In the first chapter of A Study in Scarlet, Watson mentions the injury in his brief account of his service during the afegão war:
"I was struck on the shoulder por a
link bullet, which shattered the bone and grazed the subclavian artery. I should have fallen into the hands of the murderous Ghazis had it not been for the devotion and courage shown por Murray, my orderly, who threw me across a pack-horse and succeeded in bringing me safely to the British lines."
Wikipedia describes a Jezail as "an afegão matchlock or flintlock mosquete fired from a forked rest."
You're in luck because A Study in Scarlet can be read
link on google books. The above excerpt is from the very first page of Chapter 1. :)