According to the Wiki, the Diaub is a Jakobs sniper rifle. So it should always have the side magazine block per the Parts poster linked by Vaulthunter101. I have seen snipers with cylinders, though, but I can’t think of the brand off the top of my head.
Vladoff snipers have cylinders.
Those are a little misleading, because you don’t know the rarity of the guns used for the images (see @khimerakiller’s post above). I’ve been looking at all kinds of weapons recently, and there are indeed plenty of the more common rarity snipers that have different magazines to the ones shown on the sniper posters.
Another thing I’ve noticed is that, on some of those posters, certain accessories seem to be identical. I don’t know if that’s because they actually look the same, or if the poster had some place-holder graphics on it that never got updated. Unfortunately, I don’t really have the time to go through finding every version and recording it!
Updated the rifle pics in post #11 for better resolution.
EDIT: And the quality inexplicably drops…again.
I got a Jakobs sniper yesterday with a cylinder. Not supposed to according to the poster, but it definitely had one.
As I noted above, those posters don’t always reflect what you’ll find because different rarity items can have different cosmetic details (like cylinders on sniper rifles). This mostly affects how the body of the item looks - I’ve noticed quite a few cosmetic differences on other weapons besides sniper rifles. Cataloguing all that would be a real pain, though, and doesn’t seem worth it when these don’t change the stats in any discernible way.