I had a chance to look at my fluids book, and I double checked my
calculations.
First, the drag coefficient is inversely proportional to the Reynolds
number, and the Reynolds number is directly proportional to velocity
(but this relationship is not linear).  That means that the drag
coefficient DECREASES with increasing velocity. I reviewed a plot of C
vs Reynolds number for a sphere and a circular cylinder, and in both
cases the drag coefficient approached an asymptote with increasing
Reynolds number (increasing velocity).
Second, I thought I'd tell you my assumptions for my calculations.  I
used a 125 grain weight and a 9 mm diameter.  I found in my fluids book
a slightly different value for air density than what I used in my
previous calculation, but the difference in density was only about 5%.
My second results were consistent with the first.  The equation I got
the second time was v = 148 / sqrt(C) in feet per second, where C is the
drag coefficient. So the question is what is C?
If you assume C is 1, then you get a velocity of 148 fps, which gives a
Reynolds number of about 27,000.  Looking at the plots for a cylinder
and a sphere show that for this Reynolds number the value of C is
between 0.4 and 1.0.  Using a value of C in that range will give you a
velocity between 59 and 148 fps.  Recall that the Reynolds number was
calculated based on a velocity of 148 fps, but if I use 59 fps, the
Reynolds number falls to about 10,000.  A quick look at the plots shows
the value of C remains virtually unchanged.  Thus I conclude that the
value of C is somewhere between 0.4 and 1 (and I suspect it is closer to
0.4, but that's just my gut feeling).
Let me summarize the results in a quick table.
C       Terminal Velocity
0.2         331 fps
0.4         234
0.6         191
0.8         166
1.0         148
The terminal velocity is much lower than I would have expected, and it
is certainly much less than it would be when it left the muzzle.  For
your reference, sonic velocity at a typical room temperature is about
1100 fps.
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