fire forming question
I am fire forming 19 Calhoon cases using 13.2 gr 1680. The MV is 3085 fps. Using a already fireformed 19 Calhoon with more capacity, the same 13.2 grs of 1680 gives 3258 fps. How can the larger case give more velocity with the same powder charge?
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If I understand you questions correctly, perhaps some of the energy of the powder is used to expand the case. Also, the flow of the gases during this process would probably be slowed. 0 -
Is this repeatable? Ambient temp the same for both tests? Chrono at the same distance? Same brand and lot of bullets and powder? How many samples did you test? 0 -
One thing to consider is that the "pre-formed" case volume doesn't mean much, as the effective volume is controlled by the chamber (minus the brass) volume 0 -
mbsams...yes it was all done the same day minutes apart. Same case, bullet and powder lot. Average ten shots each. Velocity is the average for each group. 0 -
I fire form 9mm Para to 9mm MAK a couple times a year. The cases are always full of powder stains. The next loading is always cleaner.
Are your form fired cases dirtier than the others? Your probably just geting some blow back. Combine this with rsnyder55's observation and it could explain the 173 fps difference.
What's the speed with the second loading of the fire formed cartridges?0 -
The fireformed case gives an excellent seal of gas pressure and correct headspace. The virgin cases do not. The expanding gas pressures are slightly reduced as the brass is formed with some escaping rearward resulting in dirty cases. Therefore, lower velocities while fireforming. Some recommend fireforming with max loads and bullets seated into the lands to achieve perfect fireformed brass. 0
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