I recently watched one of Matt Dubber's videos, where he talked about the research that Patriot (Javelin) and FX were doing on slugs. Without going into details, he said that a pointy nose is actually LESS accurate than a blunt nose, at subsonic speeds.
According to this article, it is the specific air flow, modified by the series of wadcutting shoulder, then crimp and grease grooves that make the Keith style semi-wadcutter stable as it transitions from supersonic to sub-sonic flight. The pertinent section starts just over halfway down the page, headed Aerodynamics : http://www.lasc.us/Fryxell_Book_Chapter_9_MouldsMouldDesign.htm
Quote from: Cableaddict on November 18, 2021, 07:57:48 PMI recently watched one of Matt Dubber's videos, where he talked about the research that Patriot (Javelin) and FX were doing on slugs. Without going into details, he said that a pointy nose is actually LESS accurate than a blunt nose, at subsonic speeds. I saw that video too and wondered if the statement was absolute. Or, if Dubber meant long pointy slugs were less accurate when shot from barrels with a twist rate better suited to short blunt projectiles. If the latter, I would say "duh". If he meant regardless of optimizing the barrel twist, then we have something interesting to contemplate.What I noticed a long time ago was that semi-wadcutter bullets shot rather well for those competing in centerfire handgun competition. If you note that these target load bullets were subsonic at the muzzle, and the range limited to 50 meters; I will counter with; the same rather blunt bullets work well at 200 meter handgun metallic silhouette competition. And those left the muzzle at perhaps 1400 FPS (.44 Mag), so possibly trans-sonic on the way to the 200 m targets. So, I suspect that blunt bullet are less affected by starting or dropping into the trans-sonic speed range than pointy bullets.I do remember thinking that the 1:20 twist worked well for 200 to 300 grain .44 mag bullets, launched as light loads, from a revolver or max loads from a revolver or rifle; just supersonic or over 1600 FPS. Just how the same apparently slow barrel twist worked so well over such a variety of conditions impressed and perplexed me. It occurred to be that the catalyst might be the rather blunt bullets typically used. Perhaps a similar mechanism works for airgun slugs.I see hints that the "rules" for projectile aerodynamics don't scale with caliber. Other than the difference in flywheel effect that drives twist requirements, there seems to be something else going on. Perhaps the boundary layer clinging to a small diameter projectile covers a larger percentage of the frontal area, than for a projectile twice the diameter. The nearest analogy I can think of, off hand, is comparing air flow and pressure differentials through two orifice plates: If you have a 1" ID pipe partially blocked by an orifice plate, having a single 1/2" diameter hole, measure the pressure differential across the orifice required to achieve a specific mass flow. Now measure the pressure differential to drive the same flow with 20 smaller holes, that have the same combined area as the single large one.I guarantee you that despite the same combined area, the flow with multiple small holes will be significantly less at the same pressure differential; or, the pressure differential required to drive the same flow with multiple smaller holes will be significantly higher.The reason for this is the difference in the total length of the boundary layer around the (combined) circumferences of hole edges. Air clings to the hole edge, and the air flow velocity at the hole edge is zero (relative to the stationary plate). The air flow some distance away from the edge of the hole reaches its max velocity. The boundary layers for the small holes and the large holes are the same thickness (at the same pressure differential; unless the small holes never reach the max flow velocity in the middle).The point of the above is that smaller caliber projectiles have a larger circumferential boundary layer area to frontal area ratio, compared to larger diameter projectiles. My instinct tells me that if you think of airflow past a projectile as flow through a "reverse orifice plate", then some of the observed results for orifice plate flow applies to projectiles. This should affect skin drag and the rules for skin drag VS optimized form factor, as a function of caliber. Pointy bullets of a given weight have more skin drag than blunt ones of the same weight, but blunt ones work harder pushing the air in front of them out of the way. I can see how these factors might interact with twist requirements - and the change in twist requirements as projectiles slow down, down range. Perhaps Miles can put me out of my misery, if my partial vision about this is more of a hallucination.
Thanks for sharing, this should keep me occupied for a while.
How did I miss this awesome thread? HOW?!? 😄 Anyhow, thanks for this awesome discussion. 👍🏼 And as always — Miles brings in scientific evidence (maybe the only guy who has this evidence worldwide on airgun projectiles). And Peter (subscriber) explains complicated esoteric things in clear everyday language. Great! 🔶 If I remember correctly (pre-pandemic?) someone (Bob, Niels?) explained that the hollow point in NSA slugs is not so much for expansion but for improved precision(?). 🔶 I am disappointed that so many hobby ballisticians used totally inadequate ballistic materials for expansion testing. 😟 Water is "perfect" for expansion at high impact velocities — by comparison, flesh (and ballistic gel) much less so.... 🔶 This year I wrote a short series of articles on "killing power" and the terminal ballistics involved. The link below is Part 1. It also contains the link (in Part 4) to the results of slug and pellet expansion tests in ballistic gel. Below two examples of the test results.The link to the series: https://www.gatewaytoairguns.org/GTA/index.php?topic=182421 Matthias 😊
On the comment that pointy projectiles are less accurate, I think it may be a typical wrong conclusion from observations from someone who doesn't really know his subject. There are many more parameters deciding accuracy than just the nose shape and length. Yes it is important to the aerodynamics but it also changes a lot of other things so to decide it is all down to the nose length and shape is a bit arbitrary.
Matthias, you may be amused that I had a boss 22 years ago who begged me to make it clear when I was speculating, and when I was explaining something that had been proven. Apparently my speculation sometimes gives the impression that I know what I am talking about, when it is only based on conjecture.
If I remember correctly (pre-pandemic?) someone (Bob, Niels?) explained that the hollow point in NSA slugs is not so much for expansion but for improved precision(?).
Quote from: ballisticboy on November 19, 2021, 09:29:03 AMOn the comment that pointy projectiles are less accurate, I think it may be a typical wrong conclusion from observations from someone who doesn't really know his subject. There are many more parameters deciding accuracy than just the nose shape and length. Yes it is important to the aerodynamics but it also changes a lot of other things so to decide it is all down to the nose length and shape is a bit arbitrary.I'm as far from an expert on this as you can be, but it should be noted that NASA also found that a rounded leading edge to be better than a point, during a rocket's atmospheric (subsonic) flight.Elon Musk recently mentioned the same thing, based on his aerospace company's tests.
Don't confuse accuracy (group size to most people) with drag. Drag has very little to do with group sizes unless you are shooting in very gusty conditions.
How about cutting a cross section into slug along with silicone, which I didn't have any but cut a cross section.These pics are the results.The 738 and 740 are the FPS shot from my Kral NP03 .22 caliber at 2800 psi.
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