This is not exactly what you asked for but maybe this can help you. Just talked Hambrusch hunting weapons in ferlach, austria told me that they can make some just as blanks or finished barrels.

"Dear Erik,

out chopper lump barrels be made out of blanks dia. 65 mm or 80 mm.
We can supply the barrels at any stage you require.
Please send a sketch for the dimensions.

Best regards
Peter"