Why Challenger 2 Is Still Using Rifled Barrel?

The British Challenger 2 main battle tank uses a 120 mm L30A1 gun which has a rifled barrel instead of a smooth bore one like the rest of NATO tanks. The reason for this is that the main ammunitions used by NATO tanks are APFSDS or HE rounds. These shells use fins for stability and don't need to be spin.

The British Challenger tank and the Indian Arjun tank both use rifled barrels because they also use HESH or High Explosive Squash Head shells which have no fin.

For rifled guns there is a special APFSDS round that has sabot which rotates independently of the warhead as much as possible. This is done by using a slip obturation ring that allows the high pressure propellant gasses to seal, yet not transfer the total spin rate of the rifling into the projectile.