Some great shots there Lello.
Superimposed shots are probably too long a flash duration. From Foxys post I would be aiming at using 1/128 output ( 1/41600 sec ) for maximum freezing of the action.
I guess the D300 is quite clean to allow setting 1600 ISO to allow for a reasonable aperture.
As for firing bb's or pellets then choose pellets. BB's are the more dangerous option as they are steel balls. Try for an air pistol rather than rifle as they are limited to 6 ft pounds which is half the power of an air-rifle.
Thinks .........................
I have an antique Webly air pistol. .............. try a bit of kitchen foil as a flash trigger ....... mmmmm
. Another diversion ?????????
Don