Let?s keep in mind that at the time of the writing of the Bill of Rights the most modern military rifle (s) were:
The Brown Bess;
http://en.wikipedia.org/wiki/Brown_Bess and
The Charleville;
http://en.wikipedia.org/wiki/Charleville_musket .
They were world class because you could load and fire one of these rifles, three times within 60 seconds due to their smooth bore- No rifling. They also shot a .69 caliber or larger ball.
The rifled rifle was not considered a modern military weapon as they took around 120 seconds to load and fire. And these rifles shot a .45 - .58 caliber ball- Considered at the time as a small round.
Let?s now jump ahead to 1935. It was perfectly legal to own a ?machine gun.? Then in 1935 the NFA (National Firearms Act) was passed:
http://en.wikipedia.org/wiki/National_Firearms_Act . This act was not passed because of the terrible machine gun but due to the repeal of prohibition. The Treasury Department had Treasury Agents that no longer had a job; consequently, they pushed for this law so they would not have to relinquish power, e.g. lay off agents. Also, the $200- fee collected would (In theory) add revenue to the Governments coffers.
Now let?s jump to 1968 when the 1935 NFA was amended. Up until this amendment if you were willing to pay the money for the stamp and the money for the weapon you could own anything that was used in the US military.
Then of course came the last amendment to the 1935 NFA in 1985; which prohibits US citizens from owning machine guns and other devises produced post the passing of this amendment. That is why due to government intervention a pre-ban M16 that would have cost a couple hundred dollars in 1984 will cost upwards of a $10,000- today. Or a favorite of mine a BAR- Pre 1985 would have cost $700- now will cost around $30,000-.
By the way, your local LE department is exempt to this admended law. They can buy post 1985 machine guns and other devises. The MSRP for a Colt M16 is about $1,500-. A friend of mine who is a LEO told me that their department just bought fully auto M16's from Smith & Wesson and the prices was just under $1,000- each.
In summary; let?s remember that up until 1968 the "rule" was; if a weapon was being used in the US Military and a US Citizen was willing to pay the price of the excise tax ($200-) and the cost of the weapon they could own it.