smg762 said:

apparently the 556 FABRl was 31mm so a .25 bullet should stretch to 36mm?

The L/D of the initial FABRL was 5.5 (https://apps.dtic.mil/sti/pdfs/AD0765459.pdf ; p.13) but it seems that they reduced the bullets length during the tests at 4.73 (https://apps.dtic.mil/sti/pdfs/ADA039156.pdf ; p.80).

So, depending on your taste, the .257" bullet could be 1.216" (30.9 mm) to 1.414" (35.9 mm) in length, with an ogive between 69% and 73% of the overall bullet.

A steel bullet with an AR2 shape is reported to have a weight of 5.372 g (82 grs) here https://apps.dtic.mil/sti/pdfs/AD0882117.pdf ; p.191 and +, with a mean BC around 0.201.

what was the caliber and length of this bullet? was it entirely hardened steel?

i feel a 93grain bullet which is 36mm and all-steel would have very high penetration at 3000fps velocity

By the way what was the problem with  .224 Boz 

On paper a cartridge that could be used in existing guns seemed to be the ticket.