I have read several posts about the actual differences in these two kinds of ammo, so I don't need that explained.  But my question is this:  which is more popular- among gun manufacturers and gun buyers? Is .223 rem the overwhelming choice? Thanks for any information.

No brainer here, In bolt rifles 223 SAMMI chamber specs are more prevelent. In AR15/M16s the most common chamber spec is 5.56 NATO.

Only long range "varmint" type ARs have other than 5.56 NATO chambers, RRA has a "Wylde" and some makers use 223 SAMMIs, all of which is expained in the "ammo faq" here: [url]www.ammo-oracle.com[/url].

Comparison chart, tacked to top of AR Discussions forum here:
[img]http://photos.ar15.com/ImageGallery/IG_LoadImage.asp?iImageUnq=3425[/img]

Mike

Thanks mr_wilson for your informative response.