milsig makes one and it already comes with the metal buffer tube:

MILSIG Canada - Products - The Ultimate Magazine Fed Scenario Paintball Marker

here's one on eBay without the buffer tube:
item# 330274633320

http://cgi.ebay.com/Magpul-MOE-Carbine-Stock-Mil-Spec-Black-FREE-ship_W0QQitemZ330274633320QQcmdZViewItem?hash=item 330274633320&_trkparms=72%3A1205|39%3A1|66%3A2|65% 3A12|240%3A1318&_trksid=p3286.c0.m14