The one piece double gun barrel is not a new thing, far from it.
Jean Breuil was a specialist almost 100 years ago...and sans CNC...Now, that's real skill.
If you ever see a French gun with "MONOBLOC" on it, it does not mean sleeved (that would be "canons frettes"), it means that it was built from a single block of steel.
Not common but out there none the less.
And yes there is a bit of weight improvement, albeit not gigantic.
Best regards,
WC-