[mountn man]

and which has any barrel with a bore of more than one-half inch in diameter."

An AR-15 shoots a .223.
Since it is less than 1/2", that means it's not a firearm

[FoxInSocks]

Well, another part of that section, 3(A), reads "The term 'firearm' means (A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive." No barrel-size restriction on that part.