To get the BB gun sound in a 30-06 you can only use one grain of powder.
To get the most velocity, the bullet must compress the powder.
If the bullet has to jump a long way to the rifling, accuracy is terrible.
The the old guy out in the middle of nowhere with two lathes make adapters is gone. His link went dead.
http://alexcartridge.com/
Now all that is available is that guy in Alaska. And he uses direct firing pin impingement, which makes as long a jump to riflings as possible.
http://www.mcace.com/