I put out a request among some military history buffs for any insights about what sidearms the dangerous Doctor J. Watson might have carried during his military service -- I was especially looking for some definitive answer to the question of whether or not a noncombatant like an army surgeon would have even carried a weapon in the field at that time, or would it have been a matter of choice.
A rough consensus is that a Victorian medical officer on active service in dangerous country with foes like the Pathans, who would not have likely respected an infidel doctor's status, would probably carry a sidearm for personal defence even if not engaged in direct action. And as an officer, he would be expected to purchase and use whatever type he liked, as long as it was compatible with standard issue ammunition.
I was also directed to this useful link for more discussion, which also concerns Holmes' and DOYLE'S own sidearms:
and Wikipedia has an entry for the famous "Bulldog" revolver:
Kevin H.