I believe Sellier & Bellot makes, or sell as their own, a fair bit of their own projectiles could be the case with those. They are available to purchase as a component, I had some of the 173 SPCE's in 7mm Diameter came along with a box of loading supplies from an older gent. They shot well enough, once a load was found and killed deer, only had 40 some and haven't been able to locate a source. A quick search shows they offer component 150 and 180 grain .311 soft points.
I also didn't search too hard having a variety of other projectiles that the rifle likes better and are more common.
No way , are these only 311s?? interesting. ive not actually shot groups with it as it just isnt needed, it just needs to hit where its aimed, i also searched up a bullet list but couldnt find the diametersthanks though !
just a standard cup an core SP ? is it ... slow velocity would help it look as neat as it does