We find out in A Study in Scarlet, by Arthur Conan Doyle (written 1886, pub 1887) that Watson got his medical degree in 1878 and at some point afterward he joined the British army (rank unknown), at some time and then attached to the 66th Berkshire Regiment of Foot (no doubt in the capacity as doctor). He was wounded during the 2nd Anglo-Afghan War at the Battle of Maiwand (27 July 1880). Doyle may have also lumped in the catastrophic retreat to Kandahar into the action, but probably not the subsequent Battle of Kandahar and relief itself. Dr. Watson survived the battle and his wound, and he recuperated for a few months (presumably in India or possibly South Africa) before returning from overseas to London and leaving the army. That would make it at least 1881 (with travel time added) or so that Watson meets Holmes in London, presuming they met fairly soon after his arrival.
Speaking of Watson's gun, I'd love to see someone tackle is Dr. Watson's private file box from the opening of Billy Wilder's "The Private Life of Sherlock Holmes". I'm afraid I don't have enough experience with vintage firearms to try and identify the gun he uses in the film.