Andy,
I guess an IR trigger has got to be very sensitive to register movement of something as small and fast as a pellet. May be worth starting by dropping a larger target ie grape through the beam to get some idea first. This gives the beam a slow large target to focus on. Objects can then be reduced in size so you can determine the sensitivity limits of the IR trigger.
My idea does not involve a pellet gun but I think would have been able to work on Lello's balloon as that could have been by impact trigger.
If the Vivitars fastest speed 1/30,000 is not fast enough I will have to do a job on the SC-17 contacts so I can get up to the 1/64,000 best with the SB80-DX.
I got a bit to fancy for my own good last time I tried this, so going to try something a lot easier this time.
Don