Rice offers a not very well known HC, thin-wall, barrel constructed from 4140CM steel. Only available up to 41" in length, and with a maximum breech diameter of 1.125" across the flats octagon. At a cost of $375.00. They are supposed to be VERY LIGHT in relationship to ordinary barrels of similar diameters and lengths.
The only ones I have seen online have been at the American Longrifles Forum. One in 20 gauge, 0.620" bore diameter; and one in 16 gauge, 0.672" bore diameter. Jason will make these 4140CM barrels all the way down to .45 caliber smoothbore, 51.3 gauge; and up to a 12 gauge, 0.730" bore diameter.
51.3 gauge = 0.450" bore diameter
37.4 gauge = 0.500" bore diameter
28 gauge = 0.540" bore diameter
24 gauge = 0.580" bore diameter
20 gauge = 0.620" bore diameter
16 gauge = 0.672" bore diameter
14 gauge = 0.690" bore diameter
12 gauge = 0.730" bore diameter
Each barrel will be sized according to the length desired, and the style, such as Classic Spanish proportions (2/5 octagon-to-3/5 round). Regardless, the barrel walls will be kept as thin as possible.
If the customer is willing to pay the bill for the extra work, then these smoothbore barrels may be had in traditional swamped octagon patterns, only up to 41" in length. Be warned that the cost will likely be between $500.00 to double the cost of an octagon-round 4140CM barrel.
Rice also offers jug choking, but only in 24 gauge barrels, and larger. And, only in their new production barrels.