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All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: melloroadman on June 26, 2011, 11:52:57 AM
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Here is a chart that shows tensile strength and shear strength of pure as well as some alloyed lead . Marvin
http://www.alchemycastings.com/lead-products/solder.htm (http://www.alchemycastings.com/lead-products/solder.htm)
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Interesting chart.... If you look at pure lead, and consider what can happens with a skirted pellet.... you can see why they can deform to match the bore.... You do the calculations in the same way as calculating the burst strength for a piece of tubing.... as the inside of the skirt has the air pressure pushing outward (towards the rifling).... Here are the numbers for pure lead with a tensile strength of 1780 psi....
Caliber = .25".... Skirt thickness = 0.0625".... ID of skirt = 0.125".... Pressure required to obturate = 1780 psi....
Caliber = .50".... Skirt thickness = 0.125".... ID of skirt = 0.25".... Pressure required to obturate = 1780 psi....
You can see that for a pellet with a hollow base where the diameter of the hole is half the caliber it takes 1780 psi to cause the skirt of the pellet to expand.... Actually, since the yield strength will be LESS than the tensile strength in practice it would take less pressure than that to cause obturation.... As the tensile strength increases, so will the pressure required to expand the skirt.... For 5% Tin where the tensile strength is 4190 psi....
Caliber = .25".... Skirt thickness = 0.0625".... ID of skirt = 0.125".... Pressure required to obturate = 4190 psi....
Caliber = .50".... Skirt thickness = 0.125".... ID of skirt = 0.25".... Pressure required to obturate = 4190 psi....
Note that the air pressure required for obturation increases in direct proportion to the tensile strength.... If you make the skirt thinner, the pressure decreases.... Again, for the 5% Tin alloy with a skirt half as thick....
Caliber = .25".... Skirt thickness = 0.03125".... ID of skirt = 0.1875".... Pressure required to obturate = 1397 psi....
Caliber = .50".... Skirt thickness = 0.0625.... ID of skirt = 0.375".... Pressure required to obturate = 1397 psi....
I hope this helps to understand what happens with the typical "skirted" pellet design common to diablo pellets and why the skirt forms to the bore.... This will NOT occur with flat based bullets, of course.... as there is no "radial" force (ie towards the rifling) available to cause the lead to yield....
Bob
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How much obturation took place and for what length of time ? And was it enough to enlarge to the groove diameter when forced against the bore ?Marvin
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I was just showing the math.... I have seen significant obturation with skirted (diablo) pellets in .25 cal and smaller but I never thought to take any photos.... Pellets with extremely thin/soft skirts can even have them "blown out" to completely cylindrical.... at least in the rear part where the skirt is thin.... I believe Lloyd has some photos of such pellets.... Hopefully he can post them....
There are three methods in which a skirted pellet (or any tube) can distort.... Elastically (where it recovers once the force is removed).... by Yielding (pushed past the point where is will return elastically and therefore stretching permanently) which is determined by the Yield Strength.... and by Failing (where the load exceeds the Tensile Strength and the tube ruptures).... The barrel prevents the latter from occuring.... but any taper in the skirt.... which produces a space where the lead can stretch into.... or even the grooves of the rifling.... allow a space for the lead to "stretch" into when the yield strength is exceeded.... allowing the pellet to obturate (seal) in the bore....
There is no reason that big bores shouldn't follow the same rules of physics.... As you can see, it isn't the caliber, but the ratio of skirt thickness to pellet diameter (ie ID to OD) that governs the pressure required.... and the strength of the alloy, of course.... I found a table of the Yield Strength of some alloys as well....
http://www.boulder.nist.gov/div853/lead_free/part1.html# (http://www.boulder.nist.gov/div853/lead_free/part1.html#) 1.1.
You will note that in most cases the Yield strength is quite a bit less than the Tensile strength.... Yield strength is where the material distorts permanently (stretches) without breaking.... The Tensile strength is where it fails (breaks or ruptures).... As an example, solder that is 25% tin / 75% lead has a Tensile strength of 5770 psi.... but the Yield strength is only 3426 psi....
For a pellet skirt to stretch requires four conditions to be met.... The skirt must be relatively thin.... the material must have a relatively low Yield strength.... the pressure must be relatively high.... and there must be somewhere for the "lead" to move into.... If the skirt is too thick, the alloy too hard, or the pressure too low, obturation obtained by the pellet "flowing" into the grooves will not occur.... It does occur in diablo pellets, I have seen it first hand.... There is no reason that given the correct conditions it will not also happen in big bores....
Bob
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Many have tried with less than good results . The time for this to happen is measured in M.S. It is not the same as a fixed pressure on a pipe . What has happened in the past is when the skirt is thin enough to expand with the pressures in a PCP the skirt blows off the M.S. it leaves the barrel and there is no external resistance to contain it . And when I was swagging I also swagged for my 25 caliber condor . Again solid base and skirted with no gain in energy with the skirted . And are you sure that what you are seeing is not rifling engraving .JME Marvin
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I PM'd Lloyd about the photo of the pellet obturation.... but haven't heard back so I'll steal it from his photobucket account.... Hope he doesn't mind....
(http://i226.photobucket.com/albums/dd79/loyd500/22J-SB-exact-jumbo.jpg)
.22 cal JSB 15.9 gr. Exact Jumbo.... Left two pellets shot at 720 fps.... Right two pellets pushed through the bore....
Note that the skirt on the pellets on the right which were pushed through show slight engraving from the rifling.... The skirt has not expanded out to fill the grooves competely, however....
The pellets on the left show that the thin portion at the back of the skirt has expanded and been pressed hard against the bore, completely filling the rifling grooves.... You can even notice that outside of the skirt is no longer straight but slightly "S" shaped, and the expansion continues forward nearly half way to the waist.... That would coincide to about where the hollow on the base ends.... In addition, the rear portion of the pellet is shortened somewhat as when the pellet expanded that material had to come from somewhere so the pellet got shorter....
I have seen similar results with many diablo pellets in .177, 22, and .25 cal.... particularly on soft pellets like JSBs.... I just didn't think to phototgraph them like Lloyd did.... ;D
Bob
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Bob I am not going to argue with you about this . If you will read my thread again you will see that I stated that if it is thin enough with pcp and the more the energy the worse it gets the skirt blows off . Take a good look at how the skirt is tearing through on those pellets . The base of the bullet is more important than the nose . It must be flat with no jagged edges . Many better than I have tried and failed . Field test with accuracy as well as energy for not only I have shown no gain when the skirt is held to a thickness that will hold up to the energy . If it was as you state I would think that the majority of bullets made would be hollow based . Marvin
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Bob in a e-mail I asked you to help me figure out what the TS and shear would be on 6,7, 8, 9 bhn lead . I have the info as shown on the chart for 5 and 10 . Have you any help for me in that area .Marvin
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HI Marvin....
Not trying to argue either.... I didn't say that skirted pellets were better.... only that obturation was possible as the lead will, indeed, expand to fill the grooves in the rifling if:
A. the lead is soft enough
B. the skirt is thin enough
C. the pressure is high enough
Whether it produces a better pellet as to accuracy, velocity, or any other criteria you choose to place on it is entirely another debate.... The majority of air rifle pellets ARE hollow based.... while the majority of powderburner bullets are NOT.... My opinion is, that difference in design is because of the huge differences in pressure and temperature between the two types of firearms.... I don't see where the "skirt is tearing through" on the fired pellets, I think the distortion occurred when they hit the target.... Even if that is the case, it should be possible to vary the dimensions of the skirt to allow obturation and yet avoid "blowing off the skirt".... Since there is much less pressure at the muzzle than at the beginning of the shot, the correct combination should be achievable.... The fact that diablo pellets are the working standard in airguns (up to .25 cal. anyway) is the defacto proof of that, IMO.... Nobody can deny that they have superb accuracy at subsonic velocities....
You have much more experience in regards to big-bore airguns than I do.... If you state catagorically that flat based, alloyed bullets have equal to or better velocity and accuracy than is it possible to achieve with a hollow based, soft lead bullet I will accept that in the absense of proof to the contrary....
BTW, I do not have any "formula" which can predict the Tensile and Shear strength for lead of hardnesses 6,7,8, and 9.... I would doubt that the relationship is linear.... and I would further think that it would depend on what the alloying metal was.... Small impurites (intentional or not) can have HUGE effects on alloys.... Just look at what a very small amount of carbon can do in steel....
Bob
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Thank you Bob . And if you run into the info that I am looking for please let me know . And it would need to be alloys that are common to ammo . And Bob I once thought just as you . Bought several grand worth of swagging equipment . Proved my self wrong . Lost about a grand when said and done . Found out that many had gone that way with the same results . I now try and research as much as possible .Marvin
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Hi gents!
I've been off line for a few days and just got back to my email and such. A bunch of work and house projects had to take priority for a while.
Marvin, One thing I found very interesting about that chart you referenced is that both lead and tin are very soft in their pure state, but alloyed together, the tensile strength really goes up. I didn't realize it was that dramatic a change.
You might already be familiar with this site, or have even referenced it, but it seems to have a wealth of info about bullet alloys and hardness and age hardening and alloy selection.
http://www.lasc.us/Fryxell_Book_Chapter_3_alloySelectionMetallurgy.htm (http://www.lasc.us/Fryxell_Book_Chapter_3_alloySelectionMetallurgy.htm)
http://www.lasc.us/ (http://www.lasc.us/)
Of course its geared toward pistol shooters, rather than airgunners, but still might be a little additional good info.
Regarding the pellets in the photo, I shot those with my RWS54 into a bunch of plastic grocery bags, which is how they got a little bent up. The slight notching around the edge of the skirt is probably from the rifling as you say, because these do have thin skirts and a certain percentage of the pellets in the tin are misshapen anyway. Still, these JSBs shoot very accurately in my RWS54, but I have not shot them any farther than 40 yds. These and regular .22 CPs in the box are the 2 most accurate pellets I have found for that gun and they booth shoot about the same at 40 yds.
I am curious about the swagging equipment you used to have. Was that powered equipment or a leverage type hand press? I have never seen any swaging equipment that was big industrial stuff. What calibers? From what you are saying, I take it that it wasn't worth the effort, or cost, or didn't produce good results, and something you basically struck from the list? From the wealth of testing you have done over the years it sounds like you have pretty much ruled out a bunch of stuff as junk and found some real keepers that are reliable in most applications.
I often feel like I am still fumbling around with bullets and haven't really settled on something. Or more realistically, just haven't taken the proper approach to narrow the field down. With springers, its pretty straightforward.... take a dozen different popular pellets, try them all out and pick the one or two best for the application. With the big bore PCPs, there are just so many variables. You must have been at that point once, years ago?
Thanks,
Lloyd
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Marvin,
Have you ever thought about casting some tensile test coupons and taking them to a test lab for pull testing? I'm thinking of a place that I occasionally visit that has a nice Instron tensile tester that they use for pull testing elastomers. I've watched it work, and it's pretty slick and gives a lot more info than just the tensile.
One thing about lead would be how fast it was pulled. Pull it too slow and you'd see a lot of ductility. Maybe the results just aren't very repeatable. I don't know???
Lead is probably closer to some of their elastomers than it is to most metals that get tested. After all, you are only talking about 2,000 to 10,000 psi instead of the usual 30,000 to 200,000 for metals. I bet that most pull test machines made for metal just don't work very accurately down in that low range. They'd just be starting the pull and it would be all over.
Just a thought.
Lloyd
http://www.instron.us/wa/home/default_en.aspx?ref=http%3a%2f%2fwww.google.com%2fsearch (http://www.instron.us/wa/home/default_en.aspx?ref=http%3a%2f%2fwww.google.com%2fsearch)
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Thanks for the thought . But that would be expensive I would think . Some day the info will surface . I had a corbin s press . His newest hand press . When I got started I was in to .177 and thought that would be the way to go . But then I started in on BB . and things changed real quick . I have mentioned before some of the things about lead and the likes and dislikes of different rifles so I will not go there now . But there are so may myths about AR ammo that it is not funny . I said it on another forum last week . History does repeat its self . And the things that are being said today about AR and there ammo are the same things that were said about PB when they were changing from BP to SLP in the 40 -70 and again to the magnum loads we have today . And yes it is the big jump that a small amount of tin can do to lead that I would like to surface . Just a touch changes the strains to fill a mold as well as strengthen the lead to endure shearing . But some find that hard to believe . Just trying to help others as well as my self to understand and know the truth .Marvin
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Lloyd here is a article about bhn . Many do not know that one number jump in bhn is a large jump in hardness as well .Marvin
http://beartoothbullets.com/faq/index.htm (http://beartoothbullets.com/faq/index.htm)
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Marvin,
Thanks for the additional info. No doubt about it, history repeats itself. Always a new crop of learners coming along.
The Beartooth site has some info I haven't seen before.
One of the few things I've seen that everyone agrees on is that a tiny bit of tin makes the molds fill better. ;D
Thanks,
Lloyd
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Marvin,
I've been following this post for a few days and a couple things come to mind. One is that if you had asked me or Seth or Bob...and listened, you would not have wasted a bunch of money on that Corbin press. Corbin makes good gear but it doesn't fit the airgunners needs.
You're making the same mistake now, no matter what info is presented, or how clear the math is, you have a way of deflecting to another set of questions. The fact is that thin skirted pellets obturate and often dramatically. Lots of pictures have been posted that show the effect and your hunches about how long it takes are meaningless.... it happens!
As far as the big bore ammo goes, you really should point out that your experience is not nearly as great as your professed expertise. You might want to point out to the folks here that you bought your home test bhn tester after getting told the range lead you were harvesting and trying to deal was not pure or soft.
Since than, you've made a point of calling anyone who makes a buck selling slugs corrupt (in as many words) and you've obfuscated the facts with ignorant diversions at every turn.
You've got nothing on the line as you don't dare risk an unsatisfied customer and you present yourself as an expert when you rely on internet links instead of experience. Your "testing" is superficial and the only group you've ever posted is no better than average.
Yes, bhn can be a factor but those of us that have done this for years realize that for airguns, sizing and shape are far more important and the softest lead has big advantages (and we've all played around with harder lead). If you'd listened before your range excavation and swaging trip, you'd have learned something and saved a bunch of money.
Suggesting that high power airguns are new or even close to the BP to SLP transition is just self serving. History may repeat itself but 3000-4500psi guns won't ever be like 30-60K psi SLP magnums just because you just discovered them yesterday.
Sorry to all others here for the harsh post but this dis-information is becoming annoying and doing all who heed it a disservice.
Marvin,
Maybe you ought to point out that the bhn tester you have requires exact measurements and has never proven to be exact or especially repeatable between users as opposed to suggesting that your measurements are infallible. Maybe you ought to admit that you have no idea how long it takes for a pellet skirt to expand. Maybe you ought to admit that all your "scientific" testing has never demonstrated any accuracy gain whatsoever (at least none that has ever been shown or confirmed). In fact, maybe you ought to point out that you've hardly ever demonstrated any accuracy at all!
I'm all for experiments and do it youself work but I can't stand a bunch of sophmoric &^^& being dressed up as new knowledge. You're not an expert and you don't get to discount physics just because it doesn't fit your narrative. Diablo pellets will obturate EXACTLY as the math above describes and dozens of photos have documented. Oh, BTW, pellets don't need a flat base to get pushed out the bore.... any pellet that fills the bore has the exact same surface area to be acted on as any other...yes, a really pointed tail bullet has the same area pushing it down the bore as a flat based bullet or a hollow based bullet (go ask your high school physics teacher). That's stuff you should have learned in high school. If you're wondering, look back at your posts for the huge mistake regarding the physics... Really, it's getting so darn annoying having you post mistakes about physics or material properties that are facts and not matters for personal preference. You want a hard bullet? Great! Just don't pretend that the laws of physics are matters for your preference! And don't go all over the net demanding a high school education from folks that don't buy your story! That's just silly.
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The Beartooth site has some info I haven't seen before.
I really like the Ballisticians' Corner section of the Beartooth site. I find the Permanent Wound Channel calculator particularly interesting. There is even a roundball:caliber calculator on there.
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Yes Lloyd and Chris the internet is full of useful knowledge that will help us in this hobby .Marvin
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Thank you Jerry for trying to inform people of the truth that they seem to be blind to.