As I said, that rule is not a panacea.
Maybe just a starting point.
The intent of the design is important. What is it optimized for? Can we satisfy that need for well-designed hunting rifles that are not as lethal as, say, an AR-15?

as that weapon uses undersized/underpowered rounds as compared to hunting rifle. The ar-15 DOES do it at a faster rate than a hunting rifle would though, leading to more casualties (both dead and wounded) than the typical hunting rifle.

That has been determined by the supreme court of the US.
I do hunt with a semiautomatic rifle. It is not an AR type rifle, it is a Browning 270. I also own a Remington semi-auto rifle in 270 and have a couple 10 round magazines for it.
Almost all of my shotguns are also semi-auto's and every one of them is used for hunting.
I have had several semi-automatic pistols. Not a one of them has ever injured anyone.

If you could optimize you inventory for each specific use you have in mind, could you also select weapons that would have less potential for the misuse we are seeing?
I looked up your Browning. Fine piece.
It looks longer than an AR. Heavier?. Maybe not such a 'tactical' weapon.
Designed for hunting game, not combat.
That's what I mean by a sort of baseline. What is it designed to do?

So next year there will be 300 million murders? Nonsense. Oh, and about the constitutional threshold.."In common use for lawful purposes"..Keep us up to date on the constitutional amendment..

If it is designed to kill people it should be unlawful.
A golf club can kill a person but at least we can say it wasn't designed to crush a skull.
What is the intent of the design?
Maybe we can start there. Just suggesting.

What activity would you be defending if you insisted on having a weapon designed to kill people?
Do you want to shoot targets? Get a target pistol.
Do your want to hunt game? Get a hunting rifle.
It's a question of what is the most appropriate tool for the job.

I just cannot find 20 senators that would ban sales of semi-auto weapons. Much less 51.

Would 6 shot revolvers be OK? They are really semi-auto.

What is the intention?
Is there a risk that that firepower could fall out of your control? In a way is not a gun collection similar to having dangerous materials of any kind? You can get them if you demonstrate need, but you gotta get a permit and give assurances.
Do you really need six? All at once? A gun collector might justify that.
Maybe a gun collector could submit to a little more documentation of the care and security of the inventory.

Maybe a military historian could have a Thompson sub-machine gun. Would it be worth it to him to submit to strict registration and accountability?

Clearly, many weapons are designed for military use: to engage in firefights and they are advertised to private buyers as such.
A manufacturer could change the advertising and say it was the best varmint gun ever.
You have to go the other way and start with the intended use.
What do you want to do?
- hunt. With a gun.
Then any number of fine hunting rifles are out there that are not military designs.

"Then any number of fine hunting rifles are out there that are not military designs."

Suspect your firearms history is a bit spotty.

To be brief, the most popular hunting rifles, like the Remington 700, Savage bolt actions, Ruger bolt actions, and more, are all undisguised derivatives of the Mauser 1871. (http://en.wikipedia.org/wiki/Mauser_Model_1871) In fact, the Mauser in 1874 became Germany's first repeating rifle when an 8 round (tubular) magazine was added. These only had a one-lug bolt, while the successor M1887 had TWO lug bolts that became the standard for every rifle to follow. The superior Mauser design for the extractor is duplicated almost exactly in modern hunting rifles.

I can cite MANY more examples of how EVERY RIFLE had its origins in military rifles. Go ahead, name one that you think does not have a military origin...

Hunting and warfare have been intertwined throughout history and it is difficult to separate the two.
I think you could argue that bolt-action mechanisms had military origins but rifles, in general, did not.
True rifling was developed in the mid-15th century but not explicitly for military purposes. Long rifles were used for hunting (and sniping) long before the the mid-19th century when rifled muskets became the standard infantry weapon.
The issue I was addressing was what is the intended use and how well does the weapon address that use?
I don't think that you can argue that the AR class was designed to hunt deer. It is made for fire fights. It is a military design.
A Winchester M-70 designed for hunting deer.