Get the one like in the picture where the bottle mounts directly on the gun and not the one with the long tube and then the bottle as you need all hands free really.
Just make sure you hold the thing upright as you release the trigger, otherwise it may not suck up a full dose. I have to say I prefer the long tube and bottle round my neck for this reason - although you do have to be careful with tubes around horned sheep!
TT, I just wonder whether you are thinking that the autovaccinator guns somehow automatically deliver the shot subcutaneously. They don't, you still have to place the needle correctly under the sheep's skin and hold it steady while you deliver the dose. The automatic part is the sucking up of the next dose so the needle is ready for the next sheep.