N/A Pressure and Accuracy

[Dr. Kralenstein]

So I've noticed on a couple of my guns that when the pressure drops, my accuracy will diminish, even though the velocity will stay roughly the same.

Anyone else experience this? It's especially noticeable on pistol length barrels, not so much on my rifles.

I will shoot a string over the chrony and see how many shots for a curve, and then test accuracy over that shot count and pressure setting. During the climb of velocity and through the peak, my POI is dead on. As the pressure comes down but the velocity hasn't dropped yet, my POI will shift in the direction of the rifling, and then after the fps starts to slowly drop, the pellets wander high and low.

What gives? I would expect a vertical drop and maybe some drift, but not shot gunning. I've had other guns that the POI will just slowly drop and usually go left or right in a line, running with the spin drift of rifling.

Is this caused by valve dwell since the hammer is hitting harder at the lower pressure, and maybe a longer burst of air is causing turbulence?

 

[Dairyboy]

Your last sentence was my thought maybe. At higher pressure it's more of a sip of that high pressure air to get the same speed as your low pressure your letting more air out to equal the same speed. So my thoughts are could be causing some extra "dirty" air messing with the accuracy

 

[MushroomPsyche]

I tuned my gun to 31 FPE. I have a Taipan Veteran Long and since the barrel is indeed quite long, the regulator ended up at 90 bars for the 31 FPE tune. Initially, I had a hard time getting pellet-on-pellet at 25 meters. After a few months, I removed the stock barrel shroud and put a DonnyFL Koi directly on the barrel threads. Immediately, I noticed much improved accuracy. I think this is the same thing you're seeing... When the pressure drops, the dwell time increases.

 

[Caliber 22]

Is this caused by valve dwell since the hammer is hitting harder at the lower pressure, and maybe a longer burst of air is causing turbulence?

Correct. Like the others also said. My wife's unregulated, non hammer adjustable rifle does the same.

Sometimes with regulated air rifles and pistols with a tune much lower than the knee and it goes off-reg, it will first shoot faster and higher before velocity drops and shoots lower. Once it goes off-reg accuracy may suffer, not always. I did not experience it myself but saw others with the problem and same explanation.

 

[Dr. Kralenstein]

The gun in question isn't regulated but is shrouded. I will get out today and try it with the shroud and see what happens, and then try with my Donny Koi. It's a floated barrel as well, so no barrel band attached to the air tank to cause the issue.

 

[Caliber 22]

Jip, like my wife's PCP which is behaving the same, as mentioned above.

What I did before was to reduce the spring tension by removing the spring guide. That reduced the FPS and FPE so I hade to test for another pellet. It was more accurate and gave more shots, but since the FPE reduced a lot it was not good for hunting. So I changed it back.

 

[Feinwerk]

If you are using a shroud, you could experiment with adding vent holes in your shroud back near where the barrel meets the receiver. Some of my rifles have this and I assume it's for helping to strip air from behind the projectile as it expands backwards from the baffle area into the shroud. My FX independence shroud is vented this way. The hole is on the right side so it vents away from me when I shoot right-handed. You could start with a 1/16 hole or something smaller using a numbered drill bit and have it vent on the side opposite of your head when you shoot normally to help reduce perceived noise .
I did some shroud venting experiments with my Benjamin Marauder pistol and found some improvement. I did have to tape over one hole that was too large so it can be undone but it will leave something that shows.

Something to think about. Good luck with your setup.

Feinwerk

 

[JamesD.]

You could always try a step down in-line. Bellville like this or a $400 diaphragm more consistent type.
What people normally find is that placement can change on low to higher pressure but; you will see 2 power spikes. Around 1500-1800psi it will jump (hammer and spring being the same) and then whatever starting fill pressure it is set for (2600-3200?) can vary.

Now if you decide to run it at low or a higher pressure you are going to notice the difference in sound. The wastefulness of air or dwelling of the valve depends on slug weight/OD, flat, cup, hollow base.

I had a diaphragm one for the .257 Scandalous and when air is right that’s easily a sub 2moa at 300, sub 1/4 moa @100, I just need a better top tier optic than my 16x SWFA
The Lilja barrel & Griffin slugs are perfection but, it’s the stability, air pressure, harmonics, optics and shooter that make it great or just good.

 

[Dr. Kralenstein]

I spent last night trying different springs and pressures to see where I could get my best accuracy and power/shots.

I have a 300 BAR fill on this gun, so I ended up turning the power up to around 32 fpe and it flattened my shot count out to around 840 fps with the 21gr baracudas. I'm getting 4 mags/48 shots out of a 350cc bottle and the 13" barrel, before the accuracy issue starts to happen. It's not as abrupt as it was with the other combos of springs I tried. Goes through 50 BAR on 48 shots, but that's plenty fine for hunting and pesting with. I get almost a full extra 12 shot mag after the POI shifts, and these shots are predictable enough for usage.

Here are some groups at 35 yards.
This large group was towards the end of my shooting to show what I mean by the downward right drift.

This next pic is 3 separate 5 shot groups with a fresh fill.

Here was a quick run of a full 12 shot mag

These groups were all shot from a seated position using only the bipod amd shouldered no rear bag.

Here is the gun in question.

 

[TYLERHASKINS2]

I think you're correct with your last statement about dwell time at lower pressures. this is not something I see with my airforce TalonP (12 inch barrel 25 caliber) for the most part as the pressure drops so does the group, then when the pressure gets really low (probably about 1500PSI) then the group opens up a bit but since that's the end of my shot curve ive never really looked into it much.

 

[Caliber 22]

If you are using a shroud, you could experiment with adding vent holes in your shroud back near where the barrel meets the receiver. Some of my rifles have this and I assume it's for helping to strip air from behind the projectile as it expands backwards from the baffle area into the shroud. My FX independence shroud is vented this way. The hole is on the right side so it vents away from me when I shoot right-handed. You could start with a 1/16 hole or something smaller using a numbered drill bit and have it vent on the side opposite of your head when you shoot normally to help reduce perceived noise .
I did some shroud venting experiments with my Benjamin Marauder pistol and found some improvement. I did have to tape over one hole that was too large so it can be undone but it will leave something that shows.

Something to think about. Good luck with your setup.

Feinwerk

Thanks for that. Now that I think of it, my rifle has an air stripper on the end of the barrel inside the shroud. I put some O-rings over the barrel about 10cm from the crown, big and thick enough to press against the barrel and the shroud so it help reduce barrel harmonics and it helped a lot. I then added another O-ring about 5cm from the crown and since then the long distance accuracy reduced some, but I did not connect the two as I did other things at the same time. It might be the volume for the air stripper that reduced that caused the reduced accuracy. I am going the remove the 5cm O-ring to see what the result will be.

 

[nobody1]

All of these questions and problems do not exist in a regulated air gun.

I am really trying to not buy any more unregulated air guns

In the past (5-10 years) the excuse to buy an unregulated air gun was that it had more power.
I am not sure that is the case now.

 

[Vetmx]

The only gun I’ve ever experienced this with is my Kral Empire XS. Yes, basically a little longer than pistol length barrel. I’m at my camp right now so I can’t look at my notes but I believe the gun starts to shoot slightly low when it starts to get blubbery. I think maybe the last 8 or so shots before I hit my refill number. Still accurate, but the ragged hole starts to move downward at 30 yards.

I was briefly perplexed by it but the gun is a shop gun so it’s scrutinizing days are over. I’m not missing a critter because of it but the issue is there. If the gun wasn’t so darn accurate most wouldn’t even notice it.

 

[Vetmx]

All of these questions and problems do not exist in a regulated air gun.

I am really trying to not buy any more unregulated air guns

In the past (5-10 years) the excuse to buy an unregulated air gun was that it had more power.
I am not sure that is the case now.

Regulated guns have their own issues. Go read all the M3 and DRS topics that are reg related. My Edgun Lelya had a chronic reg issue. Actually so do RTI’s and a lot of other guns. Regulated, unregulated, it’s just the PCP life. Some issues from time to time.

 

[Dr. Kralenstein]

The only gun I’ve ever experienced this with is my Kral Empire XS. Yes, basically a little longer than pistol length barrel. I’m at my camp right now so I can’t look at my notes but I believe the gun starts to shoot slightly low when it starts to get blubbery. I think maybe the last 8 or so shots before I hit my refill number. Still accurate, but the ragged hole starts to move downward at 30 yards.

I was briefly perplexed by it but the gun is a shop gun so it’s scrutinizing days are over. I’m not missing a critter because of it but the issue is there. If the gun wasn’t so darn accurate most wouldn’t even notice it.

This issue has only happened on my short barreled Krals as well. Interesting...

 

[Dairyboy]

Barrel length makes sense if you think about it. Less runway to use up that longer dwell pellet is probably out of the barrel before the valve is closed

 

[HogKiller]

Your last sentence was my thought maybe. At higher pressure it's more of a sip of that high pressure air to get the same speed as your low pressure your letting more air out to equal the same speed. So my thoughts are could be causing some extra "dirty" air messing with the accuracy

Yeah what he said, the key is THE right amount of air, consistently. Enough to get it out of the muzzle and not a lick more. There is a fine balance between air pressure and hammer/valve dwell/opening.

 

[Stubbers]

I'd say this phenomenon is not common. Most unregulated pcp's have the same POI at their starting pressure as they do their ending pressure when tuned for a typical bell curve.

850 fps @ 3000 psi may have a dwell that is around 1/2 to 3/4 of a ms. Where 850 fps @ 2000 psi may have a dwell closer to 1ms. That 1/4-1/2 ms shouldn't be detrimental to POI. In an event they are...

The culprits could be:

1) A loose floppy / free floated barrel being effected by harmonics, as the projectile leaves the muzzle from the 1/2 ms shot @ 850 fps it may be at it's highest point, and @ 2000 psi with 1ms+ dwell time, it may no longer be.

Solution: Stiffen the barrel and/or unfloat it.

2) Excessive pressure and volume of air at the muzzle due to longer required dwell time at lower pressures.

Solution: Get a longer barrel, strip the air better with a really well made moderator/air stripper combo, or tune the gun to shoot less violently at the low pressure. If the tune is too hot for the end of your fill pressure, either re-tune for less power or don't shoot down so low.

-Matt

 

[Dr. Kralenstein]

I'd say this phenomenon is not common. Most unregulated pcp's have the same POI at their starting pressure as they do their ending pressure when tuned for a typical bell curve.

850 fps @ 3000 psi may have a dwell that is around 1/2 to 3/4 of a ms. Where 850 fps @ 2000 psi may have a dwell closer to 1ms. That 1/4-1/2 ms shouldn't be detrimental to POI. In an event they are...

The culprits could be:

1) A loose floppy / free floated barrel being effected by harmonics, as the projectile leaves the muzzle from the 1/2 ms shot @ 850 fps it may be at it's highest point, and @ 2000 psi with 1ms+ dwell time, it may no longer be.

Solution: Stiffen the barrel and/or unfloat it.

2) Excessive pressure and volume of air at the muzzle due to longer required dwell time at lower pressures.

Solution: Get a longer barrel, strip the air better with a really well made moderator/air stripper combo, or tune the gun to shoot less violently at the low pressure. If the tune is too hot for the end of your fill pressure, either re-tune for less power or don't shoot down so low.

-Matt

I'm content with 48 shots at 32 fpe and then refill. I'm done tuning it. It likes expensive pellets!

 

[Dr. Kralenstein]

I just ran a 5 shot group of polymags to see if they would group going over 900fps. Yep!

I didn't adjust my scope at all, just aimed at center where it's dialed for the baracudas. The fliers on the left side target were baracuda hunters. Odd, they group great at 800 fps out of this gun, now they're running close to 900 and open way up.