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AR15.COM
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3/29/2009 3:57:57 PM EDT
What is the real purpose for the undercut in the M4 barrel?  I bought my first AR this weekend and didn't get the M4 barrel.  I wanted a flat-top M4 with flip up sites but none of the M4's were configured the way I wanted.  I ended up going with a standard flat top RRA upper with a flip up front site.  It was cheaper than the M4 configuration anyway.
3/29/2009 3:59:51 PM EDT
[#1]
For the attachment of an M203 grenade launcher.
3/29/2009 4:22:45 PM EDT
[#2]
http://en.wikipedia.org/wiki/File:M4andM203.jpg
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