Depends on the tank. The MBT-70 (or at least the American version) was designed with a smooth bore gun to accommodate both 152mm shells and Shillelagh anti tank missiles.
Though now you mention it, since i bastardized it with two barrels, one can be rifled the other can remain smooth.
To expand on this, modern tanks, with the odd exception (British Challenger 2), have smooth bores rather than rifled ones. Rifling means gaps which gasses escape through, which means loss of pressure, which means a loss of velocity, and that's a big problem when you need to penetrate ~1000mm of RHA 2km away on a moving target. In small-arms the trade-off is usually worth it, but not so with modern tanks, which use APFSDS (Armour Piercing, Fin-Stabalised Discarding Sabot) ammunition — using a sabot allows you to use a smooth bore as you're not relying on the barrel to provide the spin (to help the projectile in flight), that comes from the fins. And a sabot is needed anyway as AP rounds need a smaller diameter projectile head for enhanced penetration.
So older tanks (up until the cold-war when they began to fall out of favour) tend to have rifling, otherwise, nope. Smoothbore is just better.
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Though now you mention it, since i bastardized it with two barrels, one can be rifled the other can remain smooth.
So older tanks (up until the cold-war when they began to fall out of favour) tend to have rifling, otherwise, nope. Smoothbore is just better.