I'd say that your gas mileage is great! I normally find about 50 shots is an average number for a QB78-family air rifle in stock condition before the muzzle velocity drops enough to change the point of impact significantly (maybe 60 shots for a .177 caliber model).
Of course - as Steve pointed out - if an HDD is fitted to your air rifle, that would make a significant improvement on stock performance.
Archer Airguns Inc.
Online stores http://www.archerairguns.com