Air Gun Home :: View topic - 8.2-8.5 .177 grain pellets okay for a 1000 FPS rated break ?

Amigo · Veteran Member

RSterns asks:

Lipoly · Veteran Member

Slavia · Moderator

Amigo, thanks for PM-ing the Cardew stuff. From what you relayed I am satisfied that they experimented with a stable "settled-in" gun, and that lean burn combustion was a supportable conclusion. Combustion, but not necessarily detonation.

So the question is, do heavy pellets cause detonation? Or perhaps better put, do they significantly increase the risk of detonation? Or are there other explanations for spring damage?

Amigo, I share your desire to provide good, conservative information - not just for the sake of newcomers to the sport, but also for the protection of my own equipment. Your experience plus my own add up to a resounding "maybe." Anybody else with experiences to throw in the pot?

rsterne · Moderator

I actually posted this question on the CAF.... One very respectable member / collector (has hundreds of airguns) had ruined 2 springs in a Diana 48 using heavy pellets.... Macarri told him to use lighter ones so he switched to 7.9 gr.... still on the third spring thousands of pellets later.... Shocked

I can see where it's possible for a gun that is close to dieseling with normal pellets to diesel with heavier ones.... makes sense.... I guess a few guns are "close" to dieseling a lot of the time, depending on the amount and type of lube and so a heavy pellet can push them over the edge.... Another identical gun at a different stage in the lube cycle would sail through no problem.... Rolling Eyes

So I guess it can happen.... thanks for the education.... Embarassed

Bob

Amigo · Veteran Member

Bob:

Thanks for you last post mentioning Macarri's advise.

I think your assessment of some airguns operating in a near or on the edge of detonation condition may be occurring more often than is suspected - especially as airguns become more and more powerful.

I think the info alluded to in DeadEyes reference to "excited gasses" has a lot to do with it.

Come to think of it, I don't recall ever having an airgun detonate while shooting outdoors with a chilled gun in temperatures near or below freezing. Hmmm...might see whar some espearimints cud be in order.

Whether it be life, automobiles or airguns I have found there are trade-off when it comes to pushing the performance "envelope". It seems generally accepted that a loss of a degree of reliability (for a time) is often the price of greater performance.

Please pardon my ignorance - What is CAF?

Considering the context and place of use, I figure the "F" is Forum but I don't recognize the CA.

Needin edjicatin,

Amigo

Lipoly · Veteran Member

I really hate to beat a dead horse, but...isn't it obvious if the gun is detonating? If you are using a relatively heavy pellet, and not hearing detonation, you are likely not damaging the gun (being as it seems detonation is the source of damage and not the heavy pellet)?

Slavia · Moderator

I don't think there is any disagreement about the potential for damage from dry-firing, or from using very light pellets. From the discussion it seems to me that detonation is the logical explanation for damage with heavy pellets, given that they "stay stuck" longer, allowing pressure in front of the piston to build up. More compression means higher temperatures, and thus more risk of reaching the flash point of combustible stuff in the cylinder.

I'm still not satisfied that there is a cause-and-effect relationship between using heavy pellets and detonation. I suspect it's more an interaction between a number of factors, like pellet weight, pellet "tightness," spring strength, piston inertia, lubricant type, lubricant amount, etc.

So I was thinking about this "piston bounce" business. A "too light" pellet starts moving early, creating more volume (and therefore less maximum pressure). A "just right" pellet would start moving just as the piston bottomed out, resulting in the maximum possible compression (and therefore the highest risk of detonation). A "too heavy" pellet would be late in moving, causing the piston to bounce off the cushion of compressed air, meaning that when the pellet did start to move it would happen with less pressure behind it. But - from the standpoint of detonation there would be a moment just before the bounce where pressure was at its maximum. This would imply that it's not just heavy pellets, but everything from "just right" on.

Gaylord talks about the bounce - not relating to detonation, but rather in terms of efficiency (parts 1 and 2):
http://www.pyramydair.com/blog/2006/09/piston-bounce-when-is-pellet-just.html
http://www.pyramydair.com/blog/2006/09/piston-bounce-when-is-pellet-just_08.html

So I checked it out. This is a tuned Crosman G1 using different weight pellets, and sure enough, there is one spot in the middle where FPE is highest:

Then I checked out what was going on inside by holding a microphone to the compression tube. The waveforms have been cropped to begin at the first sign of activity out to 90 milliseconds. All have the same vertical (dB) scale:

First, activity lasts longer the heavier the pellet gets. Second, the two light pellets have two groups of spikes, while the medium and heavier pellets have three (bounce?).

So, back to the question. I'm still not convinced that heavy pellets necessarily cause detonation, since it looks like some of the same things are going on even with medium weight pellets.

Other interpretations?

rsterne · Moderator

Slavia · Moderator

Back to the original question:

rsterne · Moderator

rsterne · Moderator

http://www.network54.com/Forum/79537/message/1173260392/Bone-Dry+Spring+Gun+Part+III

That is an EXCELLENT experiment.... removing the fuel should be JUST as valid as removing the oxygen as Cardew did.... While reading all the Nay-saying posts by Chris Coleman (boy does HE believe that Cardew is the Airgun God supreme!).... where he nit-picks at every possible tiny flaw.... it occurred to me that one of the variables that Cardew introduced in their experiment was the lack of moisture in the air.... They used DRY nitrogen in their experiment.... You don't suppose that water vapour in normal air flashing to steam could be providing some of the power in springers, do you?..... Shocked

.... and no, I don't think this is the main reason for a loss of 55% of the velocity they saw.... if it was, then springers would have an "optimum humidity" velocity.... Consider this my "reverse nit-pick".... Rolling Eyes

I have personally come to the following conclusions from all this.... until someone proves me wrong, which may well happen, it has before.... Embarassed

1. A SMALL PORTION of the power of properly lubed, settled in, moderate to high powered springers MAY come from COMBUSTION.... but nowhere near 55% as claimed by Cardew....
2. Modern seals and lubes have likely reduced the percentage of combustion taking place since Cardew did the experiment, which could mean that their results were valid under their conditions (leather seal, flammable graphite lube), but the effect of combustion on todays springers is USUALLY much less improtant....
3. Logic leads me to believe that Cardew's experiment was flawed in some way (perhaps the change in viscosity of Nitrogen at high pressures mentioned in one post in the above thread, perhaps the exclusion of water vapour, maybe something else).... I base this on my doubts that springers could be as CONSISTENT as they are in velocity as the lube is gradualy used up....
4. If a large enough amount of fuel is available, DETONATION will take place.... this may be in the form of smoke (rich mixture), smoke and a loud BANG and possibly flash (slightly rich mixture), just a loud BANG (ideal mixture), or simply a much higher than normal velocity (lean mixture).... This is BAD for the gun, and may cause instantaneous damage (broken spring, bulged compression tube, burnt seal, etc.).... If the process persists, the potential for damage increases....
5. If a springer is ON THE VERGE OF DETONATION, then increasing the pellet weight MAY push it into detonation for a few shots.... possibly enough to damage the spring, etc....

I'm sure there are a few who may disagree with these conslusions.... my answer to them is (in the nicest way possible).... PROVE ME WRONG.... Mr. Green

Bob

donec · Senior Member

Most of what has been said here is well beyond me but if I am understanding the concept of Ballistic Coefficients (BC) then the higher the BC the more stable the pellet will be especially in long shots like 100 yards and more. Is this correct?

Slavia · Moderator

Rsterne, I can agree with all of your conclusions.

But I still look forward to the day some guy comes up with a new experiment that either trashes or reinforces all of this. That's how science works.

Until then that propane tank is going to be smacked by heavy pellets.

As regards ballistic coefficients: It's not so much a measure of stability as an indicator of how well a projectile slips through the air. And it's not an exact science - think of it as a ballpark estimation.

Slavia · Moderator

Still thinking about this one. (I know, stop thinking and just pull the trigger.)

There are guns offered with two interchangeable barrels (of different calibers), and there are guns offered in two models (i.e., calibers) with exactly the same internals. Wouldn't that be the same as loading a really heavy pellet?

Dead Horse

Lipoly · Veteran Member

Good point, but after thinking about it I'm not so sure. Given larger caliber rifles have more volume for the air to expand in, and also taking into consideration that such rifles usually result in similar fpe regardless of caliber (although .22 seems to regularly put out a bit more), it would seem that the internal pressure would be greater on smaller caliber arms @ similar fpe.

So, my point is that it seems that loading a 15gr pellet in .177 would pressurize the system more than a .22 pellet @ the same weight in the long run b/c as the pellet moves down the bore, the .177 is evacuating much less space than the .22 and is therefore keeping higher pressure for more time. Initially, I would think both calibers would encounter the same pressure, but as soon as the pellets start to move, the .22 would reduce pressure faster due to the size of the barrel.