Exactly. The second amendment was meant to empower citizen militias for the purpose of avoiding a standing army which they feared would pose a threat to democratic government.
2nd Amendment said:
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.
Citizens are not militia.
The 2nd amendment discusses two related cases of the inherent right of self defense.
A State has the right of self defense. It does this by forming militias. These are regulated by the State, and commanded by the governor of that State. Any State has the right to form an active militia to repel an invasion or to otherwise protect itself if the federal government refuses to act in time.
The people ALSO have the right of self defense. They may keep and bear Arms (any type of weapon, including any type of gun).
Neither right shall be infringed. This amendment applies to the States as well as the federal government. Every State, county, parish, city, or town is subject to this restriction just as much as the federal government is.
Nothing in the 2nd amendment specifies any restriction of any weapon based on size, type, size of magazine (if any), how 'scary' it looks, etc.
The best way to deal with a murderous individual (such as mass shooter) is to shoot back. Put him out of action.