As I understand from all sources I've found so far the M1 had a 105mm M68A1 gun (basing on the British L7). I have even seen images of it being loaded with this ammo.
All the people can't be wrong...
http://www.globalsecurity.org/milita...d/m1-intro.htm http://www.wideopenwest.com/~brobere/m1ipspecs.htm http://www.clubi.ie/exalted/m1abrams.htm
etc. etc. etc.