desert eagle, if you have the time this weekend, I recommend that you go to the Wash. Arms Collectors (WAC) gunshow at the fairgrounds in Monroe. You will have the chance to learn more about ARs, buy used or new magazines at reasonable prices, etc.
Don't bother with stopping at Sam's gunshop in south Everett since it is being sold. Inventory is very low. The place has an echo.
If you are looking for the ability to switch uppers on your one and only lower, as in have a full 20-inch rifle and a 16-inch carbine, then the Carbon 15 is probably not going to work. Actually, I am not sure now. I'll have to look for one at the gunshow to see if it has the usual type upper.
If you have the time to learn more before buying, then the forums at M4carbine.net and AR15.com are a good place to do some reading.
As for me, I am very satisfied with my Smith & Wesson M&P15.
“There’s a rattle in my glove box,
A Federal twelve-gauge slug.” – Adam Carroll in song Porter Wagoner