Dude, there is a 2 grain difference in case capacity between the 260 and 6.5x55. 2 grains = less than 4%. A <4% increase in case capacity will generate a <1% increase in velocity, assuming all other factors are equal. I think that when I ran the numbers, the potential velocity difference for a load at 3,000 fps would be about 28 fps.
The fact is, when loaded to the same pressure, the 260 and 6.5x55 will produce the same approximate results with all bullet weights up to an including 140 grains. Once you start using heavier/longer bullets, the long action 6.5x55 has an advantage over the short action 260 because of bullet seating depth and COAL issues, but not a lot. Simple solution, build your 260 on a long action and you eliminate the COAL issue, so they are equal again.
Where did you get the idea that the 6.5x55 case is stronger than the 260, such that it will allow the 6.5x55 to "handle HIGHER pressures than the 260"?
This is 2011 and bullet technology has improved to the point where lighter premium bullets will allow a reloader to achieve the sort of penetration on game that used to require a longer cup & core bullet of the sort that made the 6.5s famous. If it wasn't so, Hornady would still be making and selling the 160 grain RN, but they aren't.
Although I think that ESPN's Colin Cowherd is a horse's ass, his pet saying, "Live in the is, not in the was.", seems like an appropriate response on this topic.
JEff
