Why draw a line?
As others have already pointed out, weapons have changed since the Founders wrote the Constitution. Partially because of that and the lack of a workable method for "modernizing" that document, we tend to think of those men as somewhat ignorant and backwards. Nothing could be further from the truth. While they couldn't forsee computers and nuclear weapons, they could see (probably better than we do now) the history of mankind and his stormy relationships. They knew that, regardless of the weapon, there would always be some who would misuse it and that his neighbors would have to do something to correct and/or contain him.
That idea worked pretty well for almost ninety years. Then we decided-- collectively-- that some of our neighbors were being too unruly and we needed to go to war to "correct" them. That those neighbors only wanted to be left alone was lost in the rhetoric of the day. The years of war and retribution that followed gave birth to the idea that we no longer needed to take care of ourselves since the government could do it so much better. We hired police forces rather than take our own valuable time to catch and prosecute outlaws. We hired planners to tell us where and when we could build on our own property. Then we hired tax assessors to extract money from ourselves to pay those planners, police officers and the myriads of other government agents we "needed" to make our lives easier.
Now, with Government the single, largest employer in this country, it is no longer our employee but our master. Government brings us into the world, teaches us in school, hires us to utilize the skills it has taught us, pensions us off when we are too old to work and provides a plot of ground to bury us in. Is there any surprise that Government would want to protect itself from those occasional people who somehow manage to wrench their wheels from ruts that countless others have traveled and strike out on a new and different path of their own?
People who think for thmselves are the greatest enemies of big Government. Gun owners tend to think for themselves--, some more than others. But there are still those who, even though they own guns and say many of the things freethinking gunowners say, occasionally revert to their public school taught thinking. They still believe that government is here to serve them from the time they are born until the day they die. They seem to believe that, in order for Government to properly take care of them, it must limit what they can do, lest they hurt themselves. So they don't mind when they have to get a permit to do this or have their fingerprints on file to do that. After all, they reason, Government has to keep us safe.
There will always be outlaws and Government should not and cannot control them all. Life is not safe! Indeed, nowadays Government is often the outlaw, but being so big, there is nothing the people can do to correct it, let alone contain it. But those who own guns could cause minor problems, so Government finds reasons to limit who can own them. Usually, the foundations for those reasons are laid in the public schools when we are most vulnerable to statist propaganda. It happened when we didn't even know it was being done to us. Now we happily line up to ask for permission to protect ourselves from the outlaws we pay Government to control. It shows up when we feel we aren't smart enough to decide what is a prudent instrument for our own defense.
Should we own automatic weapons? If we want to. Should we own field artillary pieces? If we can afford them and if we can justify them to our wives. Nuclear weapons? That question always comes up and it's kind of ridiculous. I doubt if even Bill Gates could afford one.
To ask where a line should be drawn is to shoot ourselves in the foot. The real question we should be asking is how to erase the lines that have already been drawn.
