Handob sprites must be numbered in increments of 8, starting with 0. Anything else will fail.
For example, 370 is not divisable by 8, so it's not a valid handob number.
that is not entirely correct
the fileids should have a distance of 8
in the example
370: Resources/FourthState/PlasmaRifle/Alien/PlasmaRifleh.gif
371: Resources/FourthState/PlasmaRifle/Human/HumanPlasmaRifleh.gif
870: Resources/FourthState/HeavyPlasma/Alien/HeavyPlasmah.gif
871: Resources/FourthState/HeavyPlasma/Human/HumanHeavyPlasmah.gif
872: Resources/FourthState/PlasmaLMG/Human/HumanPlasmaLMGh.gif
970: Resources/FourthState/PlasmaPistol/Alien/PlasmaPistolh.gif
971: Resources/FourthState/PlasmaPistol/Human/HumanPlasmaPistolh.gif
there were x70 and x71 images = no distance of 8
if the distance is less the images overwrite each other
additionally:
you can use fileids > 1000 BUT the next mod thats loaded after yours will use ids 1000-1999, the next 2000-2999,...
never saw a case where 1000 ids were not enough so you should keep in that range
i would guess the id with the least "space" is bulletsprite each entry uses up 35 fileids