Why do you consider an automatic rifle the modern equivalent of a musket?
This is the wrong question.
At the time, muskets were already obsolete, ie, no longer militarily capable.
This became apparent during the revolutionary war, when rifles proved greatly superior to muskets (which generally are defined as smoothbore).
Addressing equivalency.....
The long rifle of 1780 was the most advanced military small arm of the day.
It is reasonable to state that the Constitution's framers envisioned this as protected by the 2nd Amendment.
Their intention is the balance of small arms weaponry power between citizens, government & foreign attackers.
Thus, one cannot reasonably claim that only muzzle loading rifles are covered.
The modern equivalent is the magazine fed automatic rifle.
Why wouldn't it be, say, the modern equivalent of a Revolutionary War-era small cannon or grenade? Why is the criteria the weapon's weight and not its offensive capability?
You could make an argument that even larger weapons would be covered.
I don't, because citizens typically owned state of the art rifles, but not cannons.
The American Founding Fathers were acquainted with weapons that could take out a group of people (and that were reasonably necessary to fight wars, even then); did the Second Amendment legalize those mass killing weapons at the time?
Capability of small arms has increased over the centuries.
I don't see this addressed by the founders.
But subsequent regulation has required a special license to own automatic weapons.
Such licensees happen to be few, & very responsible.
That aside, we can also deduce that its purpose was to facilitate the arming of actual or potential soldiers, with the understanding that these soldiers would be responsible for providing their own weapons for use in wartime. This implies to me that restrictions on weapons ownership that match military "fitness for duty" requirements (e.g. maximum and minimum age, mental stability, physical fitness) would be in line with the intent of the Amendment.
Standards for becoming a soldier have historically been far more permissive.
https://www.law.cornell.edu/uscode/text/10/311
(a)
The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in
section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
https://en.wikipedia.org/wiki/Militia_(United_States)#Confederation_period_.281783.E2.80.931787.29