A similar question came up in another forum. My answer may help you out. This fellow had a .40 caliber built with a swamped barrel, but he didn't stipulate the profile of the swamped barrel. As a result his .40 caliber rifle weighs over 10 pounds.
Picking the right profile based on your caliber makes a big difference in swamped barrels. Here is what I wrote:
My .40 cal weighs 8.5 pounds. It has a straight 13/16ths x 42" Green Mountain barrel.
in comparison my .45 with a straight 13/16ths barrel weighs 7 pounds even.
I don't know what profile your swamped barrel is. If I was doing a .40 I'd use an "A", or if I had to, I'd use a "B" profile.
I have a "C" profile swamped barrel on my .54 because "C" is the lightest profile I could get for .54. It is a 42 inch barrel and that gun weighs just a hair under 9 pounds.
My .50 uses a straight 7/8th x 42" barrel and that gun weighs 8 pounds.
If I ever build another .40 it will have an "A" profile swamped barrel to reduce the weight.
If yours is over 10 pounds with a .40 caliber bore, it probably has a "C" profile. Put a pair of calipers on the muzzle. If it measures 0.88 or so inches it is a "C", which is a wicked heavy piece of steel for such a small bore.
When TVM built my .54 Virginian, I stipulated that they use the lightest swamped .54 barrel available.