Essentially yes, but it's standard on certain AR-15A2s. Bushmaster makes one too for building on.

Here's an example.

http://v4.beta.gunbroker.com/Auction...=135798245#PIC

I'm pretty sure all of the original AR-15 A2s (regular and HBAR) had the exact same lower receiver as the Sporter I and Sporter II. I don't think Colt started selling them with the fencing around the mag release until around 1990. At least, that's the impression I get from looking at my Dad's old Gun Digest issues.

I guess that makes sense then. I learned something...

5 56 NATO vs 223 Remington
 

I am considering the purchase of a Remington Model 750 semi-auto rifle. Anyone have any experience with this rifle? Doing some research on other forums, there was a lot of posters that had problems with feeding. Also how accurate are they?

My dad has a Model 750 chambered in .308, on rare occasions he'll have trouble feeding 7.62 surplus stuff. Accuracy is as good as any average rifle.