Many people have requested information on this. There appears to be some back-and-forth, and there was a requirement / solicitation issued for 5,000+ (which was subsequently cancelled), but an NSN has been assigned, which means you may be able to order it:
Item: SLIDING BUTTSTOCK CONVERSION KIT
NSN: 1005-01-569-6938
Also, here' the M4-specific enhanced sliding buttstock:
NSN: 1005-01-544-9825, Part Number: 13020680, ENHANCED SLIDING BUTTSTOCK ASSEMBLY USED ON THE M4/M4A1 5.56mm CARBINE
Also see the following threads for the "blow by blow":
http://www.m4carbine.net/showthread.php?p=395705http://www.ar15.com/forums/topic.html?b ... 796&page=2There has been much scuttlebutt about who is allowed to modify their rifles, and who isn't ..... but as a data point, and on a personal note, my former company installed over 100 M4 buttstocks on our M16A2s for the 2005-06 OIF deployment and had no problems at all. I personally fired several thousand rounds through mine with no issues. It's anyone's guess what the stats are, but.... trying to maneuver around the cramped interior of a vehicle with a full-length "musket" has probably caused more deaths / injuries due to failure to quickly place fires on the enemy, compared with weapon failures caused by using an unauthorized buffer / spring assembly...