Reversible .25 cal Slug: Low friction, semi-wadcutter, hollow point, boat tail

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Greetings folks,

I hope my long projectile designation in the subject line is not out of line. 

While it is rather a mouthful, the airgun projectile I want to share with you for a potential mold “group buy”, is in fact a Low friction, rebated boat tail, semi-wadcutter, hollow point, reversible slug.

I would appreciate any comment about my design.  If there is sufficient interest, I would like to initiate a group buy for the mold.  Someone suggested we dub it the “KEG”. 

Bob Sterne suggested I post a thread here on the NOE forum; after I posted this on Gateway to Airguns:  https://www.gatewaytoairguns.org/GTA/index.php?topic=140165.0

General specifications:
.25 Caliber
.40″ long
44 grain weight
.2″ meplat diameter
.258″ driving band diameter
.238″ cylindrical mid-section diameter
.22″ effective bearing length
.2″ deep hollow cavity; tapering from .11″ diameter at the meplat.
.40″ radius for nose and boat tail flanks; arranged at an average boat tail angle of 8 degrees

Overview:
The slug’s nose and rebated boat tail have the same profile. This makes its aerodynamics the same, whether used as a hollow point slug, or reversed as a “solid” projectile.    Either way, the “front” driving band would act as a semi-wadcutter shoulder. 

Application:
The narrow driving bands are intended to minimize bore contact, and thus suffer much lower friction than cylindrical slugs.  Total metal displacement would also be minimized.  The clearance at the central cylindrical section of the projectile should also enable this slug to be fired from chocked barrels. 

While it is not the primary intent, the slug’s .258″ diameter driving bands may even span .25 and .257 caliber applications.  Certainly, .258 is “tight” for a .25 caliber; and is intended to eliminate the usual compromise between sealing, friction, alignment and a stable “rattle-free” fit – especially on loading into commercial chambers.

Inspiration:
I have taken the drag improvements achieved by Bob Sterne with his boat tails and nose shapes; and designed my slug concepts to take full advantage of the underlying aerodynamic principles. 

See Bob’s drag diagram directly below:

Aerodynamic principles:
My concept is based on the principle that a boat tail does more to reduce drag at sub sonic velocities, than a pointy or round nose does.  Also, that a large meplat diameter improves terminal performance, without a significant increase in drag.  Now, I may have taken that principle too far with a 0.20″ diameter meplat, in order to force symmetry – a positively unnatural ballistic concept for all; except those that have tried shooting diabolo pellets backwards…

A flat point also moves the center of gravity forward (over a pointy projectile).  A shorter projectile with a less rearward center of gravity is easier to stabilize.  Boat tail projectile require faster twist; all else being equal.  Making boat tail projectile shorter and moving their CG forward helps ease that.  Having an “instantly optional” hollow base moves the CG forward, and eases twist requirements further, if required (explained below).

A large flat point also shortens the projectile so that it will fit in available magazines. 

Purpose:
The idea is that, depending on the twist and velocity out of your shooting platform, that these slugs may be used hollow point forward, or hollow base aft.  Both would hit hard, due to the large flat “point”.  The large cavity suggest extreme expansion when used as a hollow point.

If the slug didn’t have a hollow point or dimple, then one could get confused about which way round to load it into the mag.  While it is designed as symmetrical, in practice there would be a small unseen difference between “nose and “tail”.  So, with at least a dimple at one end, loading as intended would be easier to confirm.

Rebated boat tail:
The astute observer may comment that the BT “rebate” is small and not very perpendicular; hence potentially negating the rebated designation.  While this is true for the unfired slug, consider this:  The material displaced rearwards at the driving band, as a result of forcing the slug down the barrel will in fact produce what looks much more like a proper rebated boat tail once the slug leaves the barrel.

If the contention is that the “rebate” is too shallow to be effective as a “gas deflector”, then I concede the “rebated” designation.  I think that with airstrippers being common on PCPs, the value of a rebated boat tail is probably a lot less than with powder burners.

Driving band diameter and shape:
The diameter of the driving bands are “large” to enable good alignment from an oversized chamber typical of commercial pellet barrels.  Yet without resulting in excessive lead displacement when loaded and fired from typical barrels. 

Even the shape of the driving bands is intended to allow easy self aligning chambering, and to minimize any lead “flash” from the trailing edges.

Effective driving band width and spacing:
The images below indicate the effective driving band contact area, at land and groove diameters of .245 and .252″ respectively.  Note that the actual contact lengths would be longer than these images indicate.  This is because the models show material cut off the diameter of the driving bands, rather than displaced; as would occur in reality.  I estimate that actual drive band effective contact length would be over 50% longer than shown, due to material displacement.  This is still very short in the absolute sense.

Effective driving band length at groove diameter of .252″:

Effective bearing length at land tops for .245 bore diameter:

Topic

[rsterneParticipant]

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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

[teemuParticipant]

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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

[subscriber]

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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…

[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.

[rsterneParticipant]

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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

[subscriber]

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Thanks, Bob.

[Author]

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