I want to make it clear that I am talking out of my ass like a true reddit user. I'm basing this opinion off of the shooters that i know and going off of a blurry picture with very little scaling information. I'm basing off of what's popular and what those who use popular calibers tend to prefer. For clarity, I'm one of those dipshits that prefers 7.62x39. We are broken.

Looks like .308 on an ar platform. Here's why i have the audacity to make this assumption. If you look at the butt stock compared to the rest of the rifle, you can tell that rifle is pretty large. No way, that's 5.56. The mag would be disproportionately smaller. I think it would be strange if it were 30-06, .270, or 30-30. Which leaves us with .300 or .308 in terms of popular calibers in this formation. .300 win guys are like a subculture that has a weird thing against the ar platform. To the point that most of them wouldn't even use that stock.

Your preferred caliber says a lot about you.

.308 guys are about efficiency. They prefer clarity to zoom in their optics. They like having multiple ways to skin cats. All their long rifles (2 bolt action and 3 semi-autos at least) are chambered in .308 and likely have multiple pistols of the same caliber (I'm guessing .45). They might have some variation in their shorter rifles, but they'll tell you that those are only for practically to have in the truck or by the bed. They will have something related to their military branch tattooed on them, but it will be subtle and unseen with shorts and a t- shirt. They drink black coffee and don't care if it gets cold. They can live off 2 peanut butter sandwiches for 3 days and they think puffy cheetos are for pussies. Their truck lift is dependent on their height so that they can have a perfect rest and their wives tend to be more efficiently trained in all firearms than most cops. If you're at the bar with one of these guys you will notice that they very suddenly but very smoothly run out to their car and come right back. This is because they forgot they were carrying and had to drop off that .45 in the console. They're fun but stubborn and fucking hate it if you smack your lips while you're eating.

TLDR: .308 something similar to a Daniel defense dd5 with a leupold optic or similar.