I don't know about using a pendulum, and the ballistic gelatin is too inconsistent for accurate velocity measurements.
I use a chronometer.
VERY simple device - there are two photoreceptors placed a set distance apart, both with white screens above them to serve as a backdrop. Any object passing across the first photoreceptor starts a timer, and the timer stops when that object passes the second receptor. Because the distance is known, the chronometer is able to calculate velocity.
An inexpensive ballistic chronometer will set you back about $90-$100. add features like multiple shot memory, printers, or large LCD displays and the sky's the limit!