I recently replaced the Carbine lower I had on it with a new CAV ARMS MKII lower receiver. It now weighs 6.8lbs loaded (5.7lbs unloaded).
I have also added a Hogue Handall Universal Grip Sleeve that I bought from Midway for $7.99 (they are actually on sale until Nov. 30 2009 for $5.99).
This lower is a pound lighter than the standard AR15 lower. Surprised that they are not more popular because they are fairly inexpensive and seemingly VERY robust, along with being much lighter.
It is also the same length as an A1 buttstock, which I like better than the A2 length.
The Warranty is great.
"CAV-15 MKII Receivers have a life time warranty. If your receiver ever breaks in the course of normal use we will replace it for free. If your receiver ever breaks through misuse, neglect, or by accident (fire, car wreck, etc.) we will replace it for $30.00 plus shipping."
A carbine buffer and buffer spring are used.
There are some differences between aluminum receivers that I am not crazy about but it is what it is. Non standard takedown pins hold the lower and upper together, and do not stay on the lower when removed, and go in from the opposite side. The front pin is slightly longer than the rear pin as well, although I am not sure why. A couple other small differences as well.
The safety detent and spring install from the top. They are easy to install but I don't know how you would remove/replace it.
My magazines don't tend to drop free quite as easily in this lower, especially PMAG's.
You lose quite a bit in the way of options for the lower. That is the major drawback. Something I am willing to lose since this is a "KISS" style superlight AR15. If you are looking to build a superlight AR15 and/or a KISS AR15, I would recommend the CAV ARMS MKII lower.
I've heard that they are working on some new ideas for the next generation of CAV ARMS lowers. Stay tuned for new developments.