I made a little U-shaped HDD like the one he sells for my QB78D, used it for some hundreds of shots. It seemed to work fairly well. Then j made an SSG and efficiency went up significantly, gave me more shots per fill. I'd recommend you consider an SSG instead. The adjustability is important so as to establish the right power level and valve dwell. Have a look at rsterne's numerous detailed posts on various forums to learn more about these marvels, which give airgunners much more refined control over hammer behaviour.

http://www.gatewaytoairguns.org/GTA/...pic=102095.840

It's a bit trickier with the QB78D as the SSG must be entirely internal unless you also convert it to cocking on open. Here's a page where I describe mine along with various other modifications.

http://gatewaytoairguns.org/GTA/inde...topic=113196.0