Without using length, width, height, weight, barrel length, or caliber.  You may use cartridge OAL if necessary, but be careful - .30" Carbine and 5.7x28mm are on both sides of the fence.

Including these:

























Excluding these:























I might add a few later, just to make things more complex.

I was gonna say something about the rear of the bolt, bolt carrier, or slide being exposed during the action of the weapon, but that fails on the Zip.22" LR, & the Rossi Ranch Hand.

A bolt or slide being exposed other than around the ejection port fails on the Zip .22" LR, the Rossi Ranch Hand, and the .30" Carbine Enforcer.

What differentiates these two types of legal handguns?

Yet we almost always shoot all firearms with both hands these days, and all of the guns on the exclusion list above can be fired w/ one hand.  Many rifles can be as well.

A mechanical handheld paper puncher of various diameters.

Then add the words practically speaking for most people, able to shoot multiple times.  I sure as heck wouldn't want to see what happens trying to fire a sawed off, pistol gripped 12 gauge shotgun with one hand.  

What's the point of this exercise?

I'm not able to come up w/ a definition that separates the two categories above.  Almost any feature, caliber, or other characteristic falls into both categories.

Are you wondering what is considered "legally" a handgun and why?   Or just trying to poke holes in what is considered a handgun legally?  Or something else?    For some reason you have my curiosity piqued as to what you are trying to accomplish here and I'm a little slow sometimes.  LOL.

And basically the response to your statement above:  Isn't that why they're all legally considered a handgun?  None of them are designed to be fired from the shoulder.  Therefore they are held in the hand/ hands.  Yes you can do that with a rifle, but the rifles has a way to mount to your shoulder.    Hence why all the ATF confusion on how to label arm braces for pistols.  

So maybe a better definition would be firearm designed to be shot with the hand / hands without a place to rest on your shoulder.   But that definition would get everyone upset about the braces.  

No, everything above except the Mossberg Shockwave is legally a pistol under Federal law, even the drum fed Ladies Home Companion in .45"-70.  But there seems to be something fundamentally different to me in a Glock vice an AR pistol.  And if you say, sure, the magazine's in front of the grip on an AR pistol, I point to the Mauser C96.

Not legally, but practically, how can we distinguish between these two fundamentally different types of handguns?

What does that mean to you?  They're all guns, you point them at your intended target and shoot.  Why do we practically need to distinguish, other than for legal reasons?

There is no "fundamental difference".
While an AR "pistol" may not look like a handgun from 1950, a handgun from 1950 doesn't look like a handgun from 1750 either.

Whether you use one hand or two is immaterial. ATF defines: "Handgun. (a) Any firearm which has a short stock and is designed to be held and fired by the use of a single hand.." Nothing in that definition changes if you use both hands to hold the handgun.