
Actually, the situation is even worse than that.  The *total mass* of the
Pluto Fast Flyby spacecraft is only 250ish pounds, and most of that is
support equipment like power and communications.  The mass available for
instruments is maybe 10% of that.  I don't think a BATSE will fit...

Actually, would you need the shielding?  My understanding is that it's
mostly there to give the detectors some directionality.  No point in
doing that if you've only got one.  I'm sure the burst detectors that
have flown on other deep-space missions haven't weighed that much.
(Mind you, they're probably still too heavy -- the PFF people would put
more Pluto-specific instruments on first, if they had any mass to spare.)