Brad,
Yeah, I wanted to ask that myself. Have owned several 6.5x55's on modern actions and various barrel length, and haven't yet owned one that got 2900+ fps with 140's. In fact the 6.5-06 I had for a while, with a 26-inch barrel, only got around 2950 with 140's.
On another subject, here's a post I made on this subject in 2014. Robert White provided a link for the thread earlier in this thread, but I want to add some further comments at the end:
"I've owned multiple rifles in all three chamberings, and my take is this:
The 6.5x55 is capable of the same sort of accuracy as the other two, and can exceed their velocities slightly, since it has more powder room and normally comes in actions with plenty of room for seating bullets out. But the chambers and brass can vary considerably, and there aren't any full-power factory loads. Most American factory rifles (when available) only come with 1-9 twists.
Many .260's also come with 1-9 twists, but the chambers and brass are far more uniform (though brass quality can vary). Cases can also easily be made from various other .308-based rounds. But if you want to shoot very high ballistic-coefficient bullets the standard "short action" magazine is a little too short for optimum seating depth. If you onloy plan to use standard hunting bullets, it works fine. Factory loads tend to be limited these days.
The 6.5 Creedmoor solves the magazine length problem of the .260 by using a slightly shorter case with a wider, steeper shoulder. Brass quality is excellent, and the factory loads are very good, though sometimes hard to find. The 1-8 twist for ultra-long bullets is common, and every factory rifle I've shot in 6.5 Creedmoor has been VERY accurate with both factory ammo and handloads.
For the record, the only one I own a rifle for right now is the 6.5x55, but it's a custom rifle built on an FN Mauser 98 action with a 1-8 twits Lilja barrel, using a tight custom reamer. Most American factory ammo or brass won't fit in the chamber, but Lapua brass does. I've had it for a while, but if I hadn't would have kept the Ruger 77 Hawkeye in 6.5 Creedmoor I purchased the first year they came out, since it was just as accurate."
Since then I've acquired a .260, a Tikka T3 Lite from Whittaker Guns. It's nicely accurate, but mostly I like the lightness and balance in a factory rifle. However, as with many .260's I couldn't seat some bullets out to the lands, due to magazine length problems, and measuring the throat length indicates it's SAAMI standard. Not all of these bullets were super-high-BC types, since among them are several "conventional" plastic-tips. So I eventually modified the magazine and bolt-stop to allow rounds with an overall length of 2.92 inches, which allowed me to seat any bullets to the lands, even 140 Berger VLD's--and accuracy did improve with most loads.
Right now I also have two 6.5 Creedmoors, a Ruger American Predator and a Weatherby Vanguard Synthetic. Am still working on the Weatherby to get it to quit stringing shots vertically, though even with the stringing it still puts 5 shots in an inch. The RAR Predator is the most accurate of four 6.5 Creedmoors I've handloaded for. It's first 5-shot group at 100 yards, using 140 VLD's seated just shy of the lands and 41.5 grains of H4350, measured .33 inch--and that's as-is from the factory.
That said, one of the dirty little secrets of the 6.5 Creedmoor is that SOME bullets still won't seat quite out to the lands in the standard 2.84 inch short magazine. The above group's rounds had the bullets were seated to 2.88 inch overall length, which won't fit in the magazine. And yes, the throat is standard SAAMI length, as is the throat in the Vanguard, though the throats of the two rifles differ by .004 inch.
Of course, this problem can easily be solved by not using certain bullets. Some of the other Bergers will seated out to the lands at less than 2.84 OAL, as will the 139 Lapua Scenar. But the fact remains that the 6.5 Creedmoor's shorter case does not totally solve the problem of long-ogived bullets in short magazines--and with a little throat wear the problem becomes worse.
Still have my custom "classic" 6.5x55, built on a commercial FN Mauser 98 action with a medium-weight Lilja 1-8 twist barrel, and a stock made of New Zealand walnut. It was chambered with a minimum specs PT&G reamer with a short throat, so there's not problem seating ANY bullet out to the lands, with plenty of magazine room remaining. On the other hand, handloads in Lapua brass don't shoot quite as well as handloads in Hornady brass do in the Ruger American Predatot. In fact, I've never come up with one that shoots anywhere nearly as well as that first group out of the Predator--but the 6.5x55 is a lot prettier.
