Subsonically "bad" bullets

[bulletbluesky]

ELD shooting, in most general terms, includes supersonic, transonic, and subsonic flight of a projectile. Some rifle bullets are better than others going transonic and subsonic phases of flight. Dynamical instabilities during subsonic flight might emerge downrange leading to increasing yawing & pitching motion amplitudes, inspite of fact bullet wasn't affected too bad during transonic phase (and sometimes it is just continuation of bad things in transonic zone). Such behaviour can be identified in Cd vs Mach number curve constructed on the basis of Doppler radar measurements data of a bullet flight. I'd like to make a (black)list of particularly bad bullets, that should be avoided in ELR shooting where subsonic flight is present.Two examples of subsonically bad bullets:

Significantly higher drag below Mach 1 than expected for a streamlined BT bullet is a clear indicator.
The projectiles fired from barrel with 1:7" twist rate.

And one example of a well-behaved BT bullet in subsonic flight (147 gr, 7.62 mm NATO bullet/ 0.308 Win, 1:12" twist rate):

 

[st1650]

The infamous 168gr SMK…
But then again, no one is shooting these bullets at ELR distances

 

[kthomas]

I'm sure there's papers on this type of stuff.

But I imagine projectiles that don't have a centered clCG would start to become unstable a lot quicker than those with a centered CG. These are the projectiles that will pitch and yaw before others.

For example, I've heard Warner Flatlines do not transition well. Amazing BC and great supersonic. But I think that rearward CG doesn't transition well. I have some Flatlines, but haven't had the opportunity to shoot them transonic yet.

 

[bulletbluesky]

I'm sure there's papers on this type of stuff.

But I imagine projectiles that don't have a centered clCG would start to become unstable a lot quicker than those with a centered CG. These are the projectiles that will pitch and yaw before others.

For example, I've heard Warner Flatlines do not transition well. Amazing BC and great supersonic. But I think that rearward CG doesn't transition well. I have some Flatlines, but haven't had the opportunity to shoot them transonic yet.

I don't have any papers specifically or library as such. Hence this topic
BTW, 168 gr, SMK, cal .308 does not belong to these categories of instabilities since it's inherently slightly dynamically unstable from the muzzle (given normal twist).

 

[kthomas]

I think Jim Boatright has some papers on this.

Here's one: https://www.researchgate.net/publication/324222518_A_Coning_Theory_of_Bullet_Motions

He's also a member of this board, but hasn't been active since May 2023 @Jim Boatright

 

[glock24]

Is there a consensus on subsonically "good" bullets?

 

[Sandow the Heretic]

Is there a consensus on subsonically "good" bullets?

If we are talking about bullets that transition and are still predictable then the list gets pretty short. The CE Lazers do well and there is plenty of evidence on that. At least in the near subsonic range, the A-Tips seem fine. I haven't seen them enough at extended range to have an opinion.

The bullets that are uncompromising on BC tend to do worse. There are choices on the boat tail especially that can give you a better BC at the cost of significantly worse dynamic stability. A radius boat tail is a good example of that. Here is a 60 year old paper on that for the curious.

-Alex

 

[bulletbluesky]

If we are talking about bullets that transition and are still predictable then the list gets pretty short. The CE Lazers do well and there is plenty of evidence on that. At least in the near subsonic range, the A-Tips seem fine. I haven't seen them enough at extended range to have an opinion.

The bullets that are uncompromising on BC tend to do worse. There are choices on the boat tail especially that can give you a better BC at the cost of significantly worse dynamic stability. A radius boat tail is a good example of that. Here is a 60 year old paper on that for the curious.

-Alex

How short or long is that list needs to be quantified yet.

Generalized drag coefficient can be written as follows:

Cd= Cdo(S,Ma) + f(S,Ma,δ,ω,Re)

where the dependences letters denote:

S... projectile geometry
Ma.. Mach number
δ... Total angle of attack
ω...Rotational frequency
Re...Reynolds number

Cdo is called a zero-yaw drag coef. while f(...) is non-negative function difficult to calculate even with all aerodynamical coefficients given. This means that the best way to accuratelly know things for sure is an experiment: real firings with ELD Doppler data during fair weather conditions. Certainly there are VLD bullets that transit from supersonic to subsonic very good. I heard that Cheytack .375 and some Bergers solids belong to that cathegory all way down to 0.6-0.7 Mach. 7.62x51 mm FMJ NATO standard ball example given above is another example: this is not a VLD bullet but BT nonetheless. It has average Cdo between 0.9 and 0.6 Mach about 0.14, while real firings Cd varies on average between 0.15 and 0.17: so that bullet holds its own too. Another example is .223, 68 gr FMJ bullet:

If you calculate from this data Cd between 0.9 and 0.7 Mach it is somewhere between 0.11 and 0.12. Pretty good I would say, no? Now if you comapare it to its colleague in the first post , Lapua .224, 69 gr Scenar, you get an idea. However, note also that I'm not interested in the list of very good VLD bullets, but quite an opposite

 

[Sandow the Heretic]

How short or long is that list needs to be quantified yet.

Don't overthink it. Modeling tells you a maybe answer and hopefully helps you avoid clear failures when it comes time to making something and testing it in the real world. We already have real world data and demonstrated capabilities so applying a generalized model is pointless at best.

Unless you are doing bullet design and having someone turn solids for you, you are far better off simply shooting a bullet in question at extended ranges and seeing if they still group for shit. The modeling will never give you a solid answer on that and the best doppler systems can't track far enough out to actually have data anyway. Anything you do is then extrapolating beyond the curve.

As a blanked warning, the more you work with modeling the less you will trust any model output. Sometimes to the point of anti-confirmation bias.

-Alex

 

[bulletbluesky]

Don't overthink it. Modeling tells you a maybe answer and hopefully helps you avoid clear failures when it comes time to making something and testing it in the real world. We already have real world data and demonstrated capabilities so applying a generalized model is pointless at best.

Unless you are doing bullet design and having someone turn solids for you, you are far better off simply shooting a bullet in question at extended ranges and seeing if they still group for shit. The modeling will never give you a solid answer on that and the best doppler systems can't track far enough out to actually have data anyway. Anything you do is then extrapolating beyond the curve.

As a blanked warning, the more you work with modeling the less you will trust any model output. Sometimes to the point of anti-confirmation bias.

-Alex

I agree that the models, no matter how carefully prepared they might be, can only serve as guidance. There are numerous cases where simuls' outputs turned to be quite different from outcomes of real world tests. As regards your remark on tracking radars, seems you're not aware of advancements corresponding technology has made during past decade. Modern tracking Doppler radar systems have a capability to track down small arms projectiles (as small as 0.2 cal.) to over 3000 yards slant distance during fair weather conditions. What used to be Science Fiction, today is reality

 

[Sandow the Heretic]

I agree that the models, no matter how carefully prepared they might be, can only serve as guidance. There are numerous cases where simuls' outputs turned to be quite different from outcomes of real world tests. As regards your remark on tracking radars, seems you're not aware of advancements corresponding technology has made during past decade. Modern tracking Doppler radar systems have a capability to track down small arms projectiles (as small as 0.2 cal.) to over 3000 yards slant distance during fair weather conditions. What used to be Science Fiction, today is reality

I know what capabilities are occasionally quoted but I've seen no actual data past about a mile on anything as small as we are concerned about. Can you show me any actual data at 3000 yards?

-Alex

 

[bulletbluesky]

I know what capabilities are occasionally quoted but I've seen no actual data past about a mile on anything as small as we are concerned about. Can you show me any actual data at 3000 yards?

-Alex

This is a sample for about mile for a 0.310 projectile, from about 20(!) years ago ,from Yuma with love:

Weibel's phased array models of today (for instance MFTR-2100/36-80) can certainly do better and more than this (+ they are not insanely expensive as they used to be)

 

[Sandow the Heretic]

This is a sample for about mile for a 0.310 projectile, from about 20(!) years ago ,from Yuma with love:

View attachment 8376666



Weibel's phased array models of today (for instance MFTR-2100/36-80) can certainly do better and more than this (+ they are not insanely expensive as they used to be)

While the radar setup that AB uses is a more approachable $100k and change, it tops out at 15 watts. Vastly cheaper than it used to be for similar capabilities. Reaching out further requires more power and more exotic and expensive equipment like the Weibel's. My guess is that the number of times someone has paid to have small arms run across a 4KW system can be counted on one hand. That is going to remain a niche small enough that it is unlikely to get cheaper or for the economics to change. How many more bullets would someone need to sell to cover a six figure radar day and would that ever be recoverable?

Would love to be wrong on this and for there to be a preponderance of long range data coming our way but I'm not holding my breath.

-Alex

 

[bulletbluesky]

While the radar setup that AB uses is a more approachable $100k and change, it tops out at 15 watts. Vastly cheaper than it used to be for similar capabilities. Reaching out further requires more power and more exotic and expensive equipment like the Weibel's. My guess is that the number of times someone has paid to have small arms run across a 4KW system can be counted on one hand. That is going to remain a niche small enough that it is unlikely to get cheaper or for the economics to change. How many more bullets would someone need to sell to cover a six figure radar day and would that ever be recoverable?

Would love to be wrong on this and for there to be a preponderance of long range data coming our way but I'm not holding my breath.

-Alex

I don't know what AB uses. I'd guess something like this:
https://www.infinition.com/wp-conte...015-Doppler-Radar-System-Description-Rev2.pdf
That can't cost that much today. I know becouse Weibel's velocity radar system SL-30033PE (with ~2000 m /.223 cal capability) outperforms it and can be afforded by 5 figure $ price. OTOH, powerful multipurpose military radar costed much more than $100k back in 2000. I wouldn't be surprised with the price like &1M