Ultrasonic transducer for small metal parts inspection

I'm thinking about experimenting with an ultrasonic sensor to detect small cracks in used ammo brass cases, before reloading. I've seen at work pretty sophisticated inspection instruments, but I would like to experiment what results I can get using cheap parts, like this: http://www.ebay.com/itm/New-1set-16mm-40khz-Waterproof-Ultrasonic-Sensor-Receiver-Transmitter-1R-1T-/300669157830?pt=LH_DefaultDomain_0&hash=item46014741c6

The idea is to place the case (butt down) on the sensor, using some gel to get better sound propagation, and the use the Arduino to compare the echo pattern of a new case vs used ones.

Is there a chance that this works using this type of sensor or am I too ambitious??? Should I use other type of sensors? Thanks in Advance

Wavelength in air of 40kHz is around 8mm, don’t know the speed in brass, but I suspect it will be too long, and the transducer will ring too long to be of much use in inspecting anything as small as a shell case.

Thanks AWOL!

I've discovered after the post that industrial transducers are in the MHz area, and they are around 40 bucks in ebay. Too much for experimenting.