It's taking me time to unpack the unspoken assumptions and category errors of that.

I rewrote this about six times.
And still you got it wrong.
Semi-automatics are a subset of pistols.
Not quite: semi-automatic
pistols are a subset of pistols, whereas semi-automatic
revolvers are, unsurprisingly, a subset of revolvers.
Pistols are a subset of guns.
Guns are a subset of firearms.
Almost right... pistols are a subset of gun,
as are firearms. Pistols and firearms have significant overlap (most pistols in circulation are firearms) but you can also have air-pistols and other types, which are not firearms. Firearms need to use gunpowder, or similar explosive powder.
And, I think the key to your confusion here is that
semi-automatic handguns are classed as pistols.
Except those that aren't, of course.
Not all - although maybe they do where you're from? In the UK the typical police officer doesn't carry anything!
If talking the US, some will carry revolvers. I'm not sure how they get to choose, or whether they are even allowed to in some places, but for sure you will have some police carrying pistols, some carrying revolvers, and some might well carry both (small revolver as a back-up, for example).
My original point was not about pistol whipping, but rather general conversation. Example: being suspended _ captain will say "Give me your gun and shield."
The captain is simply ensuring he is covering all possibilities. "Gun" is the broadest term for the typical projectile weapon. If he had said "give me your pistol" then would the officer need to hand over his revolver, if that was his weapon?
To get boring for a moment...

... as I understand the words:
- A weapon is an implement designed to cause damage / harm to another person - from guns to clubs to swords to bolas etc.
- Guns are a subset of weapon, specifically ones that expel projectiles through a tube. This would include large-calibre weapons such as cannons, and artillery, as well as things like rail-guns etc.
- A firearm is a subset of gun, specifically one that uses gunpowder (or other explosive powder these days) as the means of expulsion, but guns can also use compressed air, springs, etc. So all firearms are guns, but not all guns are firearms
- Handguns would be a gun designed for use in one hand, rather than two, such as a pistol or revolver (the difference between them being how the round is loaded). The cause of the propulsive force of the handgun would determine whether it was technically a firearm or not, as it could be a small air-pistol. In common parlance, though, I'd think handguns are generally assumed to be firearms unless otherwise informed.
- A side-arm is any weapon that is held close to the body in a holster, sheath etc, designed for quick access... so would include knives, batons if so stored, etc, and not just guns.
- A rifle is any gun that is designed to be fired from the shoulder that uses a rifled bore - i.e. the barrel is designed to spin the bullet, giving it greater stability as it travels toward its target, and thus greater accuracy. But again, it need not be a firearm (e.g. air rifle).
Don't ask me to draw a Venn diagram, though, as you may need a 3-D one by the time you get to side-arm.
