Hats off to Many Tears rescue for a really comprehensive covering of this issue on their website. More than 4,000 people have now signed their petition and I'd urge you to have a read.
Here's the wording...
I believe the practice of the RSPCA shooting dogs with a captive bolt gun to be banned. We feel that this an inhumane way to kill an animal and should not be recognized as an acceptable means of euthanasia painless death designed to cause minimal pain and distress)for companion animals.
My own unscientific research made me question if captive bolt could be a reliable way to kill dogs humanely. A friendly vet retold an anecdote that made me very worried about it. While working in the Middle East he came across a relevant case. A large dog had growled at his owner and that owner decided to shoot his dog in the head with an AK47. Half the dog's brain appeared to have been blown away and the dog's body was left in the desert. The next morning the dog woke up and ran away. An ex-pat brought this poor dog to the vet's surgery and my vet friend patched him up and the dog lived on successfully for many years despite his enormous injury. The vet explained that dogs have such large frontal sinuses it is quite possible to miss the cranium completely and that without sedation it would be very hard to accurately shoot a large dog fatally in the head with a captive bolt.