Submachine guns vs 3A ballistic Kevlar vests?

Even though the 9mm, .44, and .45 rounds are all bigger than the 5.56mm round, it doesn't penetrate body armor. However, what kind of caliber round could be made for submachine guns to still be effective in military urban combat CQB scenarios where they could get the stopping power to pierce vests? My friend and I were talking today and we were discussing possible rounds that could be more effective given a government decided they had the funds to make such rounds. I came up with the idea of a round that when coming into contact with the target, the kinetic energy from the round would be transferred to a metal pin inside of the bullet, ejecting itself foreword, piercing body armor. I've been questioning the use of submachine guns. As a home defense weapon, they seem more viable than a carbine. However, the rounds as stated above at the beginning of my post dont have to power, despite being bigger. I've HEARD that submachine guns utilize suppressors more effectively, and that for law enforcement are preferred because they dont penetrate through walls as easily as a 5.56mm round. But in an actual combat scenario, what could be done to make submachine guns more powerful weapons like a carbine but have all the benefits of being a submachine gun? My assumption was designing a new round, but since I dont have enough knowledge of ballistics, I dont know what a cost effective round capable of doing this would look like. Thanks.