Explosive flechette. Mechanism of detonation

Question

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?

Answer 1

You just need to tune the delay between the impact and the explosion by properly setting the primer to give the right amount of time. It doesn't take that much space, and can totally fit in the flechette.

Answer 2

is there a mechanism that can satisfy these two situations?

You want cheap, simple, obvious? Use two different mechanisms.

I'd take a page out of HEAT warhead design and use a piezo-electric contact trigger at the business end of the projectile, and two different means to make use of the on-contact current spike to detonate some explosive charge either immediately, or after a very short delay.

Depending on how fancy you want your guns, you could have two magazines (or a two-part magazine) with a switch to control current ammunition type, or rely on the operator to switch out separate magazines of contact or delay detonation flechettes or perhaps simplest of all... alternate contact with delay flechettes in the magazine and just fire a bunch of them at a time, knowing that at least 50% will be effective against any given target.

made all but the higher caliber conventional slug obsolete

way to complex and big for the said flechette. Not mention expensive.

You should consider the possibility that slightly larger than normal infantry caliber explosive armour piercing round (say, 20mm) could provide a one-hit-kill capability against your magical armour, and whilst the individual cost of the round may be higher the fact that it is significantly more effective and potentially exposes the operators to less risk means that in the long run it is more economical.

It doesn't even have to be an either/or situation. Consider the old OICW concept, and the XM29 prototype:

The fat upper barrel is a 20mm grenade launcher.