http://media.militaryphotos.net/photos/M4-Carbine/carbine07 http://www.fas.org/man/dod-101/sys/land/m16.htm The above _link_s show the M4 carbine. I've always wondered what the 'step' in the barrel just forward of the front sight is for. On the F.A.S site's pictures it shows a M4 with m203 grenade launcher, the launcher seems to clip on just where the 'step' in the barrel is. IIRC M16s mount the M203 without the 'step' in the barrel so why is it needed on the M4? Jeremy Thomson