There is an interesting bit from the Federalist Papers that touches on this point, and which definitely implies that the Founders intended for the Second Amendment to protect the ownership of military-grade weapons:
In Federalist No. 46, Madison wrote how a federal army could be kept in check by state militias, "a standing army ... would be opposed [by] a militia." He argued that state militias "would be able to repel the danger" of a federal army, "It may well be doubted, whether a militia thus circumstanced could ever be conquered by such a proportion of regular troops." He contrasted the federal government of the United States to the European kingdoms, which he described as "afraid to trust the people with arms", and assured that "the existence of subordinate governments ... forms a barrier against the enterprises of ambition".
In other words, the worry, amongst many of the people at the time of the founding, was that the new federal government would immediately move to create a standing federal army, which would then exercise military control over the states. The Founder's response was the Second Amendment, and the argument that so long as the states could each individually muster a militia, the federal government would never be able to effectively overwhelm any state with a standing federal army.
Now, the only way that a state militia could have stood its ground against a federal army would be if the state militia owned military-grade weapons, and since the Second Amendment was ratified in order to ensure that any given state militia would always have the ability to obtain military-grade weapons, it necessarily stands to reason that the Second Amendment protects an individual's right to own military-grade weapons.
So no, the Second Amendment is not just limited to 18th Century muskets, or the original Kentucky sharpshooter rifle, or anything like that, because those tools would not allow a modern-day state militia to stand up to a modern-day federal army.