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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: rsterne on February 16, 2016, 08:09:15 PM
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As many of you are aware, the Lyman 257420 is the standby bullet for many .257 shooters.... It looks like this....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Lyman%20257420_zpsbjrlh49l.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Lyman%20257420_zpsbjrlh49l.jpg.html)
Although listed by Lyman at 68 gr., it casts much heavier than that, running from 73-74 gr. in pure lead to low tin alloys.... I think one of the reasons this bullet works so well is the gas check rebate, which prevents any small distortions in the base caused by the sprue cutter from touching the bore as the bullet leaves the barrel.... This is something that my boattails share, of course, so I wondered what it would be like if the 257420 had a boattail.... I used my standard 8 deg. angle, and left the diameter at the GC rebate the same, and this is what it looks like.... It would be slightly lighter because of the boattail...
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Lyman%20257420%20BT_zpsh17xftbi.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Lyman%20257420%20BT_zpsh17xftbi.jpg.html)
My latest BBTs don't have classic lube grooves any more, because we simply don't need them in airguns.... So I wondered what it would look like without them, and with the mid-body reduced to just clear the lands of the barrel to reduce the bore drag.... This is the result.... It would weigh about 75-76 gr....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Lyman%20257420%20BBT_zpsiwkechqg.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Lyman%20257420%20BBT_zpsiwkechqg.jpg.html)
It doesn't look much like the original 257420 any more.... but the nose, including the Meplat, the front driving band and the rear driving band are still the same.... According to the drag program I ran it through, below 1000 fps the drag should be reduced by about a third.... Here is the comparison, calculated by the McDrag software....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Lyman%20257420%20BBT%20Drag_zpsgkigivnk.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Lyman%20257420%20BBT%20Drag_zpsgkigivnk.jpg.html)
BTW, Mach 0.9 is ~1017 fps.... So, if any of you guys that love the Lyman 257420 want to see if it can be improved.... either of these designs might be worth a look....
Bob
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Certainly looks great! Are these available?
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Love the looks of this bullet Bob. Now if someone could coordinate a group buy from a mold maker. Would love it in a hollowpoint. My biggest problem with the 257420 has been to get a clean cut with the sprue plate.
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Cut the sprue with a **THICK**gloved hand with downward pressure as you push away .mold needs to be up to temp first though. problem will be solved.I cast many 22 bullets with large sprue holes and accuraccy is as good as the two nose pour 22s I own.
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Organizing a group buy from NOE should be possible, once they get their tooling production sorted out.... We would need a concensus as to which to make, the one with the lube grooves, or the one with the straight (but undersize) mid-body.... It would take ten guys to order to get the 15% discount that NOE do on their group buys.... If there is enough interest I can do a poll to see which design gets the nod, if either.... I checked the stability with the Kolbe twist calculator, and either boattail version has a Stability Factor of 1.3 at 1000 fps in a 14" twist.... so I don't think there should be an issue running them in the standard 25-20 liner from TJ's.... which is what most guys are using on their .257s.... The Meplat is too small for NOE to run them with the RG style HP setup, but they could do them with a Lyman type HP pin in one cavity.... I have a Lyman mold that I had converted to HP by Erik at HollowPoint Mold Services, but he had to push the Meplat back to use a 5/32" pin, which shortens the bullet slightly.... Of course the more changes you make, the less it remains a 257420.... As most of you will know, I have two similar BBT's over at NOE that have not yet generated enough interest to move to a group buy....
The 65 gr. is shorter than the 257420, and will work in a 14" twist.... http://noebulletmolds.com/smf/index.php/topic,1094.0.html (http://noebulletmolds.com/smf/index.php/topic,1094.0.html)
The 78 gr. is longer than the 257420, and has an 11" recommended twist.... http://noebulletmolds.com/smf/index.php/topic,1096.0.html (http://noebulletmolds.com/smf/index.php/topic,1096.0.html)
The boattail on the modded 257420 is much shorter and the base larger.... which allows it to work in a 14" twist while still having less drag than the original 257420.... it's kind of an in between design....
Bob
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I think this design shows great promise. After I get some projects done I am already committed to I plan to work with Bob on a design like this one or possibly another one we have talked about. Good thinking Bob, this may just be a winner...
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I would be interested in the modified also.
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Not intend to argue or anything..But noticed last time when Bob did those BT designs and there was many people who was interested but now when those molds are in Group buy discussion or even Group buy there is only few who is hit the name on that list...Not near as 10...
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Not intend to argue or anything..But noticed last time when Bob did those BT designs and there was many people who was interested but now when those molds are in Group buy discussion or even Group buy there is only few who is hit the name on that list...Not near as 10...
I think it is tough to get a lot of people all at the same time interested in one mold. They have to want that exact weight be ready all when everyone else is and have funds at that time as well. I think if number of people needed for a mold to be made was lower more molds would get made and the mold makers would see more over time. The other tough part for people is they want to see people get results before they pull the trigger on a new design.
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Absolutely correct, Nick... Many guys want to sit back and wait and see how a bullet performs before pulling out their wallet.... My hat goes off to those who are willing to take a chance on a Group Buy.... which provides the tooling to make the molds.... so that others can later benefit.... Without them, our only choice would be molds made by companies who will do one-offs.... but that have restrictions on what they can and will do that won't permit many designs.... For example, Accurate won't make this mold.... They won't do less than .30 cal, or anything will a base diameter less than 0.250".... You can't blame the mold makers like NOE either.... If they made tooling for every drawing that came across their desk, they would quickly go broke.... It takes a critical mass to make it worth making the tooling to make the first run of molds.... My thanks to those brave souls who take a chance on Group Buys.... They are the pioneers....
Bob
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Yup both You guys are right. Everybody want to get working bullet. There is big BUT..everybody doesn't have same barrels etc..Well getting molds it is like second hobby to me. Got big pile different molds and if this mold goes to GB on NOE I will take it also ;D
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Yup both You guys are right. Everybody want to get working bullet. There is big BUT..everybody doesn't have same barrels etc..Well getting molds it is like second hobby to me. Got big pile different molds and if this mold goes to GB on NOE I will take it also ;D
That's the spirit!
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Then there are fewer guys doing the Big Bore rifles than the smaller lower power rifles just because of the needs of those big bores...
But once the ground has been broken it will benefit all and it will gain momentum... hopefully to the point of to rivaling black powder...
Me I am thankful that Bob took the time to design the lights and am looking forwards to the challenge of tuning the rounds to my barrels and fpe...
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Love the looks of this bullet Bob. Now if someone could coordinate a group buy from a mold maker. Would love it in a hollowpoint. My biggest problem with the 257420 has been to get a clean cut with the sprue plate.
The simple solution to this is of course custom sprue plates. NOE makes the holes the proper size for the small boolits, so it isn't needed if the mold is sourced from them. To get Lyman molds to cast well, the smaller hole is very useful. I was never able to get small boolits to cut cleanly with the huge holes Lyman runs across the board.
cheers,
Douglas
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Cut the sprue with a **THICK**gloved hand with downward pressure as you push away .mold needs to be up to temp first though. problem will be solved.I cast many 22 bullets with large sprue holes and accuraccy is as good as the two nose pour 22s I own.
You have accuracy from .224 air rifles? Details please?
cheers,
Douglas
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To get Lyman molds to cast well, the smaller hole is very useful. I was never able to get small boolits to cut cleanly with the huge holes Lyman runs across the board.
cheers,
Douglas
Do You pour lead with ladle or do You have example Lee's pro 4-20 pot? I have that pot and I haven't got any problems to get those bullet bases straight and good.Not even .225 molds.. I got Lyman,NOE,NEI,RCBS,Ideal, molds. There is Ouhaus and Arsenal molds products which I haven't able to cast yet.
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I talked to Al Nelson at NOE today.... He agreed to put the Modified Boattail version of the 257420 in the Airgun Discussion section of his Forum, so I sent him the drawing.... He said it will be a couple of weeks before he has a chance to draw it up and add it, but I will let you know when you can go over and comment about it....
Bob
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Very interested in the modified BT rendition of this bullet.... A hollow point version would be equally interesting...
Thanks Bob for your time messin around with this slug...
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NOE would be able to put a Lyman style HP pin in one cavity....
Bob
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Well...count me in on a group buy for the 257420 Mod BBT HP.
This could be a fantastic high velocity ground squirrel round here in California...
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This is sounding good
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Impressive results Bob...well done.
Have a little project of my own, wondering where you obtained the McDrag software? A search turned up a program in what looks like basic, but the Vic 20 is long gone and I've no idea how to go about entering the program on a modern pc.
Al
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Al, I had to write my own in Excel.... I printed it out in Fortran and then just used the formulas in columns in a spreadsheet.... Took FOREVER to double check and debug.... but eventually I got identical results to the JBM online calculator.... So, it's my own version of McDrag.... I never got around to doing McGyro, although I do have the printout of the program from the JBM website (where I got the McDrag also).....
The JBM online version is great, but for some reason they decided to put restrictions on things like nose length and Meplat diameter that are unnecessary.... but it makes it quite useless for many airgun bullets....
Bob
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Thanks Bob.
Al
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I would be very interested in seeing results from this new configuration!
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I would be very interested in seeing results from this new configuration!
+2
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I had Arsenal Molds redevelop the Lyman 257420. Everyone likes the round, no one likes the 2 cavity Lyman mold. I have the Arsenal 257420 mold in 4 cavity aluminum. It has a Gas Check (GC), like the original Lyman Round. We shoot subsonic. I understand aerodynamics and have bevel bases on the molds I got from Lead Bullets Technology (LBT) but saw no need to modify such a short and highly effective round. Might I ask those who are so keen on a boat tail:
What range do you shoot? I shoot long distances (547 yards have been my longest hits to date).
What advantage do you perceive in a small light round having a boat tail? 257420 has a flat tip (meplat for those who like to sound fancy), so its not Spitzer like with a sharp point. I
It has limited aerodynamics, and the air envelope coming off the front may create a shock wave that never has the tail engaged with pulling much drag.
These shots were done with 25420 rounds from my Arsenal Mold. You might want to get one of their molds instead of re-inventing the wheel.
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Arsenal also has produced a mold for the 257388 Spitzer. Carl sent me this picture of his best group that came from that mold. These shots were done at 100 yards. I know, this is a tad off topic, but hey, if you want a great mold for $75 plus shipping (mine came to $82.50 delivered), then go to arsenalmolds.com and order the 257420 and the 257388 at the same time...save a bit on shipping!
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In my previous replies I mentioned an 80 Grain Spitzer I have from Lead Bullets Technology. This bullet has a bevel base, not a boat tail, but at subsonic speed it serves the same purpose well. If you order from LBT make sure you state you want an air gun specific round, include this picture and spec. 2575
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A lot of guns can not shoot that heavy of a bullet so not everyone has a use for it.
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I sent Arsenal a picture of Bob's Modified Boat Tail 257420, as it has similarities to the rounds I get from the LBT molds, which have slick aerodynamic properties. I like the fact that it is an air gun specific round. The biggest advantage I see is the lack of lube grooves between the bearing bands. The difference between the boat tail and the GC is rather minor in my mind. I'd love it if Bob's calculation could compare the Modified Boat Tail (lube grooveless design for both) with a GC back vs the Boat Tail.
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The calculated difference in drag is in the first post.... in theory about 1/3 less drag for the boatttail in the subsonic region.... None of the drag programs address the lube grooves at all, they assume the bullet is smooth and full diameter between the driving bands (like a jacketed PB bullet)....
Bob
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I'd be interested in your 257420 modified BT in HP when you get it going...looks like sweet well thought improvement in design on top of existing winner.
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Arsenal Molds started making the 257420 in a Boattail version some time ago.... http://arsenalmolds.com/products/257-Bullet-boattail?limit=99999999999 (http://arsenalmolds.com/products/257-Bullet-boattail?limit=99999999999)
They do not, however, make HP molds, you would have to send it to Erik at Hollowpoint Mold Services to have it converted.... I have not yet purchased one of these molds, and so far Al at NOE hasn't decided to go forward with a BT version, probably because Arsenal already has one....
Bob
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Well...count me in on a group buy for the 257420 Mod BBT HP.
This could be a fantastic high velocity ground squirrel round here in California...
There's a really good chance that I'll be getting an M10 soon which would probably love these bullets, I think I've read that these have a 1 in 21" twist but I could be wrong ???
I'd be game for the hollow point cavity too 8)