I used vet gun this year. My black cows and tigers have the biggest problem with horn flies. I put out Purina Wind n Rain mineral with fly treatment. And used fly gun on cattle with heavier infestation. Vet gun works as advertised. Stand behind your truck when feeding so your targets don't see what you are up to. Hit them once on shoulders and once on the brisket.
For the worst cases, I use Cydectin with a big syringe (no needle). One squirt high, one low. Instant relief to cattle.