Did the writer of Sword Art Online do a research on guns when writing Gun Gale Online? In a number of Sword Art Offline 2 episodes in Yui's trivia corner, she explains some of the guns seen during the series talking about their real-life counterparts.

Did the writer of Sword Art Online do a research on guns when writing Gun Gale Online? Did this trivia come after the light novels and used only to give something for Yui to talk about in Sword Art Offline?
The details of the guns in the game were actually quite basic. Here are three spots in novel #5 that mention specs on real guns used in the game:  


Before the title page- Here, it says facts about Sinon's Hecate II, things like the length of the gun and the size ammo it uses. Also the fact that, in real life, the gun has been banned from being used on human targets.
Page 137- Kirito asks why the assault rifle fires smaller rounds than a sub-machine gun and Sinon explains that it's for higher speed and accuracy, a trait that has been used since the M16.
Page 140- Sinon recommends the Five-Seven to Kirito for a sidearm to use with his lightsword. She explains that it uses small bullets shaped like rifle rounds. This gives it an advantage of accuracy and penetration.


The bit in Sword Art Offline wasn't included in the light novels, but the trivia on each of the guns is indeed there.