GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: oldpro on July 22, 2022, 11:00:15 AM
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Springers and PCP guns operate on slightly different principles so my question is will adding barrel length to a springer increase FPS/FPE and will subtracting do the opposite like it does in a PCP?
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I think it's no different than a PCP in that it depends on tune or in the case of the springer the spring, and bore and stroke of the power plant. I believe in a book I read decades ago the rifle they were testing at the time only really needed 9" of barrel length.
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The pressure pulse in a Springer is very short, so there will be an optimum barrel length, beyond which there will be no gain in velocity, and eventually adding length will decrease it.... What that optimum is will depend on the ratio of swept volume to barrel length, with more powerful Springers having a longer optimum length....
Bob
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Some rifles aren't using the entire swept volume. By design, or perhaps because of reduced spring by the owner. You can't know swept volume unless you open the rifle and measure it, where the cocking slot ends x the area of the cylinder. And then that's only accurate if the piston goes right up to the end as it stops, which is really not the case. It'll slam to a stop somewhere short of that, depending on other factors. And the leftover space is practically worthless. The spring will eventually close the piston all the way, but that part is mostly useless. The spring isn't strong enough to do anything useful on its own. It's the build up momentum while air is free to shoot out the cocking slot that make all of this work.
Pellet weight/caliber is also a factor you can't overlook. A 25 caliber rifle could have the same volume (measured on the cylinder) as a 177. But if the 25 only shoot 600 fps, and the 177 shoots at 950 fps, the 177 needs a longer barrel. Even if the 25 has a stronger spring and/or longer stroke and way more power, the 177 would need a longer barrel.
If you want to boil it down to one thing, you should think velocity (of the given pellet being fired). The higher the velocity, the longer the ideal barrel length. And vice versa. That will be fairly close.
Even in the same rifle, the barrel length would be more ideal for light weight pellets or heavy pellets. A rifle can shoot both light and heavy pellets accurately, but with the heavy pellets it would have extra barrel length that isn't strictly needed (beyond the weight/balance).
Other factors change this a bit one way or the other. Gas rams generally need/use less runway for the same velocities. Also harder on scopes.
I'm sure I'll be dressed down, put in my place, and corrected with a lot of vague BS and some boast of experience resembling a job application. Coming in 3, 2, 1...
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This got me thinking more about this particular thread/rifle.
https://www.gatewaytoairguns.org/GTA/index.php?topic=201207.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=201207.0)
Despite a relatively huge swept volume, this rifle isn't particularly powerful. And despite some owners' individual experience, it seems to not be exceptionally accurate, either.
I know where you think I'm going with this. Chop the barrel, and it'll shoot straighter. Yes, I fully believe that.
But considering the large diameter piston and large swept volume relative to the stroke/spring/piston-velocity? I wonder if you couldn't make it shoot straighter AND faster by extending the cocking slot a wee bit. To increase maximum piston speed and use MORE of the swept volume on that rifle. Now if you went too far, you're slamming the piston on metal versus air, and now you're up the creek without a paddle. This would be an interesting experiment... on a very expensive rifle.
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Most of what I have read tells me longer barrel a little more power and a shorter barrel can be more accurate.
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Travis,
Below is an topic introduction, followed by a suggested test procedure (bold heading):
Principle:
What affects springer energy conversion is the shape of the air pressure curve, as a function of pellet position as it travels down the barrel. Thus, the effective expansion volume of the barrel; and the change in expansion volume as the pellet travels along it.
As Bob mentioned, springers produce very spiky short duration air pressure pulses. Energy is transferred to the pellet as the air volume behind it expands and the pressure drops. As a .22 bore has a cross sectional area 1.54 times greater that of a .177 bore, the expansion volume for a given barrel length is 1.54 times greater for the .22.
So one might expect the .22 version of a springer to produce 1.5 times as much FPE as the as the .177, if it were simply a matter of expansion volume for a given compressed air volume. Or, a simple matter of barrel length. My small sample of springers in both calibers suggest a 1.2 times greater FPE for the .22, despite the 1.54 greater expansion volume. This suggest that a .177 version of these springers would produce a partial benefit from a longer barrel, but not in proportion to the stock barrel's FPE.
A determinative test procedure starts with buying an HW85 from Krale. It is just like the HW95, but with a 500 mm long barrel, compared to the HW95's 410 mm barrel.
https://www.krale.shop/en/weihrauch-hw85/ (https://www.krale.shop/en/weihrauch-hw85/)
https://www.krale.shop/en/weihrauch-hw95/ (https://www.krale.shop/en/weihrauch-hw95/)
Then after shooting the HW85 enough to ensure the velocity it produces is stable, measure the muzzle velocity of a representative range of pellet weights. Then cut 2" off the barrel, and velocity testing it again (after a rudimentary crown for burr removal). After that, cut off another 2"; and so on; until the barrel is short enough to define the effect of barrel length on velocity and FPE. Or cut off 1" at a time (from the start, or after the data shows a significant drop in velocity for a 2" cut).
I would suggest starting with the .22 version of the HW85, as it will allow you to shorten the barrel a lot more before the velocity starts to drop rapidly. After you get to the "perfect" length for the peak FPE, I predict the .22 will lose FPE faster than the .177 per inch, because more expansion volume is lost per inch.
I predict that a 12" barrel in .22 would still produce over 12 FPE; and that only after the barrel length drops below 8" will the velocity loss per inch become significant.
You will need to use a removable length of pipe to slip over the muzzle to act as a cocking aid. I really would not enjoy cocking the air rifle after the barrel is shorter than about 15"; without help.
The barrel shortening operation should be done in a manner that does not drop steel cuttings into the barrel, compression cylinder or mechanism. If there is interest, I could explain how I would achieve that, without taking the rifle apart for each barrel length reduction.
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See charts below:
Computed and measured .177 springer velocity, VS barrel length.
And; pressure and temperature VS time.
From: www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (http://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
It is easy to infer that cutting the barrel shorter would truncate the velocity data. One could even estimate what would happen if the barrel were made longer.
Note: One can argue with the calculated values, but we may assume the measured values are real. As real as any single air rifle's measured values - even if we make no claim of that being absolutely representative. It represents at least a starting point to illustrate the principles involved.
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Somehow I and another member seem to have posted replies to the wrong thread that belong in this thread:
https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350722#msg156350722 (https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350722#msg156350722)
https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350731#msg156350731 (https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350731#msg156350731)
https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350757#msg156350757 (https://www.gatewaytoairguns.org/GTA/index.php?topic=201308.msg156350757#msg156350757)
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The pressure pulse in a Springer is very short, so there will be an optimum barrel length, beyond which there will be no gain in velocity, and eventually adding length will decrease it.... What that optimum is will depend on the ratio of swept volume to barrel length, with more powerful Springers having a longer optimum length....
Bob
Agreed, and to which I will add that the length of the compression stroke has a bearing (longer able to accelerate the pellet further in the barrel), that the spring preload can have a small but significant bearing, as can the mass of the pellet (depending on all the other factors).
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Some sort of objective numbers might help:
I wouldn't go chopping barrels to 10 inches.
This 15 FPE Air Arms springer comes from the factory with a 9.5" barrel. Possible, because the barrel is not the cocking lever. Click on the link and then the specification tab: https://www.pyramydair.com/product/air-arms-pro-sport?m=177#512 (https://www.pyramydair.com/product/air-arms-pro-sport?m=177#512)
(https://www.pyramydair.com/images/zoomed/PY-177_Air-Arms-ProSport_1630416237.jpg)
See charts below:
Computed and measured .177 springer velocity, VS barrel length.
And; pressure and temperature VS time.
From: www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (http://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
It is easy to infer that cutting the barrel shorter would truncate the velocity data. One could even estimate what would happen if the barrel were made longer.
Note: One can argue with the calculated values, but we may assume the measured values are real. As real as any single air rifle's measured values - even if we make no claim of that being absolutely representative. It represents at least a starting point to illustrate the principles involved.
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=201308.0;attach=408857;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=201308.0;attach=408859;image)
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Preload can have several effects. I've seen some posts where the velocity rises when shortening the spring. I would guess it's because that particular spring loses force when overcompressed.
In general, you would think more preload would increase velocity a little. But not necessarily in a way that's efficient. E.g., gas rams shoot differently than springs. One obvious difference is that most springs lose force as they extend. You can feel it in most of my spring rifles. The cocking force starts out smaller, and then it stacks toward the end. Gas rams are more constant. This should make the pressure spike more spikey, since that force remains high up until the end where the piston stops, suddenly. For a given cocking force, who knows which is "more efficient?" But for the same velocity of the pellet, gas rams should have sharper recoil and be harder on scopes. It makes sense that most long range competition shooters are using springs.
This particular rifle still shoots harder than I want it to. But I'm afraid of losing accuracy. Normally, I would just keep cutting coils, and if it got too slow for the barrel, I'd just shorten the barrel some more. This rifle (Hatsan Alpha) is trickier to shorten, due to the plastic shroud.
So sticking washers/nickels in your springer to increase preload might give you a couple fps, but at a cost. I have cut several springs. In one rifle, I removed all the preload, then cut it some more. If you shake the rifle, uncocked, you can feel the piston moving inside. It didn't lose as much velocity as I would have expected, and it shoots smoother, now. That's something I can't imagine how you could do with a gas piston. If you set it for negative preload, I picture the ram exploding the first time you dry fired it.
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Ken,
I think spring preload affects the equation only in the sense of affecting the average force accelerating the piston. You could have zero preload, and other than managing the spring's oscillations after the shot the effect on the piston compressing the air would remain the same. Here is an analogy:
1. You accelerate a test vehicle to 100 MPH via radio control and crash into a barrier. In this case, you keep the throttle wide open all the way to impact.
2. You accelerate a vehicle to 102 MPH, then close the throttle so the vehicle coasts down to 100 MPH at the instant it crashes into the test barrier.
Providing the impact speed of the vehicle is the same, the impact is practically the same. The additional "force" from the engine when the vehicle is at full speed is actually insignificant. It can be calculated via the engine torque through the gearing and wheel radius. This force may be a few hundred pounds, compared to tens of tons of force caused by slowing the mass from 100 MPH in the distance it takes to "shorten" the vehicle against the barrier.
Near the end of its stroke, the piston behaves like the crashing vehicle, in that remaining coil spring or gas ram preload makes little difference. The vehicle, piston and even pellet are all essentially ballistic missiles crashing into targets; albeit their functional travel distances (and reusability) are different.
You have seen how gas rams don't seem as efficient as steel springs in converting input energy to pellet energy. The average force to cock a 17 FPE gas ram "springer" is more than the average force for a coil spring version, producing the same pellet FPE. Obviously the distance your hand moves at the end of the barrel or cocking arm has to factor in, but multiplying average force by total hand travel is exactly proportional to the energy you store when cocking it.
Comparing different cocking mechanism geometries can spoil the comparison; so the same air rifle with a coil spring, compared to a gas ram would be pertinent. Cocking effort VS pellet FPE.
This general observation about gas rams being less efficient proves my point; that the force near the front end of the piston stroke is not very meaningful. It makes cocking more difficult, but adds little pellet FPE.
The reason is because this extra preload force sweeps a short distance and adds only a little piston velocity. And piston velocity is what it adds: The idea that the gas ram causes the piston to "sweep" the air out after the initial controlled crash is miscalibrated. The force trying to slow down and reverse the piston due to the high pressure air in front of it is an order of magnitude greater than the gas ram force continuing to push it forward.
Gas rams provide a crisp shot cycle without spring twang. They can be kept cocked without spring set or fatigue. They do not necessarily provide a lot more projectile energy. Not compared to how much harder gas rams can feel to cock. And the perception of actually cocking a gas ram reveals an important point:
When people buy springers online, they read that the peak cocking force is 35 lb (for example) and think that is not too much. While I agree, 35 lb near the end of the cocking stroke is a lot easier to handle, than near the start of the cocking stroke. The ergonomic ease of having your hands far apart, VS close together are not captured by "peak force".
All of that said, there is a relationship between swept volume of the piston and the barrel length (internal volume). You can't expand air in a long barrel that you did not stuff into the other end via a sufficient swept volume. If you have a large swept volume, cutting the barrel too short will waste all that extra compressed air. Caliber matters because it affects expansion volume; so the often repeated statement that "you only need" so many inches is non-sensical when spoken in absence of the other details.
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That's something I can't imagine how you could do with a gas piston. If you set it for negative preload, I picture the ram exploding the first time you dry fired it.
Correct, Ken. You can't have the gas ram slam into its internal travel limit from full speed. It would hammer itself into oblivion very quickly.
You edits (above) about negative preload on a coil spring not costing much pellet velocity prove the long winded point I was trying to make. Thanks
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Sorry, Subscriber. I have a bad habit of needing to edit my posts to make them sensible, even. (As well as occasionally trying to take my foot out my mouth before anyone else notices). At least this forum locks me out from editing after so many minutes. This is actually really good for my sanity. Good points.
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I often read my post carefully after sending them. Then see all manner of orrible mistakes :)
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Some more objective figures.
On page 82 of Cardew's book, he has the pellet accelerating only for the first five inches of barrel (though the accompanying illustration suggests nearer 6"), but he was testing with very short stoke airguns of 55mm to 65mm (the latter having an ample 45.9cc of swept volume).
Two friends of mine, one a retired chartered engineer, the other John Bowkett, independently of each other incrementally shortened the 16" barrels of .22" HW80s (81mm stroke), and found the pellet still accelerating as it approached the 16" muzzle.
I swapped the complete 'power plants' (cylinder, piston, spring, guides and preload washers) between the 13" barrel TX200 and the 9.8" TX200 HC (both .177"). One power plant had a stroke of 96mm, the other 85mm. At the shorter stroke, the two heaviest pellets, the RWS Supermag and Bisley Magnum had the same muzzle energy from both the 13" and 9.8" barrels, so both had ceased to accelerate by 9.8" but, at the longer stroke, only the Bisley Magnum failed to accelerate past 9.8". All other pellets accelerated beyond the 9.8" barrel with both power plants.
Taken together, these results strongly suggest that the length of the piston stroke plays a significant role, along with the swept volume, in the length of barrel to allow full pellet acceleration, and my TX200 test also suggests the role played by pellet mass.
Hope this is of some interest.
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Thanks, Jim.
Everything there supports the way I understand things, unless I'm suffering from confirmation bias. The only thing that surprised me is the .22 HW80 using the entire 16".
I'm still trying to figure out why gas rams have more of a brief shock, after Subscriber guided me out of the woods. And here's where I ended up:
The spring and the gas ram might have the same mass, but the gas ram is rigid. The spring is free to re-compress as the pressure spikes.
This sounded pretty good in my head. If this make the pressure spike spikier, that might be why my gas ram 22 doesn't need as much barrel as the HW80, shooting similar velocities. At least I think mine makes high 600's to low 700's with 14.3 grain pellet, going by owner data on the web. But... confirmation bias.
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More data is better.
Is it the longer stroke, or the heavier piston and the greater swept volume that goes with a longer stroke that matters?
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More data is better.
Is it the longer stroke, or the heavier piston and the greater swept volume that goes with a longer stroke that matters?
This is why I go by the velocity for a guideline comparison.
Cardew's pea shooter had plenty of volume, but not enough stroke and/or spring force. What are the numbers there? Who cares. Just go by the 500 fps or so that it shoots. Until you can put a chronograph on the piston, you don't know what it's doing when it reaches the compression chamber.
Yes, the volume of the barrel is different between calibers. And that matters, but apparently not as much as you'd think. Only a factor of 1.2 vs the expected 1.5, all else equal, in a previously posted comparison test.
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The difference in the 1.5 barrel volume VS 1.2 FPE ratio indicates that the average springer barrel is longer than required for near peak FPE. Nothing more.
A longer stroke length does make it possible to accelerate the piston to a higher speed because it travels further before there is significant back pressure. At a standard swept volume, a longer stroke also means a smaller piston diameter, meaning it would be easier to generate a higher compression pressure. It also means that the force trying to bounce the piston while there is useful air pressure acting against it would be less. So, I buy into the longer stroke argument. Or shall I say, I acknowledge the observation made by Jim.
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Oh, I had interpreted that 1.2 thing wrong.
The available "swept volume" is also easily manipulated by the length of the compression chamber.
For instance, if the rifle has a huge swept volume, but not enough piston momentum to make good use of it, you should be able to just drill some holes or grind the cocking slot longer. You basically increased the stroke length and reduced the size of the compression chamber. Piston should reach higher speed.
Yeah, Jim's observations also show that the 9.5 barrel length is kinda short for the 177 version of that rifle. The heavy magnum is the only pellet that isn't exiting too early. It should have the best accuracy potential. When the lighter pellets leave too early, they are going to experience greater variation in muzzle velocity. A slight bit of excess length ensures that things 'even out" a little better.
Of course, maybe you want to shoot magnum weight pellets and hopefully the barrel likes them. Then all is gravy. If you wanted to shoot lighter pellets, cutting down the spring should have a great chance to improve the accuracy with lighter pellets, if they aren't shooting well.
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Using an hw80 in 22, shooting at 765 fps in a 16" barrel as a reference point, here's a list of predictive barrel lengths, simply adjusting by velocity squared.
450 fps: 5.5"
500 fps: 6.8
550 fps: 8.2
600 fps: 9.8
650 fps: 11.5
700 fps: 13.4
750 fps: 15.4
800 fps: 17.5
850 fps: 19.8
900 fps: 22.1
950 fps: 24.7
1000 fps: 27.3
This actually looks fairly decent to me, until you get over 850 fps. (But do a lot of these faster rifles like JSB Heavies? Maybe the factory barrels on these magnums are designed for heavier pellets, and the big numbers are for more for comparison/marketing.)
There are plenty of other factors to consider, and maybe someone obsessed with swept volume has some of those numbers to play with.
All of the springers I have definitely gained accuracy by shortening, I ended up within 2-3" at most from these numbers. That's whether 177 or 22. I don't have any magnums. (Except for my TR-5... technically it's a bit long, still. 10" barrel shooting ~490 fps; I would have shortened it more for better or worse, but I ran out of room).
I've never lost accuracy, yet. But a couple guns I shortened were accurate to begin with. These 2 are shorter than the list says they should be, but they're still about as accurate. No chrony, unfortunately.
Actually, I should add at least one slower figure to the list. I shortened an Umarex Buckmark pistol. This pistol has no breech choke and it's a tight bore all the way through. Pellets were sometimes not even exiting the barrel. I cut it down to the end of the breech block, and it shoots consistently and accurately, now. Specs say 320 fps, and that feels right to me, with the shortened barrel.
320 fps: 2.8"
The barrel started out at 6", IIRC, correctly. I'll have to take it apart to measure, exactly, at some point. I think I removed nearly half of that.
late edit: I didn't take it apart. But measured from the outside, the original barrel was definitely between 5.8" and 6". And now it's 3.1-3.2"
It will just punch a clean chad through the bottom of a soda can.
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I said 2-3", in my last post. Let's run down the list for real.
22 Venom. I've seen chronograph numbers right at 700 fps.
700 fps:13.4" I cut mine to 12.5."
And back then I did this with a hacksaw. I'm pretty sure I made only 1 or 2 cuts. I believe I skipped right over 13.5 by a lot.
B1 rifle. I'm putting mine at 550 fps completely guesstimated by shooting a lot of stuff.
550 fps: 8.2" I cut mine to 8"
XS25 rifle. Guesstimated 450 fps.
450 fps: 5.5" I cut mine to 6.5"
Specs say 495 fps, which would be predicted at 6.7". So I'm not trying to make my numbers match. I feel like this rifle shoots only 450 with the pellets I'm using.
These are all rifles that had major problems with accuracy, and they are all accurate, now. Excluding the TR-5, which also gained accuracy, and even including the Buckmark pistol, I was withing 1" on every one!?
I swear I did not adjust my velocities to match the list. I've have these numbers sorted for awhile now, comparing my rifles obsessively. I guess I should buy a chrony.
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Your calculations that barrel lengths are proportional to velocity squared are only valid if the pressure is constant from breech to muzzle.... In a Springer that is even less true than in a PCP, because the highest part of the pressure pulse is of such short duration....
Bob
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To accurately predict how velocity is affected by barrel length, one needs the pressure profile down the barrel. Below is a pressure profile calculated by Lloyd for a specific PCP, with a specific tune. I can draw one for a springer, but that is not going to cut it :)
Ken, your velocity prediction serves as a starting point for someone contemplating a barrel chop, but it assumes constant air pressure down the barrel. A springer's pressure profile is very much a spike, rather than flat.
That said, real measurements before and after a barrel chop would be useful.
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I believe we might be looking through the kaleidoscope from opposite ends. I'm trying to calculate/predict the ideal barrel length for a given velocity. That velocity already has the pressure curve and pellet weight and powerplant factored in.
Are you looking at this from reverrse? Velocity as a function of the barrel length? Because you can do that, here, too. But it might not be what you are thinking. This would be apples to oranges.
Take the HW80, and cut the barrel length in half from 16 to 8". The ideal velocity is now square root of [(8/16)x765^2), or 541 feet per second. This is not going to equate to your PCP observation, because it means something different.
I'm not suggesting that the HW80 is going to only shoot 541 fps, now. My B1 shoots that fast with a weaker powerplant and an 8" barrel. The 8" HW80 is obviously going to shoot faster than the B1, and it's going to be wasting a good bit of air and making some serious noise. What I propose is that if you increased the pellet weight exactly to the point where no air is being wasted, the velocity is going to be close to 540 fps. That weight of a pellet might not exist, and if it did it could shoot horrible at 540 fps. But when you get to that weight where 100% of the acceleration is being used, I believe you'd be pretty close to 540 fps.
This might not be the highest energy you could make out of an 8" HW80. You might get more energy somewhere between. Wasting some air, but getting higher velocities and KE by using a pellet not quite that heavy. But probably heavier than a 14.3.
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Are you looking at this from reverrse? Velocity as a function of the barrel length?
I am trying to answer Travis's question in the opening post of this thread.
If you are concerned with wasting air, then:
If you cut a given barrel length and the velocity stays the same as with the longer barrel, then you are not wasting air with the shorter barrel. If the velocity of shorter barrel drops, then you are now "wasting air". It is a matter of degree; and what you are trying to achieve: Precision on target, adequate power, and a compact package; or bragging rights for being different?
Bragging rights with modified cars includes "slamming" them; and with trucks, "lifting" them. Such extremes don't appeal to everyone. Neither do super short or super long barrels, on pretty much anything with a barrel. However, everyone is free to do to their own property whatever they like. The rest of us more timid folk may learn something from the results.
That said; simply cutting off several inches from a spring air rifle barrel and measuring the velocity does not tell the whole story. The velocity lost due to the shorter barrel may be offset by the fact that choke is now gone; suggesting by single data point that "it made no difference". Cutting the barrel shorter an inch at a time provides much more useful data. Especially if you keep going down all the way to the breech block.
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If you have no chrony, where are the numbers coming from that you stated were close to your calculated values?
No chrony, unfortunately.
I guess I should buy a chrony.
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Cutting the barrel shorter an inch at a time provides much more useful data. Especially if you keep going down all the way to the breech block.
Yes. On my Venom, I recall being lazy. I did some googling, and it seemed safe to just cut what I recall to be about 3-3.5" off all at once. That still left a decent balance and it's still easy for me to cock.
On most of the others, I went 1" or less at a time. But not for the velocity; I was interested in the accuracy.
On the XS25, I actually stopped and shot it at 7 1/4" for 2-3 years, and I only went shorter after that pellet dried up. I only found one pellet that shot straight at this length. When the breech is already as long from the pivot as the barrel, there's not much barrel left to cock the gun with or to attach a shroud to. So I took off as little as I could, to make it work.
By the time I did the B1 and Buckmark, I already had a bandsaw. I peeled back the B1 slowly, initially trying to preserve enough length to use the cool front sight and to avoid a shroud. That didn't pan out. So I continued cutting 1" at a time until I was satisfied with the accuracy. Ditto the Buckmark. I shortened that about 3/4" per go.
Where are my velocity numbers coming from? I have a few points of reference. I am calibrated to those points, and I compare my various rifles against each other.
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On most of the others, I went 1" or less at a time. But not for the velocity; I was interested in the accuracy.
Smart move.
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And that's the cool part.
I never did a calculation before I did this. I cut until it was accurate. And graphing it out by velocity squared, I end up on the same curve as the HW80, which Jim has done surgery on and found to be about right. And which has a good reputation for accuracy.
Since there's so much doubt over my velocity numbers, I am going to try to avoid further running round the bush in circles. I did it, I shared it. And I shall strive to own a and use a chronograph before my next do-si-do around and around we go, nowhere in a hurry.
Anyone else who has cut down a barrel and improved their accuracy (and who has a chronograph to avoid the naysayers), I hope you take a quick look at my chart and I wonder if you will find you're pretty close.
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If you compare a pellet rifle to a car, the car's power delivery is a bit different from a rifle. But the acceleration of the car is very predictable. It starts out high, and it tapers off, the faster it goes. There are bumps from gear shifts and torque curves, but it pretty much starts out high and tapers off towards the end.
So if you take that car to the racetrack, say it can do a quarter mile in 12 seconds and finishes at 130 mph. If you doubled the track length, you wouldn't double the exit speed.
So for fun, let's give the car constant acceleration and suspend friction and air resistance. Now the car accelerates the exact same amount, all of the time. Say this car happens to have the exact, constant torque and mass that it does the quarter mile in... 12 seconds at 130 mph. Now, if you extend the track to half a mile, it will finish at 260 mph, right?
Of course not. Because the last half of this longer race will pass by pretty fast.
If you wanted the car to finish at double the exit speed, you would need to double the duration of the race in time, not distance. The difference in distance should be related to the difference in finishing speed squared. This is why you could estimate or calculate the length of track you would need in order to double the exit speed. You would need 4x as much distance, because 2v/1v is 2. And 2 squared is 4.
The rifle doesn't accelerate the same as either of these cars. But if you only settle on the one scenario where you are at max efficiency, pellet just leaving shortly after receiving all of the acceleration possible, then the acceleration curves will match closely. Pressure will be higher in rifles pushing heavier pellets. But acceleration is inversely proportional to mass of the pellet. Those are both higher, because they are opposing each other and cancelling out. If you end at the same exit speed, you got there the same way. The acceleration will be fairly identical.
Yes, you can slightly change the power delivery with piston mass, diameter, stroke, volume. But you also can't. There's a fairly limited sweet spot for good efficiency, which is why todays rifles shoot harder and more efficiently than before. We figured that part out. Only so far you would want to stray from that. How would you benefit?
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Actual chronograph results!
Some background in case you don't get it:
The formula is a basic physics equation that describes the relationship between acceleration and distance. It's general. It's not for springers. It can't be exact. And yeah, some springers are different from others. What you gonna do? Just close your eyes and throw a dart? Or at least make an estimate? The formula is L1 x (V2/V1)^2 = L2
The estimated barrel length is then calculated in relation to the specs I found online for a 22 HW80, at 765 fps, 16" barrel. Dunno what pellet. Dunno if that's really the ideal barrel length for that pellet/speed. It doesn't matter. It's just a data point to compare against. So L1 is 16" and V1 is 765 fps. Notice that pellet weight is not part of this equation. You use the velocity for the pellet you want your rifle to shoot.
This is the estimated "ideal" barrel length. That doesn't make it the bestest. "Ideal" is supposed to approximate the minimum barrel length that would achieve this velocity with maximum efficiency. If your barrel is longer than that, it can still shoot fine, if it's a good barrel. If it's a bad barrel that's too long, that can make your rifle shoot really bad.
(These are all the springers I have owned that that didn't shoot well; They were all way longer than they "should be" based on this model. They all improved after shortening; All good to excellent, now.)
XS25, Econ 2 7.4
1) 424.3
2) 429.5
3) 422.9
4) 419.4
5) 426.3
Average 424.48 fps.
Predicted: 4.9"
Actual: 6.5"
B1, Econ 2 7.4
1) 516.9
2) 526.2
3) 519.6
4) 521.4
5) 522.9
average 521.4 fps
predicted: 7.43"
actual: 8.0"
Venom, CPHP 14.3
1) 670.4
2) 662.9
3) 671.7
4) 673.8
5) 661.2
average 668 fps
Predicted: 12.2"
actual: 12.5"
Buckmark, Econ 2 7.4
1) 263.5
2) 269.7
3) 273.4
4) 273.6
5) 268.7
average: 269.78
predicted: 1.98"
actual: 3.1" (Can't go any lower. Wish I could try it).
TR-5
average 460 fps (got tired of doing strings, just shot 2)
est: 5.8"
actual : 10" (short as I could go; shooting pretty decent at 10" down from 16")
I tried to make this simple, for people who don't math. I reduced the HW80 data to a constant. I will call it Plink's constant.
Plink's constant: 191.25 fps/inch. Yeah, I know the units are whack, and I probably did it wrong. It doesn't matter.
So put your rifle's velocity into this equation. Estimated ideal barrel length in inches = (V/191.25)^2
If your rifle don't shoot for &^^&, and the barrel is longer than that, you should try lighter pellets. Or you might consider shortening your barrel. But be careful. If your barrel is longer than the predicted AND it shoots badly, it will probably increase in velocity as you shorten it. So your ideal barrel length will be longer than you calculated. This just give you an idea of how much you might be able to go, but you probably shouldn't chop it in one go.
You should also know that shortening a barrel by a lot will quite likely make it more hold-sensitive. Unless/until you replace that weight with a shroud.
If your rifle don't shoot, and it's shorter than the estimated, you should try heavier pellets.
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And I didn't do these calculations before cutting down these barrels. I might have done calculations years ago, on my first couple rifles, after the fact. After more googling. I'm sure I'm not the first person to use this equation on air rifles.
I certainly didn't do the calculations on the last 3 I did. I only found out the other night how eerily close many of those got to the HW80 specs. I did the calculations while posting about it. And I pulled the Plink constant because of Jim-in-UK's post about the HW80 being at least not too long, if anything. Although who knows what velocity/pellet that was with.
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Other than the Buckmark pistol, my previous estimates were all consistently high by about 6 percent. So they were totally fine for comparisons.
If the listed speed of the HW80 is inflated by more than 3 percent using real-world pellets, my guesses would have been more accurate for the comparison. They'd have already been calibrated to the marketing speeds, apples to apples.
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And I pulled the Plink constant because of Jim-in-UK's post about the HW80 being at least not too long, if anything. Although who knows what velocity/pellet that was with.
If it's of any help, the velocity figures for the different barrel lengths were expressed as a percentage of the MV at 16", which was stated to be circa 600fps, though no information of the pellet. Perhaps of note: 90% of full 16" MV was achieved at the 8" halfway mark.
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Thanks Jim
Taking your velocity data and assuming it is for a 15 grain .22 pellet;
I get 12 FPE at 16" of pellet travel, and 9.7 FPE at 8".
From the energy acquired in the first and second 8" increments, I calculated the average pressures (above that to overcome friction) for those two segments; based on the pellet base area, creating an average force to equate to the FPE increments.
The chart below shows the average pressures for the two 8" segments; with a hand drawn red line trying to approximate the actual pressure curve. The idea is to have the same average areas under the curve as the average pressure curves.
I did not do a good job of drawing the red line because I am using a mouse curser to do it, and my hand stubbornly refuses to move smoothly. It would be better to generate a regression curve, but my scribble should be enough to make my point:
The air pressure curve for a springer is very non-linear. Thus barrel length to velocity calculations need to follow a curve that sees the fast decaying pressure adding reducing incremental FPE per inch; based on the average air pressure acting along that inch of barrel. From that, calculating the velocity should be easy enough.
In the example given, cutting the 16" barrel down to 8" may only cost 10% velocity, but there is a risk that the higher muzzle pressure may start to buffet the pellet at muzzle exit - not good for grouping. Then again, muzzle pressure from FAC PCPs is much higher; but they almost always incorporate an air stripper to reduce its effects.
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Thanks Jim
Taking your velocity data and assuming it is for a 15 grain .22 pellet;
I get 12 FPE at 16" of pellet travel, and 9.7 FPE at 8".
From the energy acquired in the first and second 8" increments, I calculated the average pressures (above that to overcome friction) for those two segments; based on the pellet base area, creating an average force to equate to the FPE increments.
The chart below shows the average pressures for the two 8" segments; with a hand drawn red line trying to approximate the actual pressure curve. The idea is to have the same average areas under the curve as the average pressure curves.
I did not do a good job of drawing the red line because I am using a mouse curser to do it, and my hand stubbornly refuses to move smoothly. It would be better to generate a regression curve, but my scribble should be enough to make my point:
The air pressure curve for a springer is very non-linear. Thus barrel length to velocity calculations need to follow a curve that sees the fast decaying pressure adding reducing incremental FPE per inch; based on the average air pressure acting along that inch of barrel. From that, calculating the velocity should be easy enough.
In the example given, cutting the 16" barrel down to 8" may only cost 10% velocity, but there is a risk that the higher muzzle pressure may start to buffet the pellet at muzzle exit - not good for grouping. Then again, muzzle pressure from FAC PCPs is much higher; but they almost always incorporate an air stripper to reduce its effects.
That's a lot to take in, but I get the gist.
On the last point, the muzzle exit air pressure with my sub 12 ft. lb. Air Arms S510 is circa 195psi, and the accuracy suggests the pellet is not being buffeted, so I can't see springer air pressure causing significant issues provided the crown is true.
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Doesn't the Air Arms S510 have a shrouded barrel? Isn't the shroud air filled behind the muzzle with a baffle in front of the barrel muzzle? Or is it only the endcap on the shroud that fills the space in the shroud?
Either way, I get that a 12 FPE air rifle has less muzzle blast to upset pellets. Especially if the barrel is longer. Certainly, the 30+ FPE versions have air strippers to fill the reflex portion of the shroud.
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Doesn't the Air Arms S510 have a shrouded barrel? Isn't the shroud air filled behind the muzzle with a baffle in front of the barrel muzzle? Or is it only the endcap on the shroud that fills the space in the shroud?
Either way, I get that a 12 FPE air rifle has less muzzle blast to upset pellets. Especially if the barrel is longer. Certainly, the 30+ FPE versions have air strippers to fill the reflex portion of the shroud.
Indeed it does, and I accept that may well have a bearing, but the air can still expand to some extent inside the shroud as the pellet emerges from the muzzle.
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Hmm. Not that anyone cares, but the data on my own rifles plus physics suggests at 600 fps, you would be basically 99% there at 9.84". That would be close to ideal for the typical springer.
If at 8", it was about 90% exit velocity and down 2.3 fp, you'd get almost all of that in the next 1.84". If you look at subscribers pressure chart, you could imagine that pretty easy. After the pressure plummets at 8", you don't need much more to go.
That just shows how good the barrel was made, that the pellet continues to accelerate a few more fps over the next 6"! In two random sampled rifles! That's good workmanship. Is "a few fps" an exaggeration? Dunno. It must not have been very much, at all. At some point you cut the tail off and call it.
Out of 16" you can easily get mid 700's efficiently. No need for that at 600 fps, but now it can also shoot lighter weight pellet efficiently. That's what a good barrel give you. If you have a bad barrel, you can mostly there by trimming off the fat on the barrel and just keeping the essential part.
After cutting the spring down, my Alpha shoots the same pellets at the velocity as my B1, but with a much longer barrel. I know I could cut the barrel down a lot more and not lose significant velocity. How much more? Probably down to near 8". That's pretty consistent. But it shoots good longer, because it's a well made barrel. And in case I ever wanted to shoot 5 grain pellets, it would be good to have.
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Maybe this is the key.
It's not the powerplant why the HW80 is still accelerating at 16" vs Cardew's rifles using much less. The powerplants between the rifles is different. But not enough to do that.
The difference is the barrel.
All rifles shooting 600 fps will have a similar looking graph of pressure over distance. There will be differences but not hugely so. They will also have some pressure left in the tube beyond 10". But if the barrel is not quite right, the deceleration from friction will trump the acceleration from that little pressure left.
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My R1 (which preceded the HW80) has a 16” barrel of which only 9” is rifled.
Just get the longest rifled barrel you can find for your rifle and shoot it.
Then start loping off an inch and keep shooting.
Eventually your questions will be answered.
That is how we came up with 18” for the .22lr..
Yes the shorter barrels will be harder to click but you will have your answer.
Mike McKuen used to cut his break barrels down to 8”.
That includes .177, .22 & .25..
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To discuss the relationship between barrel length and velocity, you have to cut the tail off. The longer you leave it, the less you have to talk about.
The velocity is already high, and the additional acceleration is super low. So for gains of that additional few fps, you might need way more barrel. And then where you end up at max velocity and call that "perfect," it's only perfect for your rifle. The next rifle that came off the same line will be perfect 2-3" shorter or longer. You're leaving the results up to the whim of fate. This is like comparing and testing the accuracy of two 223 rifles at 1200 yards and declaring one rifle the winner.
After you trim the tail and stick to where each step is now a foot rather than a mile, you can make better calculations/predictions of where you need to be. And it turns out when you do that, you can fairly accurately predict the "ideal" length of the barrel just based on the exit velocity of the pellet.
Again, one rifle's "ideal" is going to be slightly different. If it's a great barrel with the perfect chokes and bores, you might want to leave on more of the tail. And go with a longer barrel for lots of good reasons. If it's not so great, you won't. But adding the tail back on after you've done with the numbers is going to be a lot more accurate than trying to calculate things with the tail still on.
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You lost me when you started talking about the "tail" of a barrel.... :o ::) ???
Bob
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It's not the powerplant why the HW80 is still accelerating at 16" vs Cardew's rifles using much less. The powerplants between the rifles is different. But not enough to do that.
The difference is the barrel.
No. The difference is the swept volume, stored energy in the spring and the piston mass.
The more air you shove into the barrel, and the more energetically you do that, the longer barrel your system can use.
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“You lost me when you started talking about the "tail" of a barrel.”
Yep
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“You lost me when you started talking about the "tail" of a barrel.”
Yep
That's a popular sentiment. So I'll take a try at this.
See the HW80 and the pressure graph, subscriber and Jim posted about.
This 600 fps rifle will shoot good with a 16" barrel. That 16" barrel is using nearly the full the pressure/energy, all the way to the last drop as it tails off. The vast majority of velocity is achieved by 10", though.
In the tail, you get variance between rifles, due to minute differences in the barrel. Insufficient breech choke or rough bores will increase the friction. So instead of gaining 8-9 fps over that last 6", you get varying amounts of deceleration. And fliers. All in an attempt to get the last drop of koolaid out of the pitcher. That last 6" is the tail.
edit: If your rifle shoots badly, and it is significantly longer than (pellet velocity/191.25)^2, in inches, then it is also using some of this tail. Accuracy of the rifle will more than likely improve if you trim some of that tail. If it did not improve by the time you get to (velocity/191.25)^2, then the source of your problem lies elsewhere, and you probably should have checked better before you cut the barrel.
You can also try a lighter pellet, to get it out the barrel before the tail starts.
If the next three posts are about Koolaid, I maybe will stop, now.
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Barrels have two ends, the breech and the muzzle.... Please don't invent your own terms that nobody else uses.... ::)
Bob
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I know that perfectly well. Did I misspeak?
Most quality springers have a choke point at the breech. If it's sized right, the pellet comes out slightly smaller after it goes through. Then the rest of the bore touches the pellet with minimal friction.
Some fewer rifles also have a slight choke at the muzzle end.
Some not-so quality rifle barrels have no or insufficient choke at the breech, and the pellet is now subject to higher friction all the way down the entire trip.
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All materials have some amount of elasticity/compression. Including lead.
If you put a 4.53 mm pellet into a 4.52mm bore, it will push back against the bore. Forever. And after you take it out, it will still be 4.53mm.
If you force a 4.53mm pellet through a 4.49-4.50mm choke point, it will permanently size the head down. After it comes back out, it will spring back to 4.52mm. Then it will glide through a 4.52mm bore without as much friction.
These numbers are made up. It'll be something like that, not necessarily exactly what I wrote. The breech choke is basically a pellet sizer made perfect for that barrel. Except when dealing with tolerances this small, they don't always end up perfect.
Up through the important bits, this friction/choke doesn't matter as much. But when you get far down into the tail end of the pressure graph, it becomes significant. If you want to use an "overlong" barrel and have accuracy, it has to have a proper choke... or you have to use very good quality ammo perfectly sized for your rifle.
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I believe this horse has been whipped enough.
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This is one of the most informative threads I've read in awhile. A good read. learning some good stuff for use later.