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This may be common knowledge, sorry if so. Was using ballistic calculator, no access to chrono so close-enough-time, contacted Hornady to see what barrel length their rated fps is for on the 6.5 creedmoor. Response was "Unless otherwise stated all velocities are checked in 24” barrels, add/subtract 25 fps to the listed velocities per inch of change in either direction for the 6.5"
I'm not sure if I knew that but I read somewhere a whole back they developed it with a 26-28in barrel in mind. But given the variables, the velocities could be identical in 26 vs 24.
Scooter
Hornady does list in their book what gun and barrel is used (as does Sierra)
Sierra uses a lot of Savages for theirs, not exclusively and military calibers (ala Mauser, Swiss) no, usually that gun.
I would guess they use Savage for most because the barrels are easy to change when they use one up!
Most all companies test in 24" barrels. The only exception I can recall, were the Ruger Compact Magnums, when they first came out, at least in part. Their comparison velocities were comparing the 300WM in a 24" barrel to the RCM in a 20" barrel. Other than that anything else i have seen referred to a 24" barrel.
With Factory Hornady Ammo I found it to be true to 24" with the cartridges I've shot.
With Handloads you can't assume your velocity will be close to that of the reloading manual. Too many factors that control pressure/velocity.
Yep, most of mine are slow. Occasionally one that is a bit faster and most of the barrel are 26 inch or better.
The 25 fps is good for small changes in barrel length, but the rate of change tends to go down for longer barrels and drops off faster for shorter barrels.
Try using this estimator for getting velocity when charge weight, barrel length, or barrel weight are changed: http://shootersnotes.com/calculator/velocity-estimator/