Tech question, though I doubt there's currently a conclusive answer (ie, too little info out there). How is the barrel in the 433 mounted in the receiver? In this set of patent images (gen 3/4 rifle, though I doubt this has changed much), it does show some of how the barrel is fitted into the receiver, and one of the illustrations shows what appears to be a G36-type barrel nut:
The invention relates to a weapon housing (5) for a self-loading firearm (1), comprising a tube receptacle (127), characterized in that the tube receptacle (127) is formed as an inner part of the one-piece weapon housing (5). The invention also relates to a self-loading firearm (1) with such a...
patents.google.com
I know that it's not like the SCAR or Bren 2 where the barrel and trunnion are integrated and bolted into the upper. And we have to remember that the G36-based XM8 had quick change barrels as a design feature (though it required a special barrel nut wrench and the rifle possibly having to be mounted in a vice for it to work). And I did see a World of Guns video on YouTube where a G36 was dis-and reassembled. On the G36, it seems that you loosen the barrel nut and the barrel slides out and interfaces with the trunnion with 6 radial lugs that interface with the trunnion and the barrel nut.
As as can be told as of now, does the 433 use a version of this system, or something else? HK did credit Sig Sauer as far as something related to the barrel in the patent documentation. But it seems that the 433 and the MCX use a different barrel change procedure. And those two lugs on the underside of where the trunnion is set looks like attachment points for the block that locates the handguard lock and where the front of the lower locks onto the upper.