The Leshiy is crazy compact, and delivers the hits. With that though, comes some draw backs... small(ish) shot count can be a pain in the field. There's other parts to this equation, but one is just to allow for more volume of air storage within the gun (duh!).
I've doubled my shot count (more on that here) with this extra air tank, but more importantly (IMO!) it addresses another issue I have with the OEM Leshiy, it's kind of awkward to hold and get a good cheek weld (I think I used that term right?!?).
Net/Net, this will add a ton more shots, and eliminate the site line crampness thing.
PLEASE NOTE: This is a Russian company, and their site is you guessed it... in Russian. Ordering can be a challenge through their site, but the best way is to just email them here: info@drozdpcp.ru They will respond in English, and provide you a paypal address.
The processing/handling took a few days, but the shipping was incredibly fast. At the time of purchase, it was $250 shipped. You should consider this instead of a buddy bottle as it nails 2 birds with one pellet (ha! good dad joke)