smg762 said:

case is about 49mm tall, and the CETME bullet is almost entirely outside the case - there's about 4-5mm of bullet in the neck.

So, you want to use a flat-base bullet? I tested recently a 5.56 mm flat-base bullet that flew very well.

Scaled up to .257" cal, the bullet caracteristics would be:

• 32.6 mm length, 22.4 mm ogive, 0.85 mm meplat.
• 92 gr steel bullet and a C7 of 0.207.

You will need to run at a very high pressure to reach 970 m/s and 2800 J of muzzle energy.

A 49 mm case + a 32.6 mm bullet length - 5 mm of bullet intrusion = 76.6 mm cartridge.

i was hoping for an extremely long CETME bullet - 36mm. 

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

i'm aware the round would be very long - more than 762 NATO

assuming a CETME or FABRL shape, i feel that a 93 grain bullet would have a very high BC.

the 7.92 CETME was 105grain, and apparently had a similar BC to 762 NATO. 

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.