DISCLAMIER: I used my Daisy Powerline Model 93. I'm not sure if it will work on other air/BB guns but feel free to try and be safe.
Step 1: Materials and what not
You'll need an airgun (Daisy Powerline Model 93)
A large rubber band
A small scope
A BIC pen
1/2'' PVC Pipe
A suppressor (referred to in movies as a silencer) (If requested I'll make an instructables on how to make one)
A 1/2''(Slip) by 3/4''(Threaded) PVC male adaptor
A small scope
A hacksaw (if you don't have the length of PVC already cut like I did)
The scope is just for longer ranges and the rubber band goes with it
The BIC pen and PVC pipe are to convert the BB gun into an airsoft gun (single shot)
And finally the suppressor and male adaptor combined with the PVC tube and pen make the gun more quiet
Step 2: The Scope
On the Daisy the top of the magazine protrudes very slightly which was perfect for my scope, the mounts held the scope in place but not at all on the gun, which is where I used the rubber band to actually keep it on the gun.
The scope I used is a Crosman brand, it has 4x zoom and is 15mm. I got it for 1$ at a discount store.
Step 3: The Airsoft converter/suppressor connector
There is a space in between the barrel and the gun that the BIC pen fits snuggly into (after being gutted of course). After you've forced the pen into the gun measure the length still sticking out, and cut the same length of PVC. Once you have your pipe (mine is a little bit longer due to my laziness and the fact it was a scrap piece) cover two ends of the pen (or more if you prefer)(don't cover the part going into the gun) in electrical tape so it fits snuggly into the pipe, then tape the pipe in black tape for aesthetics. If you're going to suppress it then leave room on the pipe for the male adaptor .