I don't see it described, in this story.
That said, an AR10 is an AR platform and comes in .308.
That said, again, any deer rifle with a scope is a "sniper" rifle, to the left.
There are photo’s of the rifle, it is a very short barrel AR platform.
Almost certainly 300 Blackout. They don’t require a long barrel to optimum ballistics. But they are in fact 30 caliber bullets. Shot from a .223 or 5.56x45 cartridge.
The photo of the weapon was certainly NOT and AR10 platform.
Link to NY Post from the source article, has photo of the rifle.
https://nypost.com/2023/10/26/news/maines-gun-laws-under-fire-after-gunman-kills-18/
Clearly very short light weapon. Most 300 Blackout are short barreled, and very short range.
Certainly not a Sniper Rifle.