...The "clone" phrasing is more in relation that it can be a multi stage pumper/PCP concept. The gun will be in bottle mode most of the time; who wants to pump all the time!?
The HDD design is similar to the o-ring idea, certainly got ideas from those, and your HDD. However, imagine having a circular ring that looks like a "V" down the profile, just as thin too. On side of the "V" rests in the gland cut, point facing the muzzle end. Like a "micro-parchute" if you will. The "bounce" causes the other side of the "V" to expand open.
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