smg762 said:

what wouold your estimate be, for the specs required to deafeat lvl 4 with a small APFSDS for example a 4mm dart with 1500lbs of energy. do you think it would have any chance?

Just some numbers because I found radar data of our 20 x 139 mm APFSDS-T round.

The form factor of this 109 grams arrow is around 0.69 above Mach 1.2, but not as good in the transonic and subsonic region.

Scaling-down this arrow to .30" caliber will produce a ~100 grains projectile with a C7 BC around 0.220.

The arrow I wanted to launch from a .408 CT was very similar in weight, with a shaft diameter of 3.7 mm and a length of ~65 mm.

You will need an aluminium "pull type" sabot to launch this arrow, just like the 120 mm APFSDS, so let's say a parasitic weight of 15-20 gr.

Due to the arrow length, you will need something like the .300 Win Mag case to launch this arrow, with a MV around 1100 m/s according to the data found on the Hodgdon Reloading Center.

So, to summarize, a 100 gr projectile with a C7 of 0.220, launched at 1100 m/s from a .300 WM case, 3950 J at the muzzle, less supersonic range than the current Mk248 Mod1, and much more wind drift.

This highly hypothetical round could defeat lvl IV at extended range, but so will do a M948 SLAP launched from the same case.