The Russian OEM refused to transfer technology vital assemblies such as the gun system (including the barrel) and turret armour plates. This eventually lead us to develop a handful of such critical technologies if not all, by our own. Similar smooth bore guns are to feature in FMBT as well. The Russians are pretty reluctant when it comes to ToT and so are others
Broadsword: FMBT Part II: India’s future main battle tank now grapples with a weight issue
Thanks to the Arjun MBT project - we didn't had to develop everything from the scratch - In fact other sub-systems developed by DRDO for the Arjun Mk-2 program are to be featured in the T-90S including CVRDE's ERA Mk-II. A DRDO developed commander's non panoramic thermal imaging sight for the T-90S has also entered production with Bharat Electronics Limited and the Ordnance Factory Board. The T-90S will also use indigenous high accuracy fibre optic gyros developed by DRDO's Research Center Imarat (RCI) and feature fire control systems originally developed for the Arjun Mk-2.
The T-90S upgrade path is similar to that followed for modifying T-72M1s to the CIA standard which includes indigenous ERA, navigation aids, an uprated 1000 HP power pack, fire suppression system, new displays and sights...