Other than what Mule Deer said, I don't think you can predict it, there are too many variables.
I don't shoot factory ammo except for chronographing someone else's stuff. Last year that amounted to shooting up a box of 300 WM for sighting in a friend's rifle with a new scope and, then, reloading the brass. They were 180 Win. PP, which averaged about advertised vel. But, the extreme spread was something like 100 fps! Most were close to the promoted 2960 and about 5 were well over 3000... a couple hitting about 3050!
A number of years ago, I sighted in another friend's 270 Win with 150gr Federals. Most didn't make 2800 from a 24".
Then, there was my experience with a new .35 Whelen. I wanted to take it hunting before the dies came in, so I bought a box of 250s. As advertised they were supposed to make 2400 from a 24". They averaged 2247 from my 22".
The only 340 WBY factory 250gr round through mine was about 2840 fps. My handloads were averaging 2997.
But, I agree, I think factories are more aware of customers expectations and are generally doing a better job with their ammo.
Edited to add: The range, at which I've been a member for over 20 years, now has over 700 members, the vast majority of which are hunters. Many are handloaders, but few of those actually chronograph their loads!
Bob
www.bigbores.ca
