Really, lower receivers can and have been made from some very low strength materials.
There are plastic lowers currently on the market.
Old Essential Arms lowers were cast from 356-T6 aluminum.
Forging is more cost effective. Many outside surfaces require no further machine work, and there is less scrap metal to be sold as chips.
Lowers machined from extruded bar stock have similar grain structure and orientation to the forged lowers. If a billet lower is machined from rolled plate, it too will have good grain orientation - but care must be taken to cut the blank from the plate in the correct orientation before machining.
There are some billet lowers out there that were not machined from 7075 aluminum. It is easier to cut 6061, and a lot cheaper too. 6061 lowers are not as strong, but are still much stronger than cast aluminum or plastic...
Lem
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