First, according to US Federal law, the militia is every able bodied male in the United States. See
10 U.S. Code § 246. Militia: composition and classes: "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..."
Read the Constitution Center's interpretation of what "Well regulated militia" means:
""Well-regulated in the 18th century tended to be something like well-organized, well-armed, well-disciplined," says Rakove. "It didn't mean 'regulation' in the sense that we use it now, in that it's not about the regulatory state. There's been nuance there. It means the militia was in an effective shape to fight." In other words, it didn't mean the state was controlling the militia in a certain way, but rather that the militia was prepared to do its duty."
https://constitutioncenter.org/images/uploads/news/CNN_Aug_11.pdf
Second, the most common mass shooting weapons are actually semi-automatic pistols, not semi-auto rifles. According to this statistic website, there were 92 mass shooting incidents with handguns, 43 mass shooting incidents with rifles, and 26 mass shooting incidents with shotguns from 1982 to 2019.
https://www.statista.com/statistics/476409/mass-shootings-in-the-us-by-weapon-types-used/ So you can't really claim that AR-15 should be banned because it is some unusually dangerous anomaly compared to the debate over other types of guns.
Third, the AR-15 is not the same gun the military uses. The AR-15 is a semi-automatic only weapon with a firing style not much different than modern pistols that also fire in semi-auto. The military uses "assault rifle" variants of the AR-15 that have to be capable of selective fire, which includes fully automatic fire (except variants such as the A-2 which has burst fire). The “ballistics” of the 5.56mm bullet is not something particularly amazing – it is a medium caliber bullet that actually has less kinetic energy than the larger bullets of other rifles, including hunting rifles. Now, if you want to talk about optics (not much different than the ones that hunters) use or extended magazines, that is an entirely different debate, as there is debate over pistol extended magazines too.
Fourth, as for weapons designed by the military to kill, civilians had top of the line rifles, explosives, and cannons, etc during the Revolutionary War and Civil War too. Whether something is used or originally designed for the military is not an indication that the second amendment does not apply. The US government actually uses surplus M1 Garand rifles to promote civilian marksmanship and allowed countries such as the Philippines and South Korea to sell hundreds of thousands of M1 Garands to US civilians. The M1 Garand is a semi-automatic battlefield rifle created for the military and used during WW2, Korea, and even Vietnam (to a lesser extent). It has a longer range than M16s/AR-15s, and fires a 7.62x51mm bullet that has TWICE the energy of a 5.56mm bullet from an M16/AR-15.
Fifth, it’s actually a Constitutional right to own military grade weaponry in relation to the militia according to the Supreme Court. In the Supreme Court case United States vs Miller, the court ruled that sawed off shotguns can not be used by civilians because it has no military purpose and has no use to a militia. In that case, the court is saying that the second amendment protects the rights of civilians to have the type of gun that would be important military and to a militia.
Sixth and finally, home defense isn't even the main purpose of the second amendment. The main purpose of arming the people was for the people to defend against threats from both abroad (foreign invasion) and from tyranny within (despotic government).
Read these quotes from Thomas Jefferson for example:
-"The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government."
-"The constitutions of most of our States assert that all power is inherent in the people; that... it is their right and duty to be at all times armed." --Thomas Jefferson to J. Cartwright, 1824.
-"The Greeks by their laws, and the Romans by the spirit of their people, took care to put into the hands of their rulers no such engine of oppression as a standing army. Their system was to make every man a soldier and oblige him to repair to the standard of his country whenever that was reared. This made them invincible; and the same remedy will make us so." --Thomas Jefferson to T. Cooper, 1814.
(It's important to note that the people conscripted in the Greek and pre-Marian Roman levy armies provided their own weapons and equipment)
-"For a people who are free and who mean to remain so, a well-organized and armed militia is their best security." --Thomas Jefferson: 8th Annual Message, 1808.
https://www.monticello.org/site/res...ople-retain-right-keep-and-bear-arms-spurious
http://eyler.freeservers.com/JeffPers/jefpco29.htm
Full disclosure: I am also an American, and I actually own zero firearms, and have no intention of owning any firearms any time soon. However, I have read up on the history of gun rights and the writings of our Founding Fathers. I recognize and respect the importance of the second amendment and its broad intention of preserving the right to firearms for multiple reasons. The American Founding Fathers encouraged the people to be armed so they can form into militias (eg. well organized able bodied Americans) to be able to resist both a foreign invasion as well as a despotic tyrant who has taken over the American government.