When I was a kid I had a black Daisy air rifle (don't remember which model, but it was one of those that came with a scope). I could get it to shoot 7-8 BBs at once. I would pull the bolt back and load a BB into the chamber. Then with my thumb on my other hand I'd hold it over the chamber while I pulled the bolt back again. This would keep the BB I just loaded in the chamber, and the bolt would pull another BB up for loading. I'd repeat till there were 7 or 8 in there.
I never used this technique when out in the woods shooting birds, but I remember one time my friend's father took us out somewhere to shoot, and we had set up a thin piece of plexi-glass type stuff. He was shooting at it with a single BB at a time and making little cracks in the plexi-glass, while I was shooting multiple BBs at a time and blowing chunks of the plex-glass apart. I remember he even started to complain about how my BB gun was so much better than his because he didn't realize what I was doing. I think we were both about 10 yrs old.