jackssmirkingrevenge wrote:A heavy piston is a killer, both of performance and your bumper.
Well, that's not as true as it sounds.
It's fairly simple to calculate the effects of piston weight on performance.
The opening force on a piston valve is easy - the point at which a piston valve starts to open has the pressure in the chamber times the outer seat area equal the pilot pressure times the overall piston diameter (i.e. the force on the piston is zero).
It's the air gaining access to the inner seat area that suddenly increases the force and "slams" the piston open. And assuming given an equal seat diameter to barrel diameter, as well as taking pressure on the projectile and piston face to be the same (it more or less will be, unless the dead volume is stupidly large), the force opening the piston is often the same as the force moving the projectile.
At that point, it's just a ratio of masses. Multiply the distance the piston needs to move to be fully open by its mass, then divide by the projectile's mass. This will give the distance the projectile moves while the piston is opening - and hence, the distance the projectile is under only partial acceleration.
At that point, HEAL's 68.7 gram piston (as an example of this type) doesn't seem quite so heavy. Sure, it's a lot heavier than many of the projectiles, but it never moves more than 20mm - and may well achieve full flow quite some time before that. (I'm not sure the D/4 rule works brilliantly for that kind of valve design) - so a 15 gram projectile might take as much as 10 centimetres before the valve reaches full flow... but at these low velocities, you don't NEED particularly high valve flow to maintain a high pressure. So it'll have effectively been at a very high acceleration for a lot of that, making it the equivalent of only a couple of centimetres difference in barrel length as compared to an instant valve.
Given I cut down the barrel by a fifth years ago (the first 80% of its length counted for ~95% of the velocity of most projectiles), meh.
As far as how hard the piston hits... well, the above note on piston force applies again. Energy is force times distance. The force and distance will be much the same regardless of piston mass, so a light piston is similarly energetic in opening as a heavy one.
A heavy one's
momentum will be higher, but it'll also strike slower, so the impact will happen over a longer time. There is a difference, but it's not as great as might be imagined.
Actually, these days, HEAL's piston doesn't even have much of a conventional bumper. Most of it is the responsibility of the rubber washers at the back...
... which partly work because of the high Poisson's ratio and coefficient of friction of rubber to work. Aside from just absorbing energy in elastic deformation, they compress outwards as the rear of the piston reaches a cursory rubber ring on the rear fitting and create friction against the valve walls. This reduces the shock of the impact on the actual fitting dramatically - and friction damped pistons will never bounce.
That said, I
will be re-engineering the piston quite a bit if I ever finish the rebuild, replacing some parts with aluminium or plastic, changing the layout to improve the effectiveness of the friction damping, and probably changing the shape of the piston face slightly.
