"Cast steel" usually refers to the Huntsman hot-rolled crucible steel process of 1742 used to make farm implements. A more modern use refers to the Bessemer process of 1856 for converting pig iron to steel. A sheet is folded over a mandrel and the long edge hammer welded to form a barrel.
National Arms was one of the Folsom import trade names
http://www.picturetrail.com/sfx/album/view/20091267

Under no circumstance would I fire that gun.

 

The William Read & Sons, Boston, Mass., catalogue from circa 1899 carries a line of cheap Belgian hammer guns under the National Arms Co. name. They were priced from $9 to $12 with back-action rebounding locks and $12 to $18 with bar-action rebounding locks.

National Arms Co. was also the name of the first company Ansley H. Fox incorporated in Baltimore, Maryland, on December 9, 1897. As far as we know they never produced a gun and the next year with new blood and capital became the Fox Gun Co., Balto., Md., U.S.A. and for the last half of 1898 and all of 1899 produced a gun designed by Ansley H. Fox.

 

First, I know nothing at all about farm implements.
I have four listings for shotguns with the name National Arms Co.
All are "Trade Brand Name" shotguns
1. Hammer double, Belgian made by Henri Pieper or Anciens Establissments Pieper (same company) of Leige Belgium
2. Hammerless double made by Crescent Fire Arms Co of Norwich,Ct.
3. Single barrel also made by Crescent Fire Arms.
4. Slide (pump) action made by Marlin Firearms Co.
Look on the bottom of the barrels under the forearm for proof marks.
If as I suspect that the gun is Belgian there will be the letters "ELG" in an oval with a crown over that. If so the gun was made some time between 1890 and 1914 when WW 1 cut off exports and it will have damascus barrels and was designed for black powder ammunition.
I strongly recommend that you do not try to shoot it.

 

Thanks for the replies. I will not be shooting this one. Interesting history though.
It does have exposed hammers and the ELG mark.
I need another wall hanger like I need a hole in the head. I'll see if my brother wants it :wink: