The thing I find puzzling is how you can have the nail in the correct position (how do you hold it or do you strap it on something?) and at just the right time that the horse runs into IT , and not some other part of your body.
The horse does't always bite at the same time and location.
Make a fist, and insert horseshoe nail in between first and second knuckles, or alternately, you can stick thumb between those fingers so that your nail sticks out. Horseshoe nail is inside your fist, and only tip of nail is out, and I mean tip...just enough to feel it when you touch hand to your face.
This will leave no mark, it is a correction, not a punishment, and the horse is the one doing the correction, not you. If it has its head/mouth/teeth where it is supposed to have them? Won't get "bit" by your correction, entirely up to the horse.
Again, NO FORCE from you at all, and no indication that you have even noticed anything going on.
You don't hold yourself in one position to do this as correction. You are going about your business of grooming, tacking or whatever, and while doing so you have the nail tip ONLY poking between your knuckles, 1/8th of inch is enough, just barely can feel it, or for that matter can use stiff brush too, and as you are working, you watch for horse to swing head to nip/bite and you casually move your hand to where mouth meets nail tip/brush. And you don't look at horse when do it. Nor talk. You continue with what you are doing, and you don't swing arm or hand into horse's mouth. HORSE moves head to bite, and finds mouth gets pricked lightly, and decides it shouldn't bite.
Same with point of elbow to jaw. NO force is used, you simply have arm bent, doing what you need to do, horse swings head around, and you have gently raised arm so point of elbow is what horse runs into. Same as trying to walk through crowded coffee shop, you are creating invisible force field if you will.
No different than females holding car key points between fingers when walking to car in parking lot.
And while the horse doesn't bite at same time and location? The horse always has its mouth at the end of its head opposite of the ears. You never find the teeth when you move the tail, nor will you find the teeth when you are reaching for the girth. Those teeth find you, and that means you can find the mouth.
You can also do the things that are going on when horse tries to bite, and do the nail/brush thing then. You know horse does it when being girthed? Be ready.
This is not rocket science people. This is basic horse handling 101.
And to OP, you are already setting yourself up for some major problems when you worry about "hurting your horsey" more than you are worried about that horsey hurting you.
Biting and nipping are major no no's and it will lead to more problems, and eventually you will get hurt badly, if you don't get over the attitude that you will never use any type of correction.