I put together a RRA carbine, with a standard lower.

I thought the lower was forged, but it looks like there are casting marks along the bottom, by the trigger guard.  

Anybody know if RRA makes any cast lowers- or maybe those are just weld marks or something?

RRA receivers are forged, those marks are forging marks.  Some companies mill them down to smooth them out in areas, and some don't.

Quoted: RRA receivers are forged, those marks are forging marks.  Some companies mill them down to smooth them out in areas, and some don't.

what exactly are forging marks?
Is that where the metal is folded over and beat into itself?
Is it pressed into a mold of some sort?

As far as I understand, forging is a process where a metal casting is heated to above a certain temperature where the metal grains soften, then the casting is put into a die matching the final desired shape of the piece.  The piece is then pressed in the die.  This re-arranges the crystalline stucture of the material, reducing its brittleness and making the metal tougher.  

The lines on the finished piece are simply the parting lines on the die.

Modern forging gets the same end result as the days when sweaty blacksmiths beat on hot pieces of metal to make horseshoes, swords, and other stuff.  The new way is a hell of a lot easier, though.

my new RRA lower has pretty bad parting lines inside the trigger finger cutout.
My old RRa receiver is cleaned up much better.