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Author Topic: Reversible .25 cal Slug: Low friction, semi-wadcutter, hollow point, boat tail  (Read 2234 times)

rsterne

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Teemu, no offense taken.... I am not an aerodynamicist, and certainly not an expert on spin-stabilized projectiles.... I got interested in developing boattail bullets for airguns simply to take advantage of the proven drag reduction of the boattail design element.... Most people (me included) thought it was primarily for high-supersonic bullets, but in fact the drag reduction of those is a much smaller percentage than when subsonic, when base drag dominates.... rather than supersonic where nose drag, and in particular the shock wave that forms around the nose, predominate.... The faster you drive a bullet, the more acute that angle, so the narrower the nose needs to be.... Subsonic, we don't have to deal with that problem....

A lot of the problem with airgun bullets is that we have such low pressures available, we are restricted to rather low Sectional Densities, or we can't get decent velocities.... This means that our bullets, of necessity, are short and dumpy.... You need a certain length in the middle of the bullet where it is supported by the bore, and if the bullet is really short, a boattail is not feasible.... I have probably erred in trying to incorporate the boattail design into bullets that are too light for caliber (low SD), but that happens when you try and improve things.... Sometimes we must take two steps forward and then a step back....

Nick Nielsen has now developed some deadly accurate hollowpoint boattails which he swages.... I know he has done a huge amount of experimenting, and is convinced that a rearward CG is necessary when Subsonic, we have corresponded a lot about various boattail designs.... I am pretty sure that if it were not for my BBTs, and my urging him to continue development, he would never have even tried.... The result is some outstanding bullets for airguns, with superb accuracy.... His .25 cal 85 gr. HPBT placed second at the EBR last year....

This is all very new technology for airguns, and I am proud to have given airgun projectiles a push in that direction.... I look forward to seeing what develops over the next decade.... After all, we are only 3 years or so along the path.... As far as designing "conventional" bullets, I have really no interest.... There are lots of good designs around, and no reason they cannot be scaled up or down to meet the needs of other calibers, like has been done by Bowman and others.... I do think that the multi-banded lube-groove design should be abandoned in favour of the mid-body style I use on my BBTs, however, with the center just clear of the rifling to reduce drag, and two narrow support surfaces at each end to provide a seal, and keep the bullet straight in the bore.... I recently found that with a proper chamber and leade that chambering a BBT that was, in theory, much too long for the chamber cut was not an issue, because the gentle slope of the back of the tangent ogive, when it met the shallow taper of the leade, engraved easily and completely, enabling the bullet to chamber without too much effort until the nose band was fully engraved in the rifling, completely past the leade.... This was in sharp contrast to a conventional bullet that although shorter, could not be chambered, even with hammering on the bolt with your fist....

I apologize to the OP for this lengthy sidetrack of his thread....

Bob

rsterne

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I would make one suggestion to subscriber, should he wish to work on an asymmetric design.... I would reduce the diameter of the Meplat to reduce the drag, and increase the diameter of the HP to aid expansion.... Recent CFD tests on pellets seem to have disproved the existence of a stagnation zone, insofar as the flow is simply sideways across the Meplat, with no circular eddies (like what exists in the waist of the diabolo pellets tested)....

https://www.gatewaytoairguns.org/GTA/index.php?topic=140314.0

I don't have access to a CFD program, so in the absence of that, I use Robert McCoy's "McDrag" program, also used in the JBM and Kolbe online bullet drag software.... When examining the nose drag, the greatest drag reduction occurs by increasing the Ogive radius, and when subsonic the Meplat diameter has little effect.... This is counter-intuitive, unless explained by the stagnation zone idea.... which is why I suggested it.... However, McCoy insists that his program agrees with real-world test results within 3% supersonic, 11% transonic and 6% subsonic.... If you haven't used his software, here is the one I use....

http://www.geoffrey-kolbe.com/drag.htm

I have my own version, which I painstakingly wrote into an Excel spreadsheet from the C+ code from the JBM download.... I used that to examine the low transonic region we try and push into with airguns (Mach 0.8-0.9), and found that there appears to be a "sweet spot" for the relationship between the Ogive and the Meplat when they meet at an angle of about 70 deg. (ie the Ogive is about 20 deg. to the bullet axis where it meets the Meplat).... I used this to develop a relationship between the Ogive radius and the Meplat (as a percentage of calibre) which I use on the noses of my designs.... In simple terms, the longer the bullet, the larger the Ogive radius, and the smaller the Meplat.... If you look at a series of my designs of different weights/lengths, you can see how it progresses.... Recent CFD analysis (done at my request by the author of the above thread on the GTA, thanks, Ron) has convinced me that a small radius where they meet, such as used in the "Ranch Dog" designs here on NOE.... can reduce the turbulence and drag at that corner, improving the BC while still allowing a large Meplat.... You might study the nose design on the RD bullets (he uses a standard nose design on all, I believe).... as his claim to fame is that the BC = SD on his designs, quite good for a flat-based design....

Bob

teemu

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Thanks both genteman's for answering. I sure hope this one works for purpose which is intented. I move back to see and waite if this goes to GB. There is quite small amount of .25 cal bullets which can use airguns so every desing is welcomed. Well my wish is there where few conventional desing also but there is probably not much need on pistol calibers. Of cource few .257 bullets are lighter and can sized down to fits right measurements.
 Well atleast that one Ranch Dog's model has 10 interested buyers and hope it goes to mill soon.

Thanks
« Last Edit: March 11, 2018, 05:13:01 PM by teemu »

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Thanks for your further input, Bob.

While emulating Nielsen slugs may work well, recreating what exists seems to defeat the purpose.

Also, while Nielsen slugs combine "sharp" noses with hollow points, this is achieved by means of swaging.  If I am to keep my double "skinny" driving bands, that dictates a cast bullet.  Cast bullet hollow points are limited to .2" diameter at NOE; unless I misunderstood.

I generated 40 and 50 grain hollow point cast flatbase derivatives of my design last night.  Was not going to show them until I was "happy with them", but their shape should provide an idea of where I am going with this.  They are .40 and .45" long respectively.  The shorter slug has a nose flank radius of .40, and the longer one; .75". 

While the shorter slug has its tail trimmed compared to the reversible slug, then the length added back with half of that between driving bands and half added to the nose length; the longer one has its nose stretched more and the flank radius increased dramatically.  Both new ones have the distance between their bearing bands increased to over an effective .25".

Their bases assume that the bearing bands will be swept rearwards.  As such, they will be more "square" on emerging from the muzzle, without having "fins" trailing off the rear.

I am not quite done with these designs, and will increase the diameter of the nose where it meets the front driving band slightly.  These don't seem to taper down enough towards the nose, with too long a parallel section, just ahead of the band. 

Just a glutton for publishing something, I guess...
« Last Edit: March 11, 2018, 07:26:28 PM by subscriber »

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Improved profile, nose flank radii: 

.40" long slug now has a .70 nose flank radius, with the termination point moved behind the front driving band.

.45" long slug now has a .111" nose flank radius, with the termination point moved behind the front driving band.

Bother retain a full .2" meplat diameter, to accept a hollow point mold pin.

The exact weights can be tuned by refining the hollow cavity shape and sizes.


rsterne

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While NOE have a 0.20" minimum Meplat diameter for their RG pin system, they can make a Lyman style HP pin in one cavity down to a 1/8" Meplat.... If you got a 2 cavity mould, you could cast 50% FN and 50% HP that way.... There is also a company called HollowPoint Mold Services that does HP conversions of NOE moulds and others and he can go down to a 1/8" pin as well.... I had my 100 gr. .257 cal BBT converted by Erik that way, and although the 0.103" Meplat was slightly too small, he just pushed it back to 1/8" when he installed the pins.... I even had the 113 gr. FN converted that way.... I think he can even go smaller than 1/8", as he recently hollowpointed the .172 cal NOE 26.5 gr. bullet for Cedric, and it comes with a 0.083" Meplat, and I'm sure he didn't push it all the way back to 1/8", I saw the bullets.... He may have used a 3/32" pin, I don't think it was larger than that.... The point is that you can get smaller HPs than 0.20", just not using the RG system that NOE make.... Here is an example of a smaller Meplat in an NOE Group Buy that has a Lyman HP pin available in one cavity....  http://noebulletmolds.com/smf/index.php/topic,1628.0.html

Bob
« Last Edit: March 11, 2018, 11:33:58 PM by rsterne »

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Thanks, Bob.

 



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