[FoxInSocks]

after possibly mistaking the teenager’s paintball gun for a firearm

Sounds like the homeowner didn't "mistake" the gun for a firearm -- it was a firearm. The definition of a firearm in the U.S. Code includes "any type of weapon . . . by whatever name known which will . . . expel a projectile by the action of an explosive or other propellant, and which has any barrel with a bore of more than one-half inch in diameter."

18 U.S.C. § 921(a)(4)(B)

[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

[Ozark Tom]

18 U.S.C. § 921(a)(4)(B)

“destructive device”