GTA
All Springer/NP/PCP Air Gun Discussion General => Projectiles => Boolit and Pellet Casting => Topic started by: ken47 on November 09, 2018, 03:09:13 PM
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I was designing a.30 cal pellet for upcoming swaging mould. The pellet was modelled around a JSB and simulated in a 30 cal Lothar Walther barrel. It was barely engaging the rifling. Then I made some changes in the diameter and checked it. It engaged. I referred to (http://www.canadianairguns.com/showthread.php?945-9mm-357-JSB-Exacts-and-Upsizing-the-30-Cal (http://www.canadianairguns.com/showthread.php?945-9mm-357-JSB-Exacts-and-Upsizing-the-30-Cal)). Rsterne you were right.
The difference 0.08mm is huge.
What do you guys think of.30 JSB's in LW barrel?
The pellet in the front is a JSB model, followed by my design and followed by Sub's design. Had to resize the pic.
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I tried the JSBs in a LW barrel and found them very loose (the head barely contacted the rifling), and Tim at Mac1 airguns tried both the TJ's barrel (which he used to win the EBR a few years ago) and the LW, and came to the conclusion that the LW barrel was oversized for the JSB pellets.... He could not get them to shoot well in it....
The resized JSBs that I made were bumped up to 0.308" head diameter to shoot in a barrel from a powderburner, which is that diameter groove, and 0.300" land.... The LW is listed at 0.306" x 0.298".... while the TJ's barrel (which Sean Pero and I designed) is 0.300" x 0.294".... The JSBs are 0.300" head diameter, and 0.308" skirt.... as are the pellets I designed for NOE....
Bob
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The first pellet you see is .300 head and .308 skirt. followed by .308 head and .316 skirt. If Noe is .308 skirt then it may not engage the rifling of LW barrel. The modelled barrel has same bore and groove. Hence the third iteration with lower friction.
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Ken, you have to remember that the skirt of the pellet obturates to fill the bore, and in fact the skirt changes shape on firing....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Pellet%20Obturation_zpsyzq1fbkq.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Pellet%20Obturation_zpsyzq1fbkq.png.html)
I have no idea why LW made their barrel so oversize.... They make barrels for the Daystate .303, and they are tighter than their standard barrels.... I was not about to produce a pellet that was larger than the JSBs, which are so successful in all of the PCPs on the market, so I copied the head and skirt dimensions in my designs for NOE.... IMO, all that is necessary is for the head to have "some" engagement with the lands, and the skirt to be larger than the grooves, so that it gets an initial seal.... I personally prefer, however, to have the head match the groove diameter, or nearly so, so that the head can't "wobble" on loading or going down the bore, which is what I think may be happening with the JSBs (0.300"/0.308") going down an LW barrel (0.298"/0.306")…. It certainly doesn't happen with the TJ's barrel (0.294"/0.300"), or with the few commercially available PCPs that do shoot well with JSB pellets....
If you are going to produce larger pellets than the JSBs, I would tread carefully, as you may end up with a pellet that ONLY works in the LW barrels.... Even if that is your goal, I would not make the head diameter larger than the LW groove diameter, which is advertised at 0.306".... I would think that a 0.308"/0.316" pellet would show the lead being scraped away by the groove, in between the rifling lands.... and I see no reason to make them that large....
Bob
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Yeah! Now I get the point. I made the changes to the design now its.303 head and .312 skirt. barely undercutting the grooves at the skirt section only.
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When you load that pellet, the skirt will collapse to the groove diameter as it engages the rifling.... With that head diameter, even if you wanted to use it in a PB barrel with a 0.308" groove, I see no reason to go larger than 0.309"-0.310" at the skirt, and certainly not for an LW barrel, at 0.306".... JMO.... Your 0.303" head diameter may work well in all available barrels, although a bit tight in a TJ's....
Bob
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I see no reason to go larger than 0.309"-0.310" at the skirt...
Sure, a much larger than PCP groove diameter pellet skirt increases loading force; but not all airguns are PCPs :)
The reason for a larger skirt may be to help springers build pressure before the pellet releases.
Also, if the chamber is oversize on a production PCP airgun, a larger skirt diameter may help point the nose closer to axial on loading.
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I suppose if the manufacturer machined the chamber oversize it might help.... but why fix a problem with another problem?.... When you chamber a pellet in a properly made gun, the skirt collapses to the chamber diameter.... It is neither larger or smaller.... Try it and see....
(http://i378.photobucket.com/albums/oo221/rsterne/BRod%20Project/78%20gr%20JSB_zps8fcgppun.jpg) (http://s378.photobucket.com/user/rsterne/media/BRod%20Project/78%20gr%20JSB_zps8fcgppun.jpg.html)
See that nice shiny, parallel band at the back of the skirt?.... That pellet was chambered and pushed back out through the breech.... It measured 0.366" before and now measures 0.359"....
Bob
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When you chamber a pellet in a properly made gun, the skirt collapses to the chamber diameter.
This assumes a PCP. When loading a pellet in a break barrel springer, the skirt remains oversize until air pressure overcomes its reluctance to start moving; then swages it down to land and groove diameter.
In background conversations with Ken, I am aware that at least one of his .30 pellet designs will be intended for springers. This includes a lower weight to get the velocity up, and a "fat skirt" to act as a "launching brake" for optimal springer air pressure profile development. Also, Hatsan .30 springers seem to be the most common, and they are reported to have loose bore diameters...
I don't see anywhere "one problem is fixed with another problem". I see history, where break barrel springers resulted in diabolo pellets with wider skirts to stop them dropping through the barrel (and act as a launching brake). PCPs became popular afterwards (Lewis and Clark's PCP from 1800 shot round ball only). Thus, for better or worse, factory made PCPs were (and are) chambered to take existing diabolo pellets.
You can argue that pumpers preceded PCPs, and I would argue that they were made to use diabolo pellets originally designed for smooth bore springers. Perhaps BB guns started as single stroke pumpers, but we are talking about pellet skirt diameters, and BBs don't have those.
Now, Bob as an astute fellow, who is able to make both his own airguns and ammo; you are free to optimize all parts of your system. Most of the rest of us, polish already oversized production PCP chambers to remove burrs, and end up making them even larger... Thus, "fat" skirts on pellets help rather than hurt.
Getting back to Ken's pellets, I can only assume that most of his pellet designs are intended to work well across a wide range of platforms. Certainly, springers over 30 ft.lb are rare, and where they exist, practically silly. So, I grant you that this suggests, new projectile designs above .25 (or even .22) caliber are most likely going to be used in PCPs; and should be optimized as such.
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I had no idea Ken was working on a pellet for a springer, where the skirt sits in a taper instead of a chamber.... .30 cal Springers seem such a waste, IMO, but to each his own.... I guess as long as that is the only gun his pellet is intended for, there could indeed be a reason for a skirt of 0.312".... I makes for a pretty specialized pellet, though.... not one I would consider the "Perfect .30 cal Pellet" as the thread is titled....
However, that is just my opinion, and worth exactly $0.00, which is the current value of 2 cents, corrected for inflation.... ;)
Bob
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I had no idea Ken was working on a pellet for a springer............
"Perfect .30 cal Pellet" ....
Ken shows three pellets in that "in-barrel" image. He is working on one perfect pellet at a time. Each for a specific purpose :)
I am sure that most .30 pellets made and sold will be shot out of a PCP. I am sure that Ken knows this too; so not to worry about one or two designs optimized for springers.
Another approach is, if a given model commercial pellet works very well out of many different air rifles, but is not available in your location, then having dies to produce copies of that good pellet would be a rather good thing. Even if they have skirt diameters larger than your barrel's groove diameter...
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Pellet skirts, to my knowledge, are always larger than the barrel's groove diameter.... JSBs run about 0.007-0.010" larger than the head diameter, depending on caliber.... I figure they know what they are doing.... The head of the pellet can be anything between groove diameter and just barely engraving on the lands and still work "OK" (obviously better in some barrels than others).... I think most problems with pellet fit are when the head is smaller than the land diameter....
Bob