Let's say a law is passed -- oh, let's call it the United Nations Arms Trade Treaty, just for giggles -- preventing civilians (meaning: non-military personnel) from owning military-style weapons. I would expect most people to think that law only affects "army stuff": assault rifles, machine guns, etc.
But here's the problem I have with that law, aside from its dubious constitutionality: poor definitions that lend themselves to slippery-slope enforcement.
You see, officers and members of the military police are regularly issued carry pistols. The regulation pistol of the US Armed Forces is the Beretta M9, a 9mm semi-auto magazine that uses a 15 round detachable magazine. It is also immensely popular in its civilian version, the Beretta 92 (probably because veterans, having returned their issued sidearm and wanting to own one for private use, decide to purchase a pistol identical to what they have trained with).
So when legislation says that "military style weapons" are forbidden for civilian ownership, it suddenly becomes a non-trivial point of legislation if this pistol is restricted because it is, in fact, issued and used by the US Armed Forces.
Is this pistol restricted because it's military issue? If not, you need a better definition than "military-style weapon".* Perhaps something like above a certain caliber, or fully automatic as opposed to semi-automatic, or any number of other things -- at which point I refer you to the 1934 National Firearms Act. **
If it is restricted, why? Is it just because it's used by the military? If that is your argument then please say yes, that I may laugh at you as I point out the large variety of pistol types used by militaries around the world. You might as well just outlaw every pistol above 9mm and .40 caliber.
Or are you going to try to stretch your definition to mean "any magazine-fed semi-auto weapon", i.e. the evil features argument? Because if you go down that road you're basically saying that civilians should only ever own revolvers, pump shotguns, and bolt-action rifles -- in which case just come right out and say THAT. I would at least appreciate your honesty in the matter. You'd still be wrong, of course, because my five-round, bolt-action Mosin-Nagant rifle was -- guess what? -- the primary infantry rifle of Russia/USSR from 1891 until 1936 or so, and still sees action to this day:
Even after the collapse of the Soviet Union, Mosin–Nagants are still commonly found on modern battlefields around the world. They are being used by insurgent forces in the Iraq War and the current war in Afghanistan. Separatists have also used the rifles alongside more modern Russian firearms in the Second war in Chechnya.
How about shotguns? They're okay, right? You'd be wrong, because the pump-action 12-gauge Mossberg 590A1 is used by the US Army.
In conclusion: The term "military-style weapon" is an arbitrary definition that means absolutely nothing. The bolt-action hunting rifles of today were (in design, if not in actuality) the infantry rifles of a century ago; pistol calibers as small as .32 have been military issue; and even manual shotguns are military-issue.
So when people say that they want to "keep military-style weapons out of the hands of civilians," please note how that definition can be stretched to encompass basically everything that doesn't fall into the very narrow category between .17 HMR and .25 ACP for pistols and rifles, and possibly single-shot shotguns in the .410 to 20-gauge range.
In other words, they want us effectively disarmed; they just don't have the courage to say so, and instead of doing it by force (which will backfire) they will attempt to do it through creeping legislation that outlaws firearms by slices as they constantly define what is "military".
* This phrase should be read in such tone of voice as to imply an unspoken "you idiot" after the main clause.
** See above.