Here is an SB1 16 ga shotgun lower I got for $50 with a Green Mountain 50 cal barrel I got for $111.
The 50 cal barrel was 1.2" in diameter. Rather than turn it down to be as small in outside diameter as the 16 ga barrel had been, I milled out the receiver to 1.2".
And it is cast iron too, as the metal shavings look like sand.
I run the pressure so high in that wildcat that I pierce the primer.
I made the forstock from an old Mauser butt.
The expansion ratio enhancer is 5/8" OD, 1/2" ID 6061 Aluminum tubing.