All the methods you mention require the reciever be it upper or lower to be machined in some fashion or another to final desired dimensions. No upper or lower is useable as is straight from the casting dies or the forging dies and requires machining or milling to bring it to final design dimensions. It is then heat treated to get the durability desired. FWIW 1100, 2410, 6061 and 7075 aluminum may all be forged, cast or machined/milled. In some instances it would be desireable structurally to have a forged piece rather than a cast or billet machined piece. This instance isn't one of them from a strength standpoint. The wear resistance comes from the alloy and the heat treat with some help from the surface coating.

Last edited by Planemech; 10/04/07.