The original standard 105mm gun tube was rifled, but ...
“The main armament is the 120mm M256 smoothbore gun, developed by Rheinmetall Waffe Munition GmbH of Germany. The 120mm gun fires the following ammunition: the M865 TPCSDS-T and M831 TP-T training rounds, the M8300 HEAT-MP-T and the M829 APFSDS-T which includes a depleted uranium penetrator. “
from : http://www.army-technology.com/projects/abrams/
Also
http://www.inetres.com/gp/military/cv/weapon/M256.html
“M256 120mm Smoothbore Gun: The M256 120mm gun is the primary armament on the U.S. M1A1 and M1A2 series of main battle tanks. The M256 is a high velocity, quick-firing, accurate, direct fire weapon used primarily against enemy tanks and hard targets. “
REM: I’ve never had the honor of firing (or loading) one, so I only ‘know’ what I read, and what I hear from my treadhead friends. I could be wrong. I don’t think you want a sabot round coming out spinning from a rifled barrel, and I think there was an issue with velocity that shifted to a smoothbore design. Actual treadheads can provide 1st-hand facts.
I thank you for the correction.