Here is my suggested improvement for the receiver end cap. Its 3D printed from rigid plastic instead of being made of rubber. This provides two advantages. Firstly the end cap doesn’t give when under vacuum providing a less mushy response in the receiver movement.
https://anonimag.es/i/EndCap1d60ad.jpg
Secondly the smooth flat surface of the end cap provides a way to integrate a valve into the cap itself. Its simply a countersunk nylon nut and bolt that holds a rubber flap (made from bicycle tire) over the holes.
https://anonimag.es/i/EndCap216d46.jpg
The valve seal is as good if not better than the standard abco one. It seems to have a much lower cracking pressure, more air throughput when the valve is open, and because of the large holes it never clogs.
Finally I should mention that the inside of the cap is slightly tapered so that when fitting it makes a tight seal with the latex liner on the receiver.
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