In watching/reading about the 8.6 Blackout cartridge, the designers have a lot to say about the 1:5 barrel twist being responsible for overcoming terminal shortcomings of the subsonic cartridge. The more aggressive spin imparts a terminal effect that makes up (in part) for the lack of velocity.

I had a hammer in my hand this morning, and was flipping it in the air, and catching it. In my goofing around, I applied a spin to the same flip maneuver, and found the impact in my palm upon catching it to be noticeably stouter. I wonder if that was the same principle in action; if so, I can certainly see how it is a real thing.

That’s an interesting claim that I’d never seen made or even mentioned before.

According to this article, the degree to which a rotating bullet is slowed by test medium can be measured and because the test projectiles were slowed in 8.5 inches of 20%* gelatin, some of the rotational energy was indeed transferred to the medium.

https://brassfetcher.com/Wound...ullet%20SpinZZZ.html

The two cartridges tested were 9mm Luger and 45 Automatic. For those two rounds there was some increased energy transfer (about 5% on average) due to the rotation slowing. Although I didn’t see it specified in my brief look at the article, common rifling twist rates for 9mm is 1/10" and for 45 ACP is 1/16". Both are therefore much slower than 1/5". Faster speeds usually decay more quickly, so I wouldn’t be surprised if the 8.6 Blackout did transfer more energy due to rotational decay.

My question, though, would be how much? Even if we grant that 5% isn’t nothing and could be of some importance, and if we assume the 8.6’s rotational energy and subsequent transfer to the target would be greater than the pistol bullets’, would it really be enough to make up for being a subsonic load?

Bullet weights and velocities vary a lot, of course, but consider a couple of data.
8.6 Blackout 300 grain bullet at subsonic velocity of 1000 fps: 666 ft-lb of muzzle energy.
300 Blackout, Hornady 135 grain FTX load MV of 2085 fps: 1303 ft-lb.

Ignoring differences in bullet weight, construction, and design that would affect ballistics performance, and looking only at muzzle energy, the 8.6 would require a huge amount of transferrable rotational energy to make up for the much greater muzzle energy of that one example 300 BLK load.

My reaction to the claim is that although there may be some truth to it, it’s hardly enough to be significant. In comparing one subsonic load to another? Perhaps, but my SIG 300 Blackout gun barrel has a 1/6" twist rate, so it’s not too far behind whatever benefit the 8.6’s 1/5 rate would confer.

Ultimately, of course, as Admiral Grace Hopper pointed out years ago, one accurate measurement is worth 1000 expert opinions. Will anyone take any accurate measurements, or will it always just be Internet claims by people with particular agendas?

Added: And I meant to include that I’m surprised anyone would hang their hat on the value of energy transfer to a discussion about a new cartridge. That tends to be dismissed by many (most?) shooters for reasons ranging from anecdotes to tradition to pictures of permanent disruption tracks in test media. But if that’s what you’ve got ….

* Most modern ballistics tests use 10% gelatin. The earliest gelatin experiments I’m aware of were conducted by the US Army after World War II and used 20%. Much later, however, the consensus was evidently reached that 10% more closely mimicked the results of flesh. The denser medium would slow the rotational speed of a bullet more quickly.

This message has been edited. Last edited by: sigfreund,

The Faxon 8.6 barrel has a THREE inch twist. Their videos show greater gelatin splash with 3 inch than 7 inch. A friend quoted an 8.6 guru to say "It cavitates five times." which must be something he sees in the gelatin splash.

I am sure there is a calculation for how much energy it takes to spin up a bullet/dump into the target.

Once upon a time the NRA compared .38 hollowpoint expansion in Colt 14 twist and Smith 18.75 twist. The Colt opened up the bullet more.

A question - Would chamber pressures be significantly higher on a rifle that has a 1/5 rate vs. one that has a 1/10 rate?

quote:

Originally posted by KSGM:
In watching/reading about the 8.6 Blackout cartridge, the designers have a lot to say about the 1:5 barrel twist being responsible for overcoming terminal shortcomings of the subsonic cartridge. The more aggressive spin imparts a terminal effect that makes up (in part) for the lack of velocity.

I had a hammer in my hand this morning, and was flipping it in the air, and catching it. In my goofing around, I applied a spin to the same flip maneuver, and found the impact in my palm upon catching it to be noticeably stouter. I wonder if that was the same principle in action; if so, I can certainly see how it is a real thing.

Changing a moving object from a single moment, to two moments, the kinetic energy is 4 times greater than the singular moment.

And why "rolling G" maneuvers in aircraft are "less desirable" than doing one moment, and then the other, to prevent overstressing the airframe.

quote:

Originally posted by sigfreund:
My reaction to the claim is that although there may be some truth to it, it’s hardly enough to be significant. In comparing one subsonic load to another? Perhaps, but my SIG 300 Blackout gun barrel has a 1/6" twist rate, so it’s not too far behind whatever benefit the 8.6’s 1/5 rate would confer.

Bingo.

This reminds me of the loud promotion that came from AAC as the 300 blackout was introduced. Silvers changed his tune over time, as others reviewed the Kool aide. No, you didn't invent the cartridge. No, the cartridge isn't leaps and bounds better than 5.56 when comparable load types are used. No, the accuracy isn't equal to the 223 Remy. No, it hasn't made 5.56/223 obsolete.

I looked at the 8.6 blackout marketing-engineering-performance when it was introduced awhile ago. Interesting concept, doubt that it will go far.

The concept is no joke-

That video certainly illustrates a difference. I feel as though I'd be more convinced if they showed multiple shots with both twist rates, and the result was consistently better with the 1:3. There's a difference there, but is it subtle enough that it could be a fluke of sorts?

Hey, would ya look at that, KSGM and Sigfreund are famous now.

It’s interesting to see this thread again, and we should point out that it was originally posted a year ago.

Thinking about the question of how much energy is transferred to the test medium reminded me of what the FBI did in an attempt to determine the most effective defensive handgun cartridges and loads. I believe I still have the results of that series of experiments in my files someplace, but because I’m too lazy at the moment to try to find them I’ll just go off my memory of what they entailed.

As I recall the tests were conducted and reported in the early 1970s. What they involved was measuring the velocity and therefore the energy of various handgun bullets before and after they passed through a block of gelatin test medium. I think the blocks were 6 inches thick, but that could be wrong. The thickness was, however, chosen to ensure that the bullets passed completely through and their remaining energy could be measured after the passthrough. Many different ammunition types were tested and although I don’t recall any specifics, hollow point bullets unsurprisingly transferred more energy to the media than did nonexpanding projectiles.

That test series was the earliest reasonably scientific attempt I’m aware of to quantify how different handgun projectiles could be expected to perform in a defensive role with the purpose of stopping threats to law enforcement officers. Their basic premise was that bullets that transferred more energy to the test medium would be better for the purpose than those that transferred less energy.

I became convinced that that premise was valid way back then, and I haven’t had any reason to change my opinion since—all else being equal. The “all else being equal” variable, however, is a significant one. Even I believe that a projectile must be capable of sufficient penetration as well as delivering some X value of kinetic energy to the threatening target. Penetration wasn’t something that was considered in the FBI’s experiments I described above, but the reported results weren’t worthless, just incomplete.*

So, what does all that have to do with the subject of this thread?

If we want to know not only how much energy a projectile has prior to target impact, but also how much energy it transferred to the target on its way through, the same sort of experiments that the FBI conducted long ago would be an easy way to answer the question: Use test medium blocks that are thick enough to provide a reasonable simulation of our intended target, but thin enough to permit complete passthrough, and measure the energy before and after the passthrough. To address the question of rotational energy, conduct the tests with bullets fired from barrels with different rifling twist rates.

It would be interesting to see the results of such a series of tests, and it would be satisfyingly ironic to circle back to a 50-year-old experimental method, and especially to resurrect the theory that energy transfer can matter.

* As an aside, those experiments and results didn’t get nearly as much attention among gun gurus of the day as the later much more comprehensive test program that led to the “Relative Incapacitation Index” (RII) using the (somewhat) infamous “computer man.” The RII was roundly denounced by a number of self-appointed experts, mainly, I believe, because their favorite cartridges and projectiles didn’t fare too well.