In a world I am currently conceptualizing, the militaries of said world make use of flechette ammunition due to advance ballistic armor which has made all but the higher caliber conventional slug obsolete. And while the flechettes don't penetrate 100% of the time (usually through deflection), when they do hit, they tend either A) get stuck in the armor or B) go into the body but make a clean hole that don't do much damage internally.
To solve this issue, an explosive packet has been placed within the flechette in order for it to detonate either with in the armor so that the next (or several) flechettes will penetrate or for it explode inside the targets body.
The issue I'm running into is that the two mechanism I know of that would detonate the the explosive package would explode on impact which wouldn't give the flechette enough time to burrow into the armor to remove a chunk of armor and wouldn't work for soft flesh; or use a timed delay system that would be way to complex and big for the said flechette. Not mention expensive.
So my question is, is there a mechanism that can satisfy these two situations?