From what I have seen there is a love or hate feeling for Olympic rifles. The first couple I looked at felt lighter built than Armalite or Bushmasters and I did not like that. Yesterday I say an Olympic carbine with A2 stock and I was impressed. It felt as sturdy as my Bushmaster. Actually better. It was very tight and fit well not to mention the color actually matched unlike my multi colored Bushy. The trigger pull was fabulous very light and CRISP again unlike my Bushy  that has a bit of gritty creep. Only problem I saw is I did not see any 5.56 stamping to show it would chamber nato ammo without problems.
Other comparisons I have are this.
Bushmaster says that the uppers are forged and so is the front sight. Inside my carry handle it sure looks like cast marks and the front sight really has alot of marks that look like cast marking. The Olympic was smooth and clean more like what I would expect a forged and machined piece to look like.

Again I am very new to these weapons and have not had a chance to handle many of the various brands.
I am trying to learn the in's and out's though. There seams to be so many variables with these weapons.

I have Bushy, Colt, and Armalite ARs and I just bought a preban Oly Arms M4 from a local police officer.  My lower has the 5.56 stamp on it.  I really love the quality of the lower, however I there is one problem with it.  I noticed that it will not accept thermold mags like my other ARs.  Also, some of my USGI mags tend to fall out sometimes.  Other than that, I love it.  Maybe someone can give me some advice on how to fix this because I believe this problem can be solved.

I now have 3 Olys and I really like them. They are highly accurate and very reliable. If it is a gun made in the last few years, it will be a 5.56. The stamping is on the top of the barrel in front of the sight tower. It wil be simply 556 or 556SS if the barrel is stainless.

Just out of curiosity, is there any reason the lower would need to say 5.56? I have a bushmaster upper that is chambered for 5.56, but my Oly lower doesn't say 5.56.  

By the way, what would happen if the upper WASNT chambered for 5.56 and you shot 5.56 through it?

Thanks guys. I knew about forged being stronger but being I use many cast parts when I build flintlocks I am used to seeing marks like I see on my Bushmaster. I did not realize forged pieces also looked like this. Guess it is a different beast. Cast parts for flintlocks take lots of work to clean up. Of course they are mostly sand cast the old fashioned way.