One thing which I often learn in this forum is that, don't argue with someone who is idiot enough to see only his view point.
Lets have a look at the ammos which are being developed for Arjun.
120mm HESH and 120mm APFSDS. Now APFSDS is a round which is best in business against a heavy armour like a MBT. Against any other light armour or in case of a urban war scenario, a HESH is good enough. You don't fire a Kinetic penetrator against a wall or a soft skin vehicle carrying anti tank ammo. So when taking all this into perspective, one could see that both smooth bore as well as rifled gun could carry out the same with ease.
Talking about T-90, our front line MBT, we could see that we do produce 125mm APFSDS and 125mm HEAT round for it. Interestingly HEAT round is not being produced for Arjun. It clearly shows that Arjun is not intended to go on and take a Anti Tank role. This role is something which has been primarily reserved for T-90 and Arjun is only on second to it as of now.
Now talking about accuracy for both, let me quote few people here.
It doesn't matter if you talk about single-shot pistols or battleship guns, a rifled barrel is always more accurate than a smooth bore.
Randy Hoagland, former Noncommissioned Officer for 27 Years, USMC & Army
It all depends on the ammunition. If your primary target is other tanks and the ammunition is APFSDS rounds, smooth ore is obviously best. If the design tgreatvisctgecfull range of possible targets, including bunkers, emplacements, buildings etc, you want to include accurate long range HE capability, and might sacrifice some capability with sabotted rounds or use HEAT for anti armor. Rifled barrel equals more accuracy, smooth bore equals best possible muzzle velocity.
Bob Keeter, Retired flight test / systems engineer / engineering lead
Courtesy : Quora
So I would put my penny on these guys rather then someone who utters shit on each and every sentence.
Now coming down to why Arjun MkII has retained its rifled main gun. There could be many reasons, but we could basically divide it into two.
1- For Good
2- For Bad.
Lets see for good reasons first.
Arjun was conceptualized in 1970. At that time we did have Vickers Mk1 (Vijayanta) with 105mm L7 rifled gun. Now if we do remember well, Arjun too had this 105 mm gun in its early prototypes. Now interestingly till 2008, the main gun barrel was imported rather then being indigenous along with other components. Please do have a look at the attached PDF.
Inspite of being a rifled gun when it was pitched against T-90 on test ground, it performed exceptionally well which prompted or should we say forced IA to order its serial production. Later when they asked for upgrade, smooth bore barrel is no where in the wish list. Moreover user never complained about the accuracy or killing power of the gun.
So apart from spare management, there must be something in doctrine and usage result of the Tank which has so far satisfied the user on Gun front atleast.
Now lets see for Bad reasons of retaining it.
Simply to bad mouth it on a later phase. It might be a deliberate attempt to keep a Achilles heel alive in the MBT to flounder later on. A rifled barrel is maintenance intensive along with being costly on production front. May be at a later date lobby would try to drag it on. But then again, with current capability to produce it in house, the ball would be rolling in user court rather then developers.
Another point which goes well against a rifled barrel is in inability to fire a CLGM. But that too had been overcame in case of Arjun with firing of LAHAT. Now IA is not satisfied with LAHAT as they wanted something with ability to engage at short distance.
So either way, IA would have a hard time to bad mouth Arjun on Gun front. They could always cry about it being over weight, but I have yet to come across a instance where they had uttered any thing against its killing power.
So basically when it comes down to have a smooth bore or a rifled gun, doctrine along with technicality plays a part. As far as firing a certain type of round is concerned, till date designers have provided a way around to overcome those. Be it specially designed APFSDS or a CLGM. So as far as it kills, no one on user front is complaining it seems.
Its just some rabid infected user here, who froth on what they want to see instead of what is the practical use of it, who keeps on barking at a single point.