As most others have said, modern high velocity ammunition would easily penetrate medieval armor.
It is likely that modern shrapnel would also penetrate. The other side of this is that the wearer’s mobility is also negatively effected.
https://www.quora.com/How-effective...nd-artillery-shrapnel-in-a-modern-battlefieldThe first, and most glaring, concern, is that it would offer essentially no protection from bullets.
Modern rifle rounds punch right through steel plate, and when faced with opponents wielding automatic weaponry, the armor would serve more as a death trap than a protector, inhibiting rapid movement and significantly contributing to fatigue.