The Sportco has some interesting design features. Three lugs, bolt head pinned to bolt body, Savage style. If you look at the bolt heads, you will see numbers. I believe these relate to sizes, for headspace adjustment.
Lloyd Nielsen made the adapter which threaded into the Sportco receiver, and incorporated the locking abutments, with threads for a conventional barrel.
Nielsen went on to make his own action, using a two piece receiver. Receiver body is a simple tube, the front piece has the locking abutments and barrel threads - same idea as his adapter.
If you stop and think about it, this system makes it very easy to machine the locking abutments. Nielsen's actions used Sportco bolt heads, with full diameter tubular bolt body.
I have a .308 rifle built on a Nielsen MN300 action. Schneider barrel, Robertson stock. Last time I had it out, it shot a 5/8" group at 100m. with 155 Sierras, 46gr of Varget in Winchester cases with CCI BR primers.