Do it whilst they're eating. What do you mean by vaccinator gun? We use a slapshot injector, before that we constrained the pigs (which are running around freely in afield

) and used normal syringes and needles which was a nightmare. We now vaccinate for erysipelas every year, getting quite the hang of it with this slapshot thingie - makes it much less fun for others to watch, though, the first few times it was hilarious if you were a spectator