280/30
No, it was developed from neither. The .276 Enfield, .276 Pederson and the .256 (Japanese 6.5mm) are nothing like the .280/30.
In 1945 the Small Arms Calibre Panel was set up to determine the characteristics of the new british infantry round. They came to the conclusion that the calibre should be .276", or if tungsten penetrators were permitted, .256".
From this two cases were designed in 1947, the .270 and the .280. The .270 did not last long and was dropped in 1948. Specimens are very rare today.
The 280 was developed in a wide variety of loads until 1948 when the base diameter was altered to that of the US .30 round and it became the .280/30. This was hoped would make it more acceptable to the Americans by offering easy conversion of Garands and Brownings to the new calibre.
Development of the .280/30 (now called 7mm) continued until 1952 when new 7mm rounds were developed by Britain, Belgium and Canada (in the so-called BBC trials) to counter US opposition, These rounds were the 7mm 1st Optimum, 7mm 2nd Optimum, 7mm high Velocity and 7mm Compromise. None succeeded and we adopted the US T65 as the 7.62mm NATO in 1954.
First Photo shows (L. to r.)
.270
.280
.280/30
7mm 2nd Optimum
7mm High Velocity
7mm Compromise
7.62mm T65 (British made)
The 280/30 First Optimum is not shown, but is the normal 280/30 with the bullet loaded slightly less deeply to allow a slight increasr in propellant.
Next shows the various .276 Enfield types and the last picture the .276 Pederson rounds. As you can see, they are completely different to the later .280 rounds.
Regards
TonyE