Please explain the metallurgical difference between the  Billet lower and a forged lower.

Which is stronger or harder, resists wear? Why the two types?

The type of alluminum used and its heat treatment are more important.  You will never, ever notice a difference in strength if they are the same alloy and same level of heat treat.  Some billet receiver can be made thicker to be stronger than forged counterparts.  Grain structure is NOT a point of failure for receivers so why wonder which part is theoretically stronger when neither would fail?  Just dont get a cast receiver and you will be fine.

The finish is what will make them more or less resistant to wear... not the underlying 7075 T6 being machined billet or machined forging.

You are looking up all the wrong trees if you thing you can get a receiver that wont wear or you are going to make a "stronger" reciever that wont break or wear out where another would... its not going to happen.  A .1% difference in strength does not mean anything if its just the carrier moving in the upper or if you drop your rifle from the roof off a 10 story building one will do nothing to your receivers and the other will destroy them.

A billet receiver is what you want for a "pretty" rifle... it has a more consistant surface.  A forged receiver will have some surface imperfections on the unmahined portions... its the nature of the beast when you make mass produced items that are forged.  For fuction you will be served perfectly with any quality forged upper be it Mega, CMT or whatever.

As far as finish you have milspec anodising like CMT, LMT etc and then there is the Mega, DPMS, etc teflon impregnated anodisation.  It a little slicker and prettier and does not need to be holding CLP to look good in pics.  Either will serve you just fine.

Some will say billet is more cinsistantly true and the rals in spec... thats an issue for your supplier, not you... you r supplier should be sending bad uppers back so if you buy from a good supplier like any of our board sponsers you will be fine.

Billet is used for low production or custom items.  Forged is more expensive to get an inital die set up but its cheaper for high volume in the long run... for instance LWRC made a new .308 rifle the SABR.  Prototypes and inital production are all billet.  It was cheaper to do CNC programming than to set up for forging initally. To save money they went to forged for future production runs for higher volume.  The inital billet is likely to be a bit "prettier" but both will be equal in funtion.

i bought an "arizona armory" lower several months back. very smooth and nice looking. i sold it off and bought a double star receiver to replace it. although it was very clean, i found that i just liked the forged better. the az armory receivers are manufactured by sun devil and i believe them to be made from 6061 aluminum, which considering the application, i'm sure was strong enough, but it felt "light" to me in comparison to the forged. (that's my excuse anyway..lol). actually, the reason for selling was more due to cosmetics......meaning...it was just too pretty for me.....

http://www.jeritopia.com/aa1.jpg

Thank you

The simple answer:

Billet parts are machined from extruded plate or extruded bar, all of the grains in the metal are uniform and going in one direction because of the extrusion process.

A forged part will be stronger because when the part is formed the grain flow distorts. A well-designed forging tool and die will control this "flow" to make the part stronger where it needs to be.

Is that part true on other gun types where the receiver has more (longitudinal???) stress?  For example the m14, is a forged receiver and people (myself included) operate under this belief that the forged receivers grain structure makes it less prone to "longitudinal" stretching than cast.  You believe this is because of heat treating and NOT the "grain" of forged parts?  Thanks for your time, material science revs my engine!

OTOH, a forged part has more likelyhood of haven hidden flaws than a billet-machined part.  

But TBH, the difference is minute.  I think it comes down to preference.

The extra strength in a forging comes from the breakup of the as-cast microstructure. This can also be acheived from rolling or extruding. The benefit of forging recievers between dies reduces the amount of machining to final product.

The final heat treatments (in AR recievers) causes diffused alloys to precipitate out of the solid solution to further act as aboundary to dislocation motion.

6061 T6 has 2/3 the strength and hardness of 7075 T6

I'm making small batch billet 80% lowers right now.  Using Kaiser's 7075 T651 this stuff is top notch.  Forging vs Billet is almost the same as long as they are the same base material.  6061 vs 7075 is like the AK rivet vs screw build issue.  One material is strong enough to function safely the other is the spec and is more durable and longer lasting.  In the small batch world I'm working with now, things are mainly 6061 for ease of production.  I'm making these to last.  No elongated pin holes.  Will not dent or ding easily.  Strong, durable and great for a precision rifle build.

One other note.  Someone mentioned that the billet allows freedom of design.  Because you're not constrained to work within the size restriction of a forging, you can get creative.  You can add style, personality and strength.

This image is from my first batch.  Made a whole whopping 43 of them.  Cut from a 12 foot long bar of 7075... (gen2 coming out Tuesday April 7)
http://www.yellowlogic.net/mm5/graphics/00000001/prod/pIMG_3286_1024.jpg