According to the description, the barrels were done by Hambrusch in Ferlach.
It looks like they're trying to get their gunmaker's fees out of the sale.
At least (if it was indeed done by Hambrusch) it's likely done right.
He's done 700 nitro rifles, perhaps he's worked this thing into semi-hotrod status.
Still, it's 'just a conversion'...

Cheers
Tinker