Why yes, an AR-15 is a musket for all intents and purposes. They both are considered firearms that are designed solely to end the lives of living things, whether it be animals or humans. Because of this, an AR-15 is not protected under the second amendment to the United States constitution.
At the time the 2nd amendment was ratified, the musket was the standard infantry weapon of the era. Today that role goes to the m16 and ak74 for most modern countries. If anything, the ar15 is a neutered version of those firearms. The only valid argument against them being the modern equivalent is they are inferior to what police and military use today.
The text of the Second Amendment states that a citizen's right to bear arms shall not be infringed. At the time the Constitution was written, this meant things like rifles and muskets. The Founders wrote this into the Bill of Rights to ensure citizens would have some kind of defense against a tyrannical government. One can argue endlessly about how tyrannical or not the current government is, but the intent of the Founders and the plain reading of the text means that citizens have a right to own weapons which would include the AR-15.
Here is the wording for the 2nd amendment: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." It is very odd that the people who believe that they can carry any type of weapon ignore the "well-regulated" part of this. In fact, gun ownership is regulated in every state. It is illegal to own fully automatic weapons in every state without licensing. So the furor is about the latest type of regulation but is using absolutist language.
While I'm sure the writer of this question thinks they are clever for their play on words, an AR-15 is not a musket. What the larger argument to be made is whether an AR-15 is considered protected by the Second Amendment of the United States Constitution. It is considered an arm and as such there is a right to bear it.
The AR-15 is owned by a niche of gun owners unlike the musket. The issue is further confused by the fact that the variety of rifles available to the consumer is much broader than in the era of the musket. It is certainly representative of a modern rifle-type firearm but is is far from exclusive.