Stanley: I made another head shot at a Starling 34 or 35yds out about a month ago and posted it here.
The 850 is a CO2 gun (12fpe) so it's recoiless. It is extremely accurate and has a pretty decent trigger which doesn't make you jerk the gun. I get 0.32" CTC groups at 30yds and 0.25" groups on my 22yd targets. It's easier to shoot accurately than a springer. Point and shoot from a rest, and never worry about the hold, like I do with my springers which are hold sensitive. I can get 5 out of 5 shots within a dime or cover them with a dime. Usually I just get 1 "big" hole the size of a dime with 5 shots.
I have missed a couple of times and posted that here too. It happens to all of us. The thing is birds are usually on the move and hardly stand still. I have shot a couple through their neck or breast feathers and watched them fly away unscathed. I see the feathers move through the scope as the pellet nicks them or goes through them.. which is the beauty of a CO2 or PCP gun as I 'm sure you now. You can actually see your quarry go down through the scope, whereas with a springer I can't do that. I usually look around the scope after I shoot a springer :)
The thing with the 850 is that because you only get 610-620fps, the heavy .22's trajectory & POI changes about every 6yds so I have done a lot of shooting at 15, 22, & 30yds to get used to its "loopy" trajectory and learn the POI, holdover/under, etc. Normally I wouldn't do that if I were shooting rabbits for ex., but with a bird, 0.5" can make the difference between a hit or a miss so I decided to study pellet trajectory more closely with the 850 since it is my primary bird gun.