Why the Smoothbore beats the Rifled gun as the armament of Modern MBTs worldwide
Conventional wisdom is that rifling allows for greater accuracy, conventional wisdom is still correct, note that every AFV EXCEPT the Tank, still fires Armor Piercing (AP,APIT,HVAP,APDS) shots from a rifled bore, eg. Bradleys, BMPs and other IFVs, however in search of the almighty penetration and the almighty cost-benefit ratio, smoothbores are ruling the Tank world.
With a rifled barrel there will always be some loss of potential power which reduces the penetration of the round. To further elucidate, if the driving lands of a shell is not fully sealed against the rifled grooves of the bore (this can be due to either manufacturing defects or fouling of the bore); some of the propelling gas while firing will escape beyond the projectile and exit the gun first.
On the other hand, the more the driving land meshes with the rifling, the more the projectile is resisting being forced down the barrel, this resistance is more pronounced in large calibers than small arms due to the sheer mass difference of the projectiles. So instead of being used to increase the shell’s velocity in the forward direction, the gas pressure has to propel the shell to move along the rifling while also increasing its velocity, this inadvertently causes a loss in energy which reflects in the relatively poor penetration of rifled cannons at long ranges compared to smoothbore Sabots (Armor Piercing Fin Stabilised Discarding Sabot).
Furthermore, all of that friction causes wear and tear on the rifling grooves of the bore, reducing barrel lifetime faster than it would a smoothbore (friction does occur in smoothbore guns but it is less pronounced). Then when the wear gets too much, its very easy and cheap to make and install replacement smoothbore cannon barrel liner than clean out or replace a rifled gun.
So, all in all, you hit harder with a smoothbore, the disparities in accuracy (MoA deviation over X metres) are compensated for by modern ballistic computers and optics. These systems also aid the gunner in faster target acquisition, engagement of the target and follow-up shots.
But, if loss in speed and consequently, penetration isn’t one’s main objective there is still an advantage to a rifled gun i.e. lobbing HESH, HEAT, HE, PCB ( the Arjun’s thermobaric round). As they are chemical based shells, they do not need speed, but only sufficient accuracy to get the job done. But for sheer Kinetic Energy based penetrators, smoothbores continue to exhibit the optimal balance of power, accuracy and ease of maintenance till date.
