Both Mounted Gun System and Self Propelled (Tracked or Wheeled) are essentially the same - SELF PROPELLED. They move on their own power and do not rely on a Field Artillery Tractor (FAT).
MGS is a more modern term as SP howitzer is a term which has been used for Tracked guns which are expected to keep pace with armored columns. Where Towed Guns would otherwise have a problem going.
Coming to why have SP Tracked, Wheeled and MGS - I think this all depends on terrain and war-fighting philosophy. My understanding of evolution of MGS in western armies is because requirement of enhanced mobility for their guns as well as expeditionary commitments of these nations. Plus, a need to reduce the logistic foot-print. And only very few nations in Europe (like Germany) now have any decent size mechanized force. So, overall requirement for SP-Tracked is limited.
And while a SP Arty brings attributes like mobility and protection for the crews, it is not an ideal solution. For one, they're too heavy to permit easy deployment across the globe. And second, they're an over-kill.
MGS is a more ideal solution in terms of logistic foot-print and ease of deployment. Not to mention the cost.
In Indian case, MGS fills role between SP-Tracked/Wheeled and Towed Guns. IA requires 100 SP-Tracked and 180 SP Wheeled guns.
While bulk of SP-Tracked/Wheeled are expected to be with armor heavy formations like armored divisions, MGS gives flexibility to fill out other armor centric formations like RAPID and (I) Armor Brigades. Not to forget Artillery Divisions. The former would've otherwise required investment in more expensive SP-Tracked platforms.
Further, MGS and Towed Guns allows IA to mix and match these in a given formation depending on requirement of terrain and role.
@rohitvats ....
Bharat Rakshak • View topic - Artillery Discussion Thread