Why is the AR-15 Considered the 'Weapon of Choice' for Mass Shooters?

  All mass shooters have used either semi-auto handguns or rifles or both. Magazine capacities varied. AR-15s are not the only semi-auto rifles manufactured or used; but it shows up more because they are, according to statistic, the "weapon of choice" to firearm owners. Anti-gun folks say it is because it is a semi-auto version of the US military weapon that is used extensively around the world. One of the reasons why the AR-15 is so popular is that it is easy to disassemble & assemble for cleaning and little maintenance is required. Another feature is that the AR-15 can be altered with custom stocks (butt & fore-end), high-quality bolts, and standard models of 5.56mm (.203 caliber) are relatively inexpensive - but not qualified for hunting big game. In recent years that has changed because now manufacturers offer the AR-15 in a wide variety of calibers, 6.5 Creedmoor for big game hunters and long-range competition shooters and even the Russian & Chinese AK caliber of 7.62x39mm which makes it competition against the AK rifle. 

  Once the AK was the cheap way for shooters that wanted a caliber heavier than the 5.56mm. It became the weapon of choice for terrorists and rebel forces in several conflicts. It also is a semi-auto version of the famous military rifle used by US enemies in Vietnam and elsewhere today in fighting against Jihadists. It is more readily available than the M16, thus makes it just as popular despite weighing more and more complicated in disassembly.

M1 Garand

  There are other popular firearms that are semi-auto replicas of WW2 firearms that include the famous Thompson carbine in .45 caliber. 

Thompson 

  WW2 paratroopers were issued a carbine version of the M1 rifle, M1A1 with a folding stock.

M1A1

  The rifle used before the M16 was the M14.

M14

  The M14 after being replaced by the M16 was reconfigured to become a sniper rifle because it was chambered for 7.62mm ammunition and there was a surplus stored in warehouses. It was then designated the M21 & M24. The M14 is hard to find, sought after by gun collectors. Reason is that when the M16 replaced it as a service rifle, it was converted to semi-auto and configured to be used as a sniper rifle or sold to foreign nations. So unlike other military weapons, the M14 is a rarity because it was never fully put into surplus. Of course, if the M14 has an auto capability, a special license is required by federal government to obtain and own one, the same paperwork one must go through to purchase and own a suppressor ("silencer" as designated by US government). The semi-auto version of the M14 is an awesome rifle. It, designated as M21 (later M24), was my primary weapon when assigned as Infantry company sniper.

M24

  The M21 was the sniper rifle used in Vietnam War and in 1988 replaced by the M24. Both being alterations of the M14 semi-auto rifle.

  The AR-15 is known by anti-gun advocates as an "assault rifle", which is absurd and demonstrates their ignorance about firearms. Socialists like to manipulate wording to favor their agenda and provide the element of emotion over reason, which is their tool to control the populace. "Assault rifle sounds ominous. They ignorantly claim that is what the acronym "AR" in AR-15 means.

  Nonetheless it is true that primarily a semi-auto rifle or handgun was used in mass shootings. 

 Semi-automatic rifles such as the AR-15 have been used in a number of mass-shootings including the Waffle House shooting in Nashville, Tennessee, the Marjory Stoneman Douglas High School shooting in Parkland, Florida, and the El Paso, Texas Walmart mass shooting. 

  Statistics between 1982 and 2019 does show the AR-15 has been used more than other weapons, but that is because of their popularity and relative inexpensive price for standard models without customization. Does that mean that mass shooting will end with the ban of the AR-15. Absolutely not because the problem is not the firearm or type of weapon, it is the people among us, the nut jobs who have decided to take their frustrations out on innocent people randomly. 
  I have been able to glean information about weapons used in mass murders between 2000 and 2018, but not have been able to find information as to what model or make of rifles were used. Here is the chart from TGC: 

Handgun: 28Rifle: 11Rifle and handgun: 9Shotgun and handgun: 6Rifle, shotgun, and handgun: 5Rifle and shotgun: 3Shotgun: 2

  The media has not been helpful in obtaining information as to how many times the AR-15 has been used that has become unreliable in obtaining information more regularly lately. Regardless, their rhetoric against the AR-15 is not justifiable, and like Senator Feinstein are basing their gun ban on the appearance of a firearm as well as magazine capacity. No mention of the Thompson carbine that has magazines with capacities from 10 rounds to 100 rounds.