Your saying what the forefathers believed is, indeed, speculation into how they intended the language to be interpreted. Using definitions of that time as comparison is useful--I certainly agree--but it still involves speculation into the minds of men that are no longer here to defend the meaning behind said language.
Speculation?
Americans have the right and advantage of being armed – unlike the citizens of other countries whose governments are afraid to trust the people with arms. – James Madison
The Constitution shall never be construed … to prevent the people of the United States who are peaceable citizens from keeping their own arms. – Samuel Adams
The best we can hope for concerning the people at large is that they be properly armed. – Alexander Hamilton
A strong body makes a strong mind. As to the species of exercise I advise the gun. While this gives moderate exercise to the body it gives boldness, enterprise and independence to the mind. Let your gun, therefore, be the constant companion of your walks. – Thomas Jefferson
To preserve liberty, it is essential that the whole body of the people always possess arms, and be taught alike, especially when young, how to use them. – Richard Henry Lee 1788
To disarm the people is the best and most effectual way to enslave them. – George Mason
...arms...discourage and keep the invader and plunderer in awe, and preserve order in the world as well as property. ...Horrid mischief would ensue were (the law-abiding) deprived the use of them. -- Thomas Paine
We should not forget that the spark which ignited the American Revolution was caused by the British attempt to confiscate the firearms of the colonists. - Patrick Henry
I'm not speculating the founders intent one bit. Although they are no longer here, their words are. They make it crystal clear