Originally Posted by codybrown
What's the difference between the two? Any difference in ballistics? From what I understand from the names they're both based off the 30-06 case, one's just a .257 and the other is 6.5mm. From a metric to standard conversion standpoint a .257 is equal to 6.527mm, so theoretically they are about identical from what I can see. Am I missing something here?
You need to be aware that 6.5 mm bullets have a diameter of .264, so they are larger than 6.5mm Its just called 6.5mm.
Now, in and of itself, .257 and .264 are really close. So are .277 (270) and .284 (7mm)
And in both cases, we have pairs of 30-06 based cartridges. 25-06/6.5-06 and 270 Win/280 Remington. Their powder capacities are very similar, as you note.
But in both cases, particularly with .257, the lower popularity of the .257 caliber means that bullet availability is much much poorer.
And in the .257 and .264 comparison, there are two very important historical issues. The 6.5x55 swede, sort of the pacesetter for 6.5mm cartridges was originally loaded with a very heavy long bullet for military use. This required "fast" rifling to spin the bullet fast to stabilize it. So 6.5 class bullets have been able to be shot in many 6.5 guns that have had fast rifling. Consequently, a lot of really excellent, heavy bullets are available.
For the .257, the opposite is sort of true -- the 25-06, was more of a varmiter gun that could take a deer when needed. But the rifling twist is more optimized for lighter, varmint class projectiles.
So at the end of the day, off the shelf guns and projectile selection differences between a 25-06 and 6.5mm based guns are very different.
Now, you could custom build a 25-06 with a fast twist, but there are few heavy aerodynamic bullets out there, so you would be far better off for long distance work to step up to the 6.5-06. Conversely, building a custom 6.5-06 and using a slow rifling twist (better for light bullet work) also makes little sense -- you would not be able to take advantage of the big heavy 6.5mm bullets. Building a 6.5-06 varmint gun makes little sense.
Hope this makes some sense....