>>64115194
Recoil, I'd assume. They originally made a 5.56x45 carbine to go with the INSAS but it was uncontrollable in auto so they shitcanned it and developed the 5.56 MINSAS cartridge. Also, the case is only the same length as 5.56 MARS, it's not the same diameter. Here's the cartridge photo from DRDO scaled to match the length of the MARS in your photo.