I looked up Ramshot data with Magnum and 120-grain bullets:

6.5x55, 2821 fps, 55,000 psi
6.5/.284, 3245 fps, 62,000 psi

So yeah, the 6.5/.284 is getting 300 extra fps but at considerably higher pressure.

Here's Hodgdon's data for 120's:

6.5x55, 2913 fps
6.5/.284, 2979 fps

Hodgdon data for 129's:

6.5x55, 2792 fps
6.5/.284, 2910 fps

Nosler data for 120's :

6.5x55 (23" barrel), 3002 fps
6.5/.284 (26" barrel), 3175 fps

Please note the barrel length difference in the Nosler data.

My experience is that barrel length makes a difference of 25-30 fps per inch on average, not 20 fps. This is with cutting the same barrel, not by comparing different barrels.





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