Nucleas rifle action

[Dark Hunt]

It's a half round lug instead of a flat surface.

The lug's bearing surfaces (backs) are flat. Look up "Toroid", there are no flat surfaces on a toroid.

If the bolt bearing surfaces were rounded to fit half round cut outs in the receiver then the bolt thrust would cause the bolt to rotate in the open direction on firing, which would be stupider than calling it "Toroidal".

[ar15barrels]

Is there something that prevents them from machining the rail and receiver as one piece?

Other than cost, no.
You can buy a Surgeon or a Defiance receiver with a rail and lug that are all one piece.

[sigstroker]

It would be a little heavier because the rail would be all steel. The rail they use is aluminum iirc.

[ar15barrels]

It would be a little heavier because the rail would be all steel. The rail they use is aluminum iirc.

That's not a bad thing.

Most aluminum rails are too soft to use with steel rings.
The steel rings will actually deform the soft aluminum rail when tightened.

The general rule on this is to not use a ring that is made of a harder material than the base.
So, you can use any ring on a steel base, but should only use a 7075 or 6061 ring on a 7075 base or a a 6061 rings on a 6061 base.