You might find that the smaller bores are actually HEAVIER. Often they use the same size octagon stock steel and just bore out less material. There are smaller sized octagon stock called "A" weight barrels, but even so, the small the bore, the heavier the barrel. You might consider a .40 in an "A weight" barrel, and then just use lighter loads.
Then to make an even lighter weight gun, consider the swamped barrel. They have a slow taper down 4/5ths of the length and then flare back out at the muzzle. My Colerain "B" lost 1.8 lbs of steel when it was swamped down from a parallel sided barrel.
