I tested the corpse drop rates using savegame scumming - 10 events for noncursed vs blessed ammo of hunting against a corpse leaving target (intrinsic granting if I can find it).
Findings: 10 trials of killing a giant slug with uncursed hunting ammo using the same level 18 archer (who did not posses the food preservation skill) at the same starting turn had the following results
3 corpses
7 without a corpse
Same PC starting about 6 turns later (for blessing the ammo, lining up the shot) against the same slug with blessed ammo of hunting
6 corpses
4 without a corpse
I briefly toyed with calculating the number required to reach statistical significance, but decided it's probably not worth the time. The fact of the matter is that the number of trials I did cannot even begin to give something resembling a statistically significant result. Since I don't have the time to calculate the required sample size to achieve... oh, I'd be content with a 60 per-cent power to detect, I'm going to round to the nearest fifty per-cent.
So my conclusion, is that it is probably a 50 per-cent drop chance with a slight possibility that blessed hunting ammo gives more corpse drops