I've been looking for a perfect autoinjector for ages and finally found one. There are many pens for MS and diabetic patients but because there isn't B12 cartridge (only ampoules) none of the pens are good to us. Anyway, here is one that pricks the skin with a touch of a button and you only need to push it in which I can. This video shows a dog owner using it for her dog but it's ok for human too. It uses 1ml insulin syringe and it only does subcutaneous but people who self-inject says subcutaneous works just as well.
In the UK, it can be purchased from diabetes-express.co.uk/cont...
FAQ on the manufacturer's site
I don't know if this has already been produced yet but really gagety thing.