There have been threads written on this subject here in the past:
Here is an interesting thread that goes deeply into the variables.
The late Bob in NH wrote what I think is a good explanation:
Re: 7mm Rem Mag at 64-65,000 PSI ?
bwinters#165561109/06/07
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BobinNH
Caampfire 'Bwan
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Joined: Jan 2007
Posts: 35,900
The real difference between the 7 rem and Weatherby is, I think, the freebore. I've built 3-4 7 Rem Mags with "long throats, that is, long enough that the 160 Nosler Partition was seated with its base even with the base of the neck. It takes a H&H-length magazine box to do this. By the way, ALL my custom barrels have been Douglas 9.5 twist, or Kreiger 9 twist. Loads were similar to the 7 Weatherby.
Set up this way, it was very easy to get 3130-3150 with a 160. Friends did the same with their rifles, so we shot thousands of rounds set up this way without a problem. I hunted with a rifle set up this way for about 10 years all across the west, no problem.
Federal emailed me in response to a similar question to say they load the 7 Rem Mag to 58,000; the WSM to 63,500 psi.
I think this pressure spike issue has nothing to do with the cartridge itself, but to the wide variation of chamber, throat, and barrel styles in the various rifles chambered to it after it came out. I say this because I have recorded chronograph data showing 2 different 7 Rem Mags giving velocities varying over 200 fps from the same ammo, same day, same everything except the barrel. I have found, for example that my Douglas barrels have given higher velocities with the same charges than the Kreigers do; said another way, it takes heavier charges in the Kreiger to hit the same velocity levels.
I have even built a couple of 7 Rem Mag barrels with "oversize" groove diameter, ie. 285 instead of 284 groove diameter.(I have done the same things with a number of 300 WM's and 270's)This actually worked very well, the barrel not being prone to touchy pressure increases as near as I could tell, and accuracy still excellent. The charges will. however, be a good deal heavier than what the manuals show.
I have generally used the cartridge with 140's at 3250-3300, and 160's at 3050-3100 without any incident. Don't know if this helps, but there it is.
link this this thread:
https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/1662047/1