Why an AR? Same ammunition as the police and military, cheap enough to ensure continued proficiency training.
Why a 12 ga Pump? 12 ga is the most popular gauge, pump will feed and fire anything including Federal and Aguila mini-shells. Semi-autos are great for the trap range and three gun competition, but can be too ammo finicky to trust when your life is on the line.
Why a Glock, and specifically a 26? Glock is the most common handgun in America, bar none. 9mm is the most popular caliber. And the 26 can use ANY Glock double-stack 9mm magazine ever made. Ammunition is cheap enough to ensure continued proficiency training, and Glock magazines are a dime a dozen and stone cold reliable.
If I could go back 140 years to Ogden, Utah, I would tell young John Browning to not worry about inventing automatic firearms. they are just too unreliable.
Armies will never use them, just too unreliable.