It would seem the value provided in the XML doesn't do anything, the base chance seems baked into the code. Fiddling with the 'perc_set' operation didn't seem to do anything, however, inserting a new line with the operation 'perc_add' seems to have worked. My limited testing has every arrow sticking into 5 or 6 zombies including using one as a pin cushion till it despawned, no arrows broke.
That should work flawlessly for nails and bolts as well, dunno if it will do anything with guns though, I doubt they have a base stick chance in their code.