Armor Piercing Flechette is +2P damage, no AP change and resisted by Impact armor, 18F and cost 18 nuyen per bullet. Oh, and they reduce the effective range of the weapon by 20%.
So the The average Arsenal shotgun becomes a 9P(f) weapon with -1 AP (against Impact Armor) and ranges of 8/20/32/48 meters. You know what, I'll stick to being afraid of the guy shooting SnS out of his sniper rifle... 6S(e) + net hits vs half-impact armor... and for 8 nuyen (less than half the price) at 150 meters without penalty.
But, if you're all that worried about it, drop a few nuyen into the SoftWeave PPP System, the cheap and easy way to bring your impact well above your ballistic armor. Impact typically only trails Ballistic by 2 points, it's not that big a shortfall.
And don't forget, nothing stops you from putting that AP Flechette into the new Battle Rifles, modded of course, to fire full-auto. You know, if you just want to get nasty. Or an HMG, for the -3 AP, I recommend the Stoner-Ares M107.