Why Does no-one make a Rail Mounted Piston System?
Two things: first, you want the DI redirection of gases to be contained as they will be going very fast and can cause damage. To do this, something must create a tight, and robust ( see handles the pressures well) enough clamp to the barrel. This is traditionally the block which is bolted or pinned to the barrel of your DI rifle over a drilled hole. So since the block is already there to capture the gases, the rest of the system is there on the barrel as well. Even on the most secure of these, you often see slight carbon offshoots from the escaping gasses barely getting through the gap. In your design, if I understand correctly, you would somehow redirect the gas to the piston chamber in the rail. You would still need to somehow direct the gases from barrel to the rail mounted block, and this would have the same structural effect as a barrel mounted block. It would be rigid by necessity.
Second, piston systems are both harder on the hardware due to the impact forces, as well as the vibrations caused during firing. To place the piston chamber off the barrel and direct gas upwards into the chamber would mean moving the impinged gasses not coaxial to the bore and then moving the piston's mass at a 90 degree angle much higher off the bore axis than one mounted to the barrel. This would be a non-ideal design decision from a mechanical engineering perspective. Generally speaking, the less movement of parts which are not par-axial ( not in line with) the bore, the better. The more movement and thus vibration you introduce in the firing process tangential to the movement of the bullet, the more you deteriorate accuracy of the system. From what I gather, your design would not introduce any significant benefits whilst making some significant engineering assumptions which could pose problematic. How do you both seal the gas at the barrel, and redirect it to the piston block in the rail? Maybe I missed something and you have a different view than the one I was able to visualize.
One concern I just thought of as well is for an AR style platform you would need a exact rail length to match each barrel length, and this is not practical given the barrel width and length combinations possible.
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