Thanks everyone, you've answered some questions I've had for quite awhile.
I've owned several lightweight AR-15's and I am specifically interested in building a quiet, lightweight AR-15 based rifle in a higher caliber than .223. Originally I thought about going with a higher caliber than .300, but one of the things I am considering seriously is the addition of a suppressor. I know it's expensive and a pain to get the paperwork thru, but I really like the idea of not having to wear a lot of ear protection and the (slightly) increased accuracy and reduced recoil that goes along with fitting a suppressor. Besides ... it's something new and unusual :)
In the higher calibers, the Beowulf evidently does not come in a twist rate that works with subsonic cartridges, and in any case, the ammo is QUITE expensive. The SOCOM might possibly be adapted, but the ammo is even MORE expensive. The .300 Whisper/Fireball seems like a fairly good choice for what I want ... quiet, relatively low recoil, inexpensive ammo (thanks to the ability to convert .223 brass). It also gives me an excuse to learn more about reloading.
There appear to be several lightweight AR-15 lowers on the market, the FAR-15 (4.9 pounds for the complete rifle), and Carbon-15 (3.9 to 4.3 pounds complete from Bushmaster) come to mind. I've owned several of the C-15's and I absolutely loved the low weight. According to BM tech support, the C-15 is not completely compatible with a 'normal' AR-15 design but "if using the Carbon 15 lower with its short buffer system you would need the shorter Carbon 15 bolt carrier and action spring" to match it with an AR-15 upper. The C-15 lower is a phenomenal 0.9 pounds. My goal, if possible is to get a suppressed .300 in the range of 5 to 6 pounds. For the subsonic stuff, I would also like to use an adjustable gas block or tube to have the ability to switch between sub and super sonic ammo.
Anyways, that's my plan ... just gotta wait till I get more info (and permission from the wife :))
Any advice and/or suggestions always greatly appreciated.
:D
Matt
Alaska