GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: EdinGa on October 28, 2023, 08:26:00 PM
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I was running a 1800 psi regulator and a stiff hammer spring on my bottle Bandit and it was making 18 fpe. I decided I want to tune it down some and went to a 850 psi regulator and a Chaser hammer spring and only lost 3 fpe. I was expecting it to drop considerably more. I haven't done a complete shot string yet, but I think it will be a lot higher than the 48 I was getting with the 1800 psi regulator. Live and learn, I guess, but I sure didn't expect the efficiency I'm getting. Is it possible that the short barrel is the limiting factor on the power ceiling?
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It's more complicated than just barrel length. Transfer port size has an effect ( to big or to small), dwell time, and the flow rate through the valve. Flow rate through the valve is not always optimal, it can be better or worse at different pressures.
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It's more complicated than just barrel length. Transfer port size has an effect ( to big or to small), dwell time, and the flow rate through the valve. Flow rate through the valve is not always optimal, it can be better or worse at different pressures.
That's true, and now that I think about it I was getting more power before I added the drop block and tank. The entire pressure tube is plenum now, so it isn't starved for air. I'm more interested in shot count than power so I'll probably leave it alone or maybe play with the hammer spring to drop the power a little more.
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Yes, the short barrel has a limiting factor on power output. The same as a longer barrel does but more so. This is because you want to tune the valve to be the most efficient at any given regulator set point. You want to tune so that the valve is closed back just before the ammo exits the muzzle to prevent wasted air from exiting behind the ammo. This means the valve in a shorter barrel has to close a little faster than in a longer barrel. So, not as much time for the shorter barrel to build as much pressure compared to a longer barrel.
When tuning I would find the max velocity for the current set point and then back off on that max velocity by 5% by adjusting the hammer energy. If you don't like the end result of this velocity, then adjust the reg and start over. When doing this you'll find the gun to be in its sweet spot and tune just below the knee of the bell curve. You should notice the highest shot count and the quietest report as well. You may also notice an increase in accuracy if the prior tune was letting out enough air behind the ammo before the valve was closing. This can cause the ammo to skew when exiting the muzzle.
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Yes, the short barrel has a limiting factor on power output. The same as a longer barrel does but more so. This is because you want to tune the valve to be the most efficient at any given regulator set point. You want to tune so that the valve is closed back just before the ammo exits the muzzle to prevent wasted air from exiting behind the ammo. This means the valve in a shorter barrel has to close a little faster than in a longer barrel. So, not as much time for the shorter barrel to build as much pressure compared to a longer barrel.
When tuning I would find the max velocity for the current set point and then back off on that max velocity by 5% by adjusting the hammer energy. If you don't like the end result of this velocity, then adjust the reg and start over. When doing this you'll find the gun to be in its sweet spot and tune just below the knee of the bell curve. You should notice the highest shot count and the quietest report as well. You may also notice an increase in accuracy if the prior tune was letting out enough air behind the ammo before the valve was closing. This can cause the ammo to skew when exiting the muzzle.
I think I'm pretty close now. It's quieter, and accuracy has improved. I'm not chasing power so I'll probably look at shot count and maybe go with a spacer on the valve spring if it doesn't go up.
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A spacer on the valve spring probably won't make the valve close faster, because the closing pressure on the poppet is probably greater than the spring rate, even spaced. A bigger poppet diameter would close faster, all things being equal.
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A spacer on the valve spring probably won't make the valve close faster, because the closing pressure on the poppet is probably greater than the spring rate, even spaced. A bigger poppet diameter would close faster, all things being equal.
I'm running the stock valve and spring that was set up for a 3000 psi fill. I'm using a 13ci tank with an 850 psi regulator and using the stock air tube as a plenum. I was thinking a spacer or a stronger spring might help it close faster. I shot it today and got 35 shots from 3000 down to 2000 psi at 15 fpe so I might just leave it alone as it is. Accuracy is great too. Here's a 5 shot group at 18 yards with Hatsan Vortex pellets.