking and shooting.