I have a Masterject, which is a cross between a caulking gun and an air pistol and you can put down a bit of feed and inject from outside the pen. Other folks like the Slapshot, which works differently and is much cheaper. Otherwise it's a minimum of three hefty chaps pinning the pig behind a strong metal hurdle and someone who doesn't mind 150 decibels of screaming pig next to their ear while they administer the jab.