gonna argue this one...
Machine guns are probably a very good option, if not the most effective home defense weapon available.
I don't see you (or anyone) getting much public agreement with that statement.
because its simply not true. Not in our society today. With nearly 80 years of Federal restriction on legal machine gun ownership, and even longer brainwashing of the general public by the entertainment media, the overwhelming majority of people who support gun rights don't support private machinegun ownership. They are just too scared. Machineguns are
only used by BAD GUYS, and, of course, Federal Agents! That's what they have been taught, and what they believe. It is an emotional issue, and one where to the majority of people, nationwide, the facts simply don't matter.
And, of course, among those who oppose gun rights, machine guns are the worst of the worst.
But, lets assume for a moment that is not the case (fantasy, but just for the purpose of discussion), machine guns are not in common use. Very, very few people without military training have any idea how to use them properly. And a great number of those
with military training don't know how to use them properly, either!
Let's leave out belt fed machine guns, after all, how suitable are they for self defense, other that against a mob?
So it comes down to assault rifles and submachine guns. These, in unexpert hands are the personification of the phrase "spray and pray". And in our society, where the individual is responsible for each and every round fired in a self defense situation, how is it even remotely responsible to advocate full auto fire?
Imaging the homeowner, woken in the middle of the night by a break in. Instead of grabbing a revolver or even an auto pistol holding 15rnds, they grab a SMG instead? And if they fire, instead of firing one round they fire 6 or 8 or 20 with a single trigger pull? How many of those rounds are going to be likely to actually strike the attacker? 2? 3? 16? and where are the rest of those rounds going to go?
These are some of the questions that are going to be asked, and played to the hilt by everyone opposed to the idea of more full autos in civilian hands!
Just look at recent history, SEMI automatic firearms have been consistantly demonised in the media, just because they LOOK like full auto military weapons!
Even the most die hard 2nd Amendment supporter in our govt, who may totally agree with our right to own full auto firearms cannot and will not publically support it. Someone said its the "third rail", and in the sense that it is political death, they are right. Anyone honest enough to admit the truth has to admit that no matter what our rights actually are, the reality is different, and huge numbers of people with no knowledge or stake in the matter, but with the same vote each of us has will bury anyone who proposes "allowing more machineguns on our streets".
We risk losing the small legal avenues for ownership we currently still have. If we make this a public issue, at this time, or for the forseeable future, all we are going to do is have to work like hell to keep what we currently have, and the odds are, we wouldn't be able to do even that.
The basic idea that a full auto increases hit probability, and therefore ought to be a useful and effective defensive weapon simply will not fly in the US as it is today. One making this argument seriously would be lucky to escape with all the creditability they had before making the argument. Also add in the fact that by bringing up the subject, a loss (virtually certain) would also damage all our gun rights gains.
I cannot urge you strongly enough to abandon this particular argument, particularly in a public forum. It simply is not a good idea.
May I remind you that Congress is locked in budget-cutting negotiations and the WH is looking for ways to raise revenue without upsetting their core constituents? What have we achieved if we reopen the registry but have to pay almost $4,000 for a tax stamp? 
