Though I have never opened up a BB shell to see the powder, I always suspected it just might be Lil-Gun inside, Lil-Gun REALLY turns on with heavy for caliber bullets, driving them at the highest velocities, usually with FAR less pressure than any other powder, As I said, I have never seen the BB powder.
And IIRC BB came on the scene just about the time Lil-Gun did.
And I know for a fact that in 180gr. 357's Lil-Gun will duplicate the 180gr. BB loading to a "T"! Without any signs of excess pressure.
Food for thought