Tuning air efficiency Q? what is a best to do?

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bigHUN

bigHUN

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Jan 16, 2020
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I am preparing my FX Impact MK2PP 700mm .25 cal (heavy modded/ported) for long range benchrest shooting but still waiting for my new Huma external adjustable regulator to arrive for fine tuning and tethering later along the line.

So far I got to 141 bars on the reg (about 240-250 bar in the bottle). Tuning with JSB MK2 Heavies 33.6 grain (sorted) because I have these the most, I have not tried slugs yet.

PW at #2 I got 916 fps with ES about 4 fps.

PW at Max I got 975 fps with ES about 5 fps.

I need about 910 fps for these MK2s (I guess) ???

Q: Leave the reg as is @140 bars with HS turned down to #2 (for 916) or make that 916 speed my Max (turn down the reg probably about 5%)?

(I understand the value of the shortest dwell time...I am looking for proven ratio)

So far the FX bottle max 240ish bar drops down to 150ish bars after two mags (2x28). Not really into saving air I just don't want to waste like a drunken soldiers, I have two 14L SCBA and one 9L Acecare fills for disposal.
 
From all that I have read and done the best efficiency has come from higher reg pressures and shorter dwell times. I used to do the opposite when I did not know better and the Impact shot just fine just not quite as efficiently.

I have tuned for the 33.95 at many velocities from 740fps to 960fps. Accuracy was stellar at 740, and 830-860 and a couple spots in between. As I pushed them a bit faster into 890s and 930 ranges accuracy would degrade.

My slug A barrel loved the NSA slug in 33.5, 36, 36.2 38.5 and the VK 34 all pretty much on the same tune as the JSB 33.95 pellets. And often shot .5 inch groups at 100 yards with the .25 liner.

It seems to me that the Superior liners like a little more speed than the older regular STX barrels. But I have some more shooting to do to prove that to myself as yet.

Scroll down a ways in the link to see the 100 yard groups at 740fps

https://www.airgunnation.com/topic/installed-my-new-700mm-slug-liner-on-my-impact-and-started-tuning/?referrer=1
 
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I have not shot long ranges with this MK2 yet, since I got it last year all gunclubs were closed and only now opening slowly, this what I am preparing for.

I only remember partially read it long ago that the .25 JSB MK2 Heavies like the 910 fps range from the STX liner, and I am not even sure do I have the A ir the B liner...I shall pull it out to check the twist rate I have. So far it did not make a sense investing into Superior liners if nowhere to practice. Also I collected couple kilogram pellets but no slugs yet, I see a shortage on these at my place.
 
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Efficiency will always be highest with high pressure and short dwell, but cranking up the pressure will seldom deliver the highest shot count because it diminishes the headroom (difference between reservoir pressure and the regulator’s setpoint).

It also can adversely affect the extreme spread, particularly if the low dwell is achieved by way of backing off the hammer spring tension. This downside can be mitigated with the valve limiter on an Impact but most rifles do not have this feature.

In terms of finding the best possible accuracy, harmonics have the final say so, like you, I don’t get too fixated on efficiency as a primary goal. It’s just a way to recognize if something is wildly off. I don’t much care if it’s 1.2fpe/ci or 1.7fpe/ci if the gun is grouping well. 
 
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I just did a quick re-tune last night in my garage and for now satisfied withe 4fps ES @ 910, at the range I will work on it more for sure when the elements start messing with the tune.

What I forgot to ask...the Edgun digital gauge...

Right now I have it in my Reg (the original FX is on the bottle side). I am getting a new Huma mini digital gauge planning in putting that one for the Reg and the Edgun back to the bottle.

Anybody had satisfactory info from Edgun calculation for the air consumption? As per design the Edgun shall give you some estimates about possible air wasting when turning the PW ???
 
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