Of course forged upper and lower is next thing to stainless, however I feel a cast lower and forged upper is as good.  The real stresses from firing are on the upper not the lower.

Well am I full of it or what?

I know I am usually but I mean on this topic.

I'll concur.  I don't see a problem with cast, sans running it over with a car or something.  Cast aluminum can be quite brittle, but I just don't think it's an issue as far as shooting stresses go.

a casting should work just fine.
but they are just uglier than a forging

I would tend to agree with your assessment.

If you have an unmarked upper, any way to tell which it is?

DoorKicker6

You can tell the differance quite easily.  The forged part is smooth as a babies butt and the cast feels rough like it needs a shave.  The forged uppers I have seen are a bit darker black due to the coating being smoother while the cast is porus.  One thing I can tell you is that if you can't tell the differance then whatever you have is same upper an lower for the diff is obvious.

"Brittle?"  How can it be brittle?  A forging is more brittle than a cast...'

Please explain how you came to that conclusion.

I am waiting for the initial person to explain their brittle theory.

Just remember, in general, the harder something is the more brittle it becomes.  With alloy's we modify the point where a metal becomes brittle, but to keep making XYVZ metal harder and harder, it will become more brittle.

I'm pissed right now cause I had a huge reply typed and the ARFCOM server logged me out and wiped the post... This is getting ridiculess

Most cast items are made using [url=http://www.matls.com/search/SpecificMaterial.asp?bassnum=MAC35Q]A356T6[/url] - this is due to its grain structure more brittle than [url=http://www.matls.com/search/SpecificMaterial.asp?bassnum=MA7076]7075T651[/url] which is the alloy most common for AR forgings.

The T# referes to current hardness and temper state.

While the cast items [u]can[/u] be further heat treated to improve there grain structure and physical properties most mfg don't because of the high chance of warpage.  Most often the parts are machined as cast.  Note: the above link for 356 is listed as sand cast. Most AR related parts would not be sand cast, they would be investment cast AKA lost wax.

Forged frames have a more ductile grain structure and therefore are less brittle than a cast frame.

BTW - I have made over 100 investment cast AR frames. They are well up to the task.

I went back and did the research and the Prof is correct.  My apologies for the confusion.

I had to go look up the definition of brittle to see that I was defining it incorrectly.

Thank you for taking the time to clarify my mistake.

What I tell ya!  Next time don't question me until you do your homework! [;)]

Uh, yes look here:

http://www.ar15.com/forums/topic.html?b=1&f=74&t=155892

This was my weapon.  It now belongs to Olympic Arms.  I am still waiting to recieve my M4 marked with the same # as the plinker.

Any questions you can E me.