I recently rebarreled my FN commercial 30-06 to 6.5x55, and more recently bought an autoloading rifle in 6.5 Hornady Creedmore for my business partner. The latest Revised Lee 2nd Edition Reloading book has some 6.5 Creedmore data so thought I'd familiarize myself with it and check the data for the 6.5x55 at the same time. To complete the ballistic trilogy, I looked at the 260 Remington. All of these cartridges are very close to each other which is no secret, but I did notice something new.
The new 6.5x55 data lists loads into the 55,000PSI class where old data tended to top out at 51K. This makes a significant change in the comparison between the 260 and the 6.5x55. They have become virtually identical in ballistics. In much of the data, the 6.5 moving a the same top speeds, (or faster) at less pressure, but using more powder than the 260 Remington.
Using a 140 grain bullet and IMR 4831, the velocities are within 15 fps.
The charge is 44 grains for the 260 and 46.3 grains for the 6.5x55.
The pressures, however, are 57,000 PSI and 50,335 PSI, respectively.
A comparison of minimum OALs shows that the 6.5 is not only a larger case, but the bullets are seated farther out. (The min OAL for the 260 is .250” shorter than the 6.5x55.) A combination of these two things probably contributes to the powder/pressure balance between the two cases with powders of appropriate burning rates.
Just a Christmas morning rambling.~Muir
PS: The 6.5 Hornady Creedmore edges both the 260 and the 6.5 but not by much, and operating at a slightly higher pressure, still.