OK, we can eliminate nitrate fumes from "recoil headaches" easily...next time you anticipate shooting the firearm that induces the headache...drink a generous amount of vitamin C in the form of orange juice, grapefruit juice etc...before, during and after the shooting session. Miners that suffer from dynamite headaches (and ammonium nitrate too) usually have 3 or more of those little juice cans in their gut bucket.
No headache? It was the nitro fumes. Headache? it was the recoil.
