The mini-Hail Fire System weighs in at considerably more than today's 450 pound limit...but remember, in the story, we are dealing with the Predator IV in 2010.
Then it really wouldn't be "mini" (unless the full-sized Hail Storms are quite a bit heavier than 1000 pounds :-)
Still, nothing like some depleted uranium to ruin the GIR's entire month.