You may notice a theme to the 2012 mass shootings: they tend to involve the use of semi-automatic rifles and handguns.  To the extent that I have not listed the specific weapon used, it does not mean a different type of weapon was used: it means that the police have not released that detail or I have not yet found mention of the type of weapon used.  

Feb. 21: Norcross, Ga.
4 killed, plus shooter
A man who had been asked to leave his family’s business returned and killed his sisters and their husbands before shooting himself. The man had a history of violence, but had purchased the gun legally. Why could someone with a history of violence purchase a gun legally?

Feb. 27: Chardon, Ohio
3 killed, 2 injured
A sophomore at another area school walked into the Chardon High School cafeteria and fired 10 shots at four students sitting at a table. He used a Ruger MK III Target .22 caliber semi-automatic handgun, reportedly stolen from his uncle. 

March 8: Pittsburgh
1 killed, plus shooter, 7 injured
A former Duquesne University teaching assistant barred from campus began shooting using two semi-automatic handguns in a psychiatric hospital. He was shot dead by police. Police said at least one gun was stolen.

April 2: Oakland
7 killed, 3 injured
A former nursing student who had dropped out returned to the school, pulled an administrator into a nursing classroom, lined up students against the wall and began shooting them.  He purchased the  .45 caliber semi-automatic handgun legally.

April 6: Tulsa
3 killed, 2 injured
Two men  targeted random black men in four locations as they drove around town in an Easter weekend shooting spree.  The weapon has not been identified.

May 30: Seattle
5 killed, plus shooter, 1 injured
The shooter who had a history of mental and behavioral problems was asked by a barista to leave a coffee shop.  He stood up and opened fire using two .45 caliber semi-automatic handguns.  The guns were purchased legally and Stawicki had a concealed weapons permit.  Why would someone with a history of mental and behavioral problems be able to obtain a concealed weapons permit?

July 20: Aurora, Colo.
12 killed, 58 injured
A former grad student  reportedly entered a mall theater during a midnight showing of “The Dark Knight Rises,” set off gas canisters and opened fire with an M&P15 (AR15 type) semi-automatic rifle with a 100 bullet clip, a Glock .40 semi-automatic handgun with a large clip, and a pump-action .12 shotgun.  He also bought 6000 rounds of ammunition on the internet.

Aug. 5: Oak Creek, Wisc.
6 killed, plus shooter, 3 injured
A white supremacist walked into a Sikh temple and opened fire just before Sunday services. The gun was purchased legally.

Aug. 13: College Station, Texas
2 killed, plus shooter, 4 injured
A man opened fire on police who approached his house to serve an eviction notice about two blocks from the Texas A&M campus. Police shot the gunman and he died in custody. Police have not publicly stated how Engeldinger acquired the guns.

Sept. 27: Minneapolis
6 killed, plus shooter, 3 injured
A man  being fired from his job at a sign company pulled out a gun and shot his two managers, the owner, other employees and a UPS driver as he walked around the building before shooting himself. The gun was purchased legally.

Oct. 21: Brookfield, Wisc.
3 killed, plus shooter, 4 injured
A former Marine whose wife had obtained a restraining order against him three days earlier entered her workplace and shot her and six other women before killing himself. The restraining order made it illegal for Haughton to buy a gun, but he bought it from a private seller; no background check was required and no waiting period was enforced.

Dec. 11: Happy Valley, Ore.
2 killed, plus shooter, 1 injured
A former gyro shop employee  opened fire randomly inside Clackamas Town Center shopping mall with a AR-15 semi-automatic rifle. His gun, reportedly stolen from an acquaintance, jammed during the spree but he killed himself with a final shot.

Dec. 14: Newtown, Conn.                                                                                                                                                                         27 killed, plus shooter, unknown number injured

A young man killed his mother and then entered Sandy Hook Elementary School, shooting and killing 26 people, including 20 children, using an AR-15 semi-automatic rifle and two semi-automatic handguns, and then killed himself. The shooter had stolen the guns from his mother, but his mother had legally purchased those guns.

Dec. 21: Frankstown Township, Pa.

3 killed, plus shooter, 3 injured
A gunman shot a woman through a church window, a man in a nearby home and the driver of a car he had hit while fleeing. He then hit a police car, injured three troopers, and was killed in a shootout.

Dec. 24: Webster, NY                                                                                                                                                                                                                                                    3 killed, plus shooter, 2 injured                                                                                                                                                                                                                                  Sniper shot firefighters and his sister, setting blaze to house to ambush firefighters, using a semi-automatic AR15 Bushmaster rifle, another rifle, and handgun.  The shooter was an ex-con who could not legally buy firearms; uncertain how he obtained them.