JP>On the subject of firing wax or rubber bullets with primers:
JP>Can anyone explain to me just WHY it is neccesary to drill the primer
JP>hole bigger in order to do this?  I understand that the primer will back
JP>out and lock up the gun if you don't, but I DON'T understand WHY this
JP>is.
JP>After all, when you've got powder in the case, the pressure generated by
JP>the burning powder tries to force itself out through the primer hole and
JP>push the primer back.  Seems to me that a primer only case should be
JP>LESS likely to force the primer back, not MORE likely.
The primer always backs out when you fire a revolver (or any other
centerfire weapon.)
What happens is in the first microseconds the pressure INSIDE the
primer pocket is high enough to back the primer out.  This is because
the primer gasses build up faster than they can escape through the
flash hole.
Then, when the powder ignites, and pressure inside the case builds up,
the whole case is shoved backward, against the frame, and the primer is
re-seated.
This is why people are so mystified about getting "high pressure signs"
with reduced loads -- if they reduce the load too much, the case will
not set back and re-seat the primer, leaving them with the primers
backed out, and the belief that "The pressure was so high it popped the
primers."
Drilling out the flash hole allows primer gasses to expand into the case
as fast as they build up, preventing primer back-out.
Of course, if you use these cases later for full charge loads, the
reverse gas flow though the now-enlarged flash hole will be likely to
blast the primer out, or pierce it, so you should always mark cases with
drilled-out flash holes to prevent accidentally loading them with powder
charges.
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