If you can make it through the whacky military files for a while, you keep running across references to the M-4, but M-3 seems to mean the .45 H&K buzz-gun they were going to adopt a while back. Still haven't found it.
No way the part of their M-4 description (that mentions the M-3) is proof enough for him...?
Re:
"CG, TACOM (DIR, ACALA) Carbine, M4 TACOM-ACALA PFDOS IV The M4 Carbine is a smaller, lighter version of the M16A2 rifle. It is a 5.56mm, gas operated, air-cooled, magazine-fed, selective rate shoulder fired weapon. It is fed by a 30 round aluminum magazine and is designed for use in close quarters. It is the fleet replacement weapon for the .45 caliber M3 submachine gun and selected M9 and M16 series weapons."
[url]http://www.sarda.army.mil/acat/mda_seq.htm[/url]