Yeah, the exact number is probably classified and precision does fall off with use.

The only NATO tank with a rifled cannon is the Challenger 2. The Brits preferred HEASH ammo, a rifled cannon improved accuracy. It took some development to get HEAT and SABOTs that can be fired from a rifled cannon. No idea what the Russian tanks are using.

EDIT: Just looked, the T-72 has a 125 mm smoothbore cannon.

Current models do, but the original M1 and M1IP were armed with the M68 105mm rifled gun (basically a licensed British L7).