Inspired by Load Toad, I did a bit of back of the fag packet (OK - I used a calculator) calculation using very old O Level physics - having heard the same story myself in my youth.
I know this is very flawed, but I used 1100 fps as the "speed of sound" (OK, that's ICAN Sea Level) and 32 fps/s as g.
Using basic formulae (OK, I know that's in a vacuum!) of v=gt and d=1/2 x g x t x t, it gives an absolute minimum of about 19 000 feet, which makes claims of 3700 fps from 12 000ft seem very unlikely.
Clearly effects of air density (which would vary) and coefficient of drag (which wouldn't) would probably increase this figure - the CD certainly would.
After this my brain started to hurt ......