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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 degreesOverview:
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:
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rsterne
I think this design has some merit, and is certainly worth the experiment…. Let’s see is there is enough interest to move it to a Group Buy…. I will send Al a PM to bring this to his attention…. and perhaps get him working on an “official” drawing, with dimensions, for the tooling…. Don’t be surprised if you hear from him to get a dimensioned drawing from you….
Bob
I don’t know if Al has any experience with faceted HP pins or not…. Perhaps he can let us know that…. Making the front of the HP cavity larger, with a smaller rim around it on the Meplat, would increase expansion a lot…. but I realize you also want it to not expand from the air pressure if shot as a hollow base…. a definite compromise there….
Bob
Yes, Bob; the base needs to be robust against air pressure, and against the loading probe. At least the probe pushes straight from the rear; without a radial component – assuming a flat probe point.
While the “lip” or edge of the hollow point starts expansion, the whole cavity may be considered as an “annular cantilever bending beam”, with a certain “limb” profile. As this cavity is quite deep, the “limb” is quite long and therefor “soft”. That is by design.
Of course, as soon as there is any expansion, the force driving expansion goes up and the process “feeds on itself”… This all assumes sufficient velocity, and that the target contain or behaves like an incompressible fluid.
I hadn’t thought about the loading probe problem…. That will definitely affect the seating depth, depending on if the probe is thin, and penetrates the HB cavity, or large and pushes on the outer rim…. Since there is a large variation in the bolt nose on different PCPs, that could be problematic…. especially since many guys thin out the probe to get more airflow around it….
Bob
I hadn’t thought about the loading probe problem…. That will definitely affect the seating depth, depending on if the probe is thin, and penetrates the HB cavity, or large and pushes on the outer rim…. Since there is a large variation in the bolt nose on different PCPs, that could be problematic…. especially since many guys thin out the probe to get more airflow around it….
Bob
People who consider this slug will have to take into account their probe shape. That is, if they need it to be reversible; as I intended. The fact that I am using narrow, but very tight driving bands means the slugs will need to be pushed into the chamber with more than a feather touch.
The post below is a “cross-post” from GTA. I think it asks and attempts to answer an important consideration, and there is no guarantee that interested parties are reading both forums on this topic:
Have you done any calculations on the yield strength of the hollow base?
Thanks for helping me do a design FMEA on this, Bob.
No calculations until just now – see below. Calculations indicate that the peak wall stress directly due to air pressure at 3000 PSI is only 1380 PSI (yield stress for pure lead assumed as 2000 PSI). Note that this peak stress value corresponds to directly opposite the rear driving band. The next highest stress value is 1362 PSI. Perhaps a more realist value, due to a total lack of support from the driving band in that section:
Now, either the rear driving band is going to seal, so the cavity sees the full pressure differential; or the driving band is going to sit away from the chamber wall. If the driving band is in hard contact with the barrel wall, then bulging due to gas pressure in the cavity is even less likely due to the support offered by the driving band swaged down to groove diameter (shown in image). If the rear driving band sits in the chamber away from the barrel wall, then air will rush past it to some extent and the cavity won’t see the full pressure differential. Once down the barrel, the rear band will seal much better, and the air pressure will decay as the expansion volume increases.
This simple analysis does not consider the stress due to the combined bending moments that would result in the wall, opposite the unsupported areas of of the hollow cavity. However, as each section seems to be able to support itself, it would not transfer much bending moment to the neighboring material. Certainly, at a yield stress for pure lead assumed at 2000 PSI, this cavity design should be robust enough.
I determined the cone angle of the cavity by “what looked right”; and out of consideration for ease of withdrawing the core pin from the a freshly cast slug: Note from the Excel table, how the wall stress increases slowly towards the largest portion of the cone shaped cavity. The walls are angled at a 16 degree included angle. If the wall angle were to have been at perhaps a 12 degree included angle, the maximum wall stress would no doubt have been at the minor diameter of the cone; and much higher than I would consider “safe”.
So, I got lucky; or my engineering judgment is not too shabby…
Thanks for motivating me to test the assumed dimensions!Also, making a hex cavity as large as I modeled may leave little margin. If the hex cavity is “right on the edge”, that should ensure maximum expansion potential.
Then again, this is based on the properties of pure lead; not with one percent tin.The standard bolt probe on a Gen2 MRod, for example, is very thick ( 5/32″? ) and would hopefully not be a problem…. However, it severely restricts airflow, so many shooters thin it down…. particularly when searching for the airflow necessary to launch such a heavy “pellet”…. On a .25 cal, I typically use 3/32″-7/64″ diameter, and anyone slimming an MRod probe would never use more than 1/8″…. Since the entry to your HB is 0.110″, a 1/8″ probe would only catch 0.007” of the edge, and the smaller ones would slip inside (and might stick on the shallow taper)…. Hatsans use a probe 3mm or less, IIRC….
Since it is a boattail design (either way it is loaded), a flow-through probe as used in a Gen1 MRod could also be problematic…. I have never had one, so don’t know the ID of the probe, but if large enough for good airflow through it, it might not catch enough of the outer edge to allow proper and straight loading…. If the ID was 3/16″, that would leave only a 0.006″ rim around the edge for the bolt to push on, not enough to prevent damage…. If the hole in the probe is smaller than that, it would not flow enough air for the FPE necessary for heavy pellets…. I am using the MRods because they are so popular, and so frequently modded, they are a large part of the .25 cal. market….
If neither of the above will work properly, then you are left with a large diameter retractable probe, where either the probe itself, or the entire bolt, is withdrawn after loading the pellet…. I am not aware of any widely manufactured airguns that have that feature, although possibly the AirForce Condor may be OK, I think the pellet is inserted directly flush into the breech with your thumb, as the airflow is axial instead of through a side port…. None of this is a criticism of your design, but these are practical considerations for people wanting to try it in the HB orientation….
Bob
Bob, those are important factors to consider.
If an Mrod owner is contemplating this slug and wants to be able use it as a “solid”, then he should modify the gun’s probe so that it matches the rear cavity shape. At a 16 degree included angle with steel on lead, the probe should not pull the slug back out of the chamber. That angle is too steep to stick; especially considering the driving bands would be jammed into a very shallow angle cone.
If a cavity matching tapered probe is problematic, perhaps a probe that is thinned out, but leaves a flat head on a rounded cone of sufficient diameter might work. Yes, it would have flow disruption, but not be as restrictive as a full length, full diameter probe.
One thing that we won’t know until we try is how much force it would take to load this slug. If that is so high that the back end is distorted, then the outside diameter of the slug could be sized down a few thousands. Now, the driving bands are extremely short, so I think this slug should load with perhaps twice the force of a diabolo pellet; at most. Yes, that is a guess.
If the MROD owner is into shooting slugs, he probably has deepened his chamber and used a shallower leade angle. That would make a huge difference.
Has anybody prototyped lead projectile designs by means of lathe turning? I might be tempted to buy a Sherline CNC lathe to do just that. Would turn “wet” to keep dust down.
Asking five people to commit $100 each for a mold, for an unproven bullet design would seem to be a stumbling block.
Perhaps there would be more interest in larger calibers…
I am certainly not beyond modifying a bolt probe to experiment, but I kind of doubt the average Joe would be willing to change his gun around unless the slug is proven, and he knows his gun has the FPE capability to handle it….
The Group Buy system works, and I think most guys figure that $100 +- is acceptable for an experimental design…. and in fact enjoy being part of the program…. I think that .25 cal is probably the place to start…. JMO…. Your idea of producing prototypes is great, and shows a lot of dedication and confidence…. Once proven, getting 10 guys together for a Group Buy would be a no-brainer….
Bob
teemu
Now I’m not trying in any ways to bash or underrate BBT’s or this desing…But. Just wondering…Is this bullets designed to mainly hunting? Because there isn’t or not published any significant accuracy on BBT’s and Wadcutter model usually. I’ve been testing .30 WC and 9mm WC both lack of accurate atleast on my and few others who did tests. This is quite like similar desing like Wadcutter.. Altought that .30 cal BBT was quite accurate on Beaumont and some guns on EBR. Meaning those .25 cal or .22 BBT’s isn’t showing good results on punching paper or shooting groups. Or I’ve been missing results..
You Bob had skills and wisdom to do designs any kind of bullet so my question is: Is there some reason why You not desing something like conventional bullets? We both have seen how accurate those are. Referring Carl’s, Bowman’s Tofazfou’s and few other shooters test’s and results.
I hope You getting my point not trying argue or underestimate anything just curious. And hope my English is good enough to You get my point.
ThanksHi teemu,
No offense taken.
Yes; hunting would be the primary purpose of this slug. An unanswered question is, what the typical user range might be? Would it be limited to 50 yards, or might it extend to 100+?
Anyway; this reversible slug concept is unproven. As such, your questions about accuracy are valid.
Now, .38 Special wadcutter projectiles used to be popular for centerfire handgun bullseye shooting. If they were inaccurate, they would not have been used. That said, they were only used to 50 yards. Their accuracy beyond that was not considered important. So, there may not even be much data about that.
I did speak with a successful airgun projectile manufacturer about making a quantity of my slugs; as shown in this thread. That I could then distribute for evaluation. The cost would be about the same as 5 NOE molds. The gent was intrigued, but based on his experience, he predicted poor accuracy at much over 50 yards. He said that in order for spin stabilized projectiles to be accurate beyond 50 yards, they required the center of mass to be 40% from the rear of the projectile. Apparently, this is so that aerodynamic forces can act on the projectile, and gently lever its nose to follow its gravitational trajectory point first.
As such, your concerns align with his.
I am still thinking about what aspects of the concept can be carried over to a more conventional projectile; should I not follow though and prototype this one. As retained energy at long range is only of academic interest, unless it is combined with practical accuracy, I may stretch the projectile, cut off the boat tail and widen and deepen the cavity. Hitting the target should be first priority; retained energy has to come second.
For a derivative design; I would retain the “tight” low friction driving bands. Also, the very blunt nose with a huge cavity, for maximum expansion at airgun velocities. I think that heavy “blunt” projectiles are not necessarily poor performers at 950 FPS: Consider how successful semi-wadcutter bullets are for revolvers, for a variety of applications from hunting to target shooting; even at comparatively long ranges. I will design some slugs along those lines to see if I can achieve the preferred CG placement, and forgo the reversible aspect.
I will post an update here when I have more information. Either of obtaining a mold, or turning the projectiles; or abandoning the concept in favor of another.
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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