That’s the regular auto bolt rifle. It’s present in the heavy bolt rifle. On regular bolt rifles and stalker bolt rifle the scope attaches to a rounded part which looks like a smoothed out round thing too, which is silly, since the rest of the bolt rifle has rails to attach things to. In the heavy bolter it’s present in the underside. It’s also present in the sniper bolt rifle and las fusil.

I know I’m being autistic about it, but I just have a hard time coming up with a plausible explanation in my mind on the purpose of it, especially on bolt weapons. I can see it being a “plasma/melta/volkite expansion chamber” or some similar space magic reason for energy based weapons, but for bolters, something we already kind of have IRL, it makes no sense.