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There’s a deeper explanation than simply “rifling tends to wear out”. You see, tanks were first designed to fight fortified infantry. That required lobbing high-explosive (HE) rounds which, well, exploded. Excellent for destroying bunkers and killing the infantry inside. The US Army in World War II took this idea to a ridiculous extent by claiming that tanks were not meant to fight tanks; that job was best left to dedicated “tank destroyers”. It didn’t work out too well, but that’s another story.
Anyway, it was around World War II that the primary purpose of the tank became fighting other tanks. For a while, HE rounds worked quite well in that role. But as armour got thicker, people began to realise that HE didn’t cut the mustard anymore – rounds began to get heavy and unwieldy – and something else was needed. That “something” turned out to be the kinetic energy (KE) penetrator. The basic concept is to use a projectile made of hard, heavy metal sharpened to a fine point and fired at a high velocity. This stuff penetrates heavy armour fairly easily and does all sorts of nasty stuff once inside the compartment. It may spall and kill the crew, it may become a fine dust that ignites to kill the crew, it may create a pressure wave that kills the crew… you get the point.
Okay, so you now have the beginnings of a solution. For this new round to do more than just tickle an enemy tank, you need two things: mass and velocity. Mass you get by using copious amounts of dense metals/alloys like tungsten carbide or depleted uranium. Velocity is a little more challenging. You want a large gun so that you can pack enough propellant to impart great velocities to the round, but you want a thin penetrator to minimise the effects of drag. See the problem here? You can’t have all three (heavy dart, thin dart, large gun) all at the same time, not easily.
Well, one bright spark decided to solve the problem by using a long, thin projectile; and surrounding it with a light “discarding sabot” that would fit snugly in the barrel. The sabot would form a nice, tight seal between the barrel and the round right until the point it exited, after which it would be [discarded](http://www.army-technology.com/contractor_images/cime_bocuze/alloy1.jpg). Ingenious!
Okay, so you’ve designed a round, but how do you make it accurate? With an HE round, a rifled barrel imparts “spin” to the round that stabilises it as it flies long distances in much the same way that Johnny Manziel does with his ~~football~~ handegg. With a KE penetrator, it was realised that the rotating motion was likely to make the long, thin round tumble. Something else had to be done. The answer lay in using fins to keep it flying straight and true, much like an arrow. Since the round was much thinner than the barrel, there was plenty of space for fins. Super duper! And the good thing about fin-stabilisation was, they no longer needed rifled barrels to spin the round. That was even better! They could use smooth barrels, which were cheaper and suffered from less wear.
And that is why you have smooth barrels on modern tanks with a few exceptions here and there. The Challenger and Arjun use rifled barrels. That’s because the Brits and Indians still like their HE and love their HESH (high explosive squash head). In spite of the popularity of KE rounds, HE still has its uses. For example, it is very good at exploderising soft-skinned vehicles that a KE round would simply pass through without causing damage. HESH is excellent against bunkers. It’s like a cake of plastic explosive that first splats onto the concrete and then explodes. The blast wave generated fractures the concrete and makes it spall, killing the occupants inside. Both are fat, heavy rounds that need spin stabilisation. But we know that spin is bad for KE rounds. The designers got around that problem by using rotating sabots that spun independent of the penetrator so that the rotation was not transferred to it.
Edit: Holy crap, thanks for the gold, kind stranger! Dense, heavy metal. Shall put it to good use making KE penetrators… muhuhahahahahaha!