AR-15's Don't Shoot 7.62mm.

https://www.tmz.com/2023/10/26/maine-mass-shooting-lewiston-dead-fleeing-safety-bowling-alley-suspect-loose/

So it's probably something based on an AR-10.

Despite being a mistake everyone made amid all the melted barrels of Mandalay Bay/Vegas Strip shooting. It seems to be a reference to the NATO M14/M16/FN FAL fued.

Basically. The US adopted the M14 making 7.62mm the standard NATO round. Forcing everyone to buy FN FAL's.

Almost immediately the US them adopted the M16, which shoots 5.56mm. Eventually everyone in NATO also switched to 5.56mm, including in the pacifist's assault rifle of choice - The SA80.

In Afghanistan NATO discovered 5.56mm is a seriously underpowered round. Forcing them to buy loads of US made L129's - shooting 7.62mm.

It's a subtle difference but one which becomes gradually more important. Depending on how far away from the business end you're standing.

https://twitterwithbraincells.blogspot.com/2023/07/with-both-sides-now-shooting-556mm-its.html

https://m.youtube.com/watch?v=a-XjUvFk_mQ&pp (A Walk In The Light Green, Vagabond Crew, Lines, Audio, Topic).