As others have stated, the Baikal SxS DR certainly does have a marked reduction in barrel diameter immediately in front of the chamber. The U/O is even worse!
Whether or not lengthening the chamber would create a dangerously thin section at this point could only be determined by measuring a new rifle.

It doesn't look like chamber-reaming the SxS to .45-90 or .45-110 would cause the barrels to fall off the front of the chambers,
but reaming for the oft-suggested .450 No.2 conversion just might!

Look, it is what it is! This sow's ear will never be a silk purse, whatever the chambering, so why risk your personal safety, or at the very least your warranty!

Besides, the damn thing only weighs 3.35 kg!!