Not to start any "flamers"... Being an Army vet from the '70s, my "issue" weapon was an M16 w/203. I have "conventional" GI ARs including MRP CQB and am happy with all of them. The only reason I got interested in GP AR conversions at all (climb into my Nomex suit) is that I am VERY pleased with my Kel Tec PLR-16. Kel-Tec put GP on their 223/556 pistols and carbines and ELIMINATED THE RECEIVER EXTENSION (buffer tube). Their pistols are shorter than many AR-15 pistols, and their SBRs have TRUE FOLDING STOCKS. They are relatively inexpensive, lightweight and VERY VERY RELIABLE. The PLR-16 uses AR-15 mags and are both accurate and very light weight. Function has been flawless with several different types of commercial and surplus 223/556 ammo. It is sufficiently reliable that it has taken its place as one of my two SHTF firearms for home defense. (The other is a Kimber 45ACP). My PLR-16 has the optional foregrip (which puts a Picatinny rail under the fore-end, in addition to a Picatinny rail on top of the receiver). A Phantom flash suppressor does wonders for eliminating flash for night firing. You can read more about the PLR-16 here if you like:
http://www.kel-tec-cnc.com/plr16.html
If you MUST have a rifle stock, Kel Tec makes carbines and SBRs using the same action as the PLR-16. The SBR SU-16D is available with both 9" and 12" barrels. I have no personal experience with either the SBRs or carbines so cannot comment on them:
http://www.kel-tec-cnc.com/su16d.html
The cost of either the Kel-Tec PLR-16 or SU-16D is on par with many GP conversion kits. However unlike the GP uppers/conversions you can probably buy one at your FFL dealer NOW, if you don't want to wait... The prices (retail $665/$874) are heavily discounted, and you may be able to pick up a new one for much less than a 1000 rnd case of surplus ammo. (I do not want to get into a Kel-Tec vs AR argument here. They both have their places, and I respect both Eugene Stoner and George Kellgren as brilliant designers)
The cost of the ultra-light, Carbon 15 GP upper I ordered from BFI was a tiny bit more than the cost of a 16" LMT GP barrel assy for the MRP (and weighs less than the CQB MRP upper with GP barrel installed). The new BFI GP upper will "share" a BFI Carbon-15 M4 type lower (ACE stock, MAGPUL grips, match trigger) with a MRP CQB and a 24" SS HBAR match upper.
Note: I am not planning to shoot matches with the BFI GP upper, so please no flamers about lightweight vs heavy barrel GPs for accuracy, or GP vs GI. There are plenty of posts on the other thread if you want to get into that. I'll be sure to update the board on the ultralight GP AR (when I eventually get it and shoot it. It may be months...