Or that it's a joke gun.

Hmm, we've talked about him shooting somebody he cares about then being horrified (desparately trying to put together the pieces we've seen!).   I wonder if they'd use shooting again - quite possibly, as they do actually use guns quite a lot in the show.    This appears to be shooting in the head (like Magnussen) so presumably shooting to kill?  Can we deduce anything from the height?   To me it looks like he's shooting somebody around his own height, which might rule out shorter cast members (Mrs Hudson, John, etc.).  Mycroft?

 

The height might still be relevant - for instance, if he's hallucinating an image of a real person.  I agree he looks panicked - quite different to shooting Magnussen, even though he's very emotional there. 

Possibly, although he's shorter than Sherlock (not much shorter, but Sherlock appears to be taller than Benedict Cumberbatch!). 

Of course it could be anyone if they're standing on a chair.

I'm just imagininig somebody standing on a chair while Sherlock shoots at him / her - that scene I'd love to see, especially what lead up to it. :D But then again, I want to see the scene anyway!

That is a distinct possibility.

I think either that, Nakahara, or he's shooting at the wall in anger/frustration.

I'm going with hallucinations.

Yes, I wouldn't be surprised if the trailer turns out to be very misleading. 

I've never seen the teasers or trailers for any of the first 3 seasons.  I should like to see them now!
 

I am almost certain the trailer and teaser will be misleading. They are expert trolls