I can understand your irritation with the use of the word "pistol" as a semi-auto as opposed to a revolver, but, colloquially, it is convenient to use that word to make the distinction between the two. "Pistol" techinically is used to describe any handgun, but that would leave us with having to say "revolving pistol," "repeating pistol," "semi-automatic (or self-loading) pistol, "single shot pistol," "machine pistol," etc.
Languages are fluid and words take on different meanings through the ages.