I was concerned when I noticed an increase of velocity after the 1st round of loads in my 25-06 and 6.5 STW with the same loads. By using a peak strain meter I recorded 5 shots with same bullet, seating depth, powder charge, batch of brass, and primer with new and fireformed brass. I noted the peak pressures were about 5000psi greater in the fireformed brass. Presumably, part of the energy of the powder is used to fireform the brass leaving less energy for pushing the bullet down the barrel. I know somebody has made reference to this phenomena before. This has significant ramifications for someone who develops hot loads with new brass. Has anyone else tried to quantify this difference?

GRKLDOC,
Depending upon the cartridge, fire forming cases seems to require enough pressure to reduce velocities between 25 fps and 75 fps. Out of curiosity Ive taken five cases that had been fireformed and fired them over a chronograph and recorded the velocity. Went home and full length resized the cases and went back out and fired them again. Went home, neck sized them, went back out and fired them over the chronograph again. All on the same day to insure temperatures didn't change things. That's how I settled on the 25 fps to 75 fps variations. It seems as though the higher the pressures of the cartridge, the greater the loss of velocity. I thought that the reverse would have been the case. I hope that this is of some help. Good luck.

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BigBob

I believe higher pressures would cause more deformation and therefore greater change is pressure readings between new and used brass. I understand the yield stress for brass is about 64000psi (point at which plastic deformation occurs) which is why we have the existing pressure limits in modern fire arms. Perhaps if we kept pressures in the mid 50's this would be less of a factor. Unfortunately I have to change my loads after I fireform because the powder charge/velocity relationship changes and therefore the barrel vibration pattern and grouping.