No, I don't know what you mean by retractable either? I know you can get spring loaded needles which retract after a dose is given, but I've never seen ones which extend AND retract?
[member=210785]wildandwooly[/member] , this might be controversial, but let me throw it out there:
I really struggled with injections initially. For example, you're always told to inject into the neck because it's a nice cheap piece of meat in case you cause an abscess. However, it's also a really wriggly jumpy bit, right? So, what I do now for breeding animals is to do intra-muscular injections into what would be their bum muscle if they had bums (!?). What I mean by that is that if you feel a triangle of hip bone, top of tail and top of leg bone, then go vertically downwards right in the middle of that triangle, and you're into muscle (You might need somebody to show you the first time).
My logic with that one is that any loss due to leg meat being condemned is offset by increased convenience and safety for me when injecting. I can keep a sheep immobile against a fence or gate with my thigh and other knee, and then reach over to give the dose. Immobilising a neck is a much harder proposition.
Vaccinating lambs is similar. For wether lambs that are never going to be clipped*, I hold them still by putting their neck between my calves (facing the opposite direction to me) and then inject over the ribs. For ewe lambs, I again hold them still between my legs (facing the same direction as me) and inject them into the side of the neck.
* heptavac usually leaves a raised scar which would be nicked by the shearer.
I'm happy to be corrected on all this by others, but I find it works for me. Also critically, it means I can do these things on my own, so there's no risk of sticking a needle into my helper!