Tikka 223 77 TMK keyhole..

[Harvey_NW]

I'm stumped. A buddy of mine sent me these results asking if I could answer why, and I can't. I've listened to the podcasts about stability and this seems like an anomaly, so maybe I'm missing something. Hopefully @Formidilosus can chime in with a technical answer.

Specs: Tikka 223 8 twist chopped to 20" and threaded, 77 TMK @ 2780fps. Shot in 40° @ 500' elevation.

He confirmed it is an 8 twist with a cleaning rod. Stability calculator seems good. No evidence of baffle strikes. He said it shot great groups at 500, shot plates out to 700, but at 840 yards 3/4 impacts keyholed the 4'×4' plywood target, with the 4th missing completely. Thoughts?

 

[TaperPin]

That’s pretty cool. Speed of sound is something like 1100 fps - is it getting messed up crossing into subsonic?

 

[hereinaz]

Could be that it’s not making it through the transonic phase without tumbling. Not much else would do that mid flight, so likely scenario, though there could be other factors because not all rounds tumble.

Red is transonic per AB. Don’t know your pressure, but what I used is likely lower.

 

[Harvey_NW]

That’s pretty cool. Speed of sound is something like 1100 fps - is it getting messed up crossing into subsonic?

Could be that it’s not making it through the transonic phase without tumbling. Not much else would do that mid flight, so likely scenario, though there could be other factors because not all rounds tumble.

Red is transonic per AB. Don’t know your pressure, but what I used is likely lower.
View attachment 818126

So that's where I'm perplexed, because it definitely seems like it. However, Jayden Quinlan jokes about "transonic instability and transonic gremlins" as though it's a commonly "observed" phenomenon, that doesn't actually happen as often as people believe. He even says that bullets actually get more stable the further they are in flight, assuming they're stabilized properly from launch. Based on the situation, I don't understand what would cause them to tumble.

 

[huntnful]

Looks like it would make a hell of a wound channel. He’s good to go IMO

 

[hereinaz]

So that's where I'm perplexed, because it definitely seems like it. However, Jayden Quinlan jokes about "transonic instability and transonic gremlins" as though it's a commonly "observed" phenomenon, that doesn't actually happen as often as people believe. He even says that bullets actually get more stable the further they are in flight, assuming they're stabilized properly from launch. Based on the situation, I don't understand what would cause them to tumble.

You're probably safe to say that the noises outside the tent at night are nothing to worry about. But, sometimes it really is a grizzly bear ready to eat you...

Given the factors, there are very few gremlins that can make a bullet keyhole at 850 if the bullet wasn't tumbling before that. As far as I know, the transonic phase is one of them. And, of course, you can't know if it is "properly stabilized" until it isn't. There's probably another factor at play, specific to his rifle/load which induces the tumbling at transonic.

I'd be curious to hear more, which is one reason I commented. There's lots of Tikka 77 gr shooters here.

 

[Harvey_NW]

Sorry for commenting on an old post, but I am currious. What was your conclusion?

Still formulating, I posted this like an hour ago

 

[sndmn11]

Sorry for commenting on an old post, but I am currious. What was your conclusion?

How old was it?

 

[sndmn11]

Still formulating, I posted this like an hour ago

It's a bot

 

[solarshooter]

I'd bet money it's transonic instability. The symptoms you described and the ballistics table posted above certainly point to that being the culprit.

He even says that bullets actually get more stable the further they are in flight, assuming they're stabilized properly from launch

This is true for supersonic flight down to transonic range - speed and dynamic pressure decrease, but RPM decreases more slowly. As you enter transonic range (~Mach 0.8-1.2), the fluid dynamics are transitioning rapidly and inconsistently from supersonic to subsonic flow. The aerodynamic center of pressure on the bullet changes drastically and dynamically (even back and forth) during this regime. Bullets absolutely need to be designed for these regimes to work well, and there are some famous ones (Sierra 168gr Matchkings for instance) that have a well known transonic instability (the 175gr Matchking was partially redesigned to address this, Litz writes about it).

A faster twist can help increase stability through this regime.

 

[Harvey_NW]

I'd bet money it's transonic instability. The symptoms you described and the ballistics table posted above certainly point to that being the culprit.


This is true for supersonic flight down to transonic range - speed and dynamic pressure decrease, but RPM decreases more slowly. As you enter transonic range (~Mach 0.8-1.2), the fluid dynamics are transitioning rapidly and inconsistently from supersonic to subsonic flow. The aerodynamic center of pressure on the bullet changes drastically and dynamically (even back and forth) during this regime. Bullets absolutely need to be designed for these regimes to work well, and there are some famous ones (Sierra 168gr Matchkings for instance) that have a well known transonic instability (the 175gr Matchking was partially redesigned to address this, Litz writes about it).

A faster twist can help increase stability through this regime.

I can't find which of their podcast episodes he addressed it, so I went back and listened to the segment in the Vortex podcast about stability where he explained going transonic and the magnus force that can occur to cause dynamic instability. So it sounds like my buddy had the perfect combination to produce a result that he sees significantly less than 1% happen in his testing. With how many RSS there are in the wild, I'm surprised it hasn't come up sooner, or at least I haven't seen it.

 

[waspocrew]

Interesting to see that!

 

[Formidilosus]

I'm stumped. A buddy of mine sent me these results asking if I could answer why, and I can't. I've listened to the podcasts about stability and this seems like an anomaly, so maybe I'm missing something. Hopefully @Formidilosus can chime in with a technical answer.

Specs: Tikka 223 8 twist chopped to 20" and threaded, 77 TMK @ 2780fps. Shot in 40° @ 500' elevation.

He confirmed it is an 8 twist with a cleaning rod. Stability calculator seems good. No evidence of baffle strikes. He said it shot great groups at 500, shot plates out to 700, but at 840 yards 3/4 impacts keyholed the 4'×4' plywood target, with the 4th missing completely. Thoughts?

View attachment 818099View attachment 818101View attachment 818102

Taken as it is shown, it’s becoming unstable- probably through transonic. However I/we have shot tens of thousands of 77gr TMK’s from 1:8” twist barrels at negative DA’s without issue, and they generally go through transonic well.

 

[JAC8504]

a rokslide special not working?! Blasphemy

 

[Wapiti1]

Has he verified the rifling twist is 1:8 by measuring it? What's on the barrel isn't always what's in the barrel.

Worth checking to see what the twist actually is. Even Tikka could mis-mark a barrel out of the thousands of rifles they build.

Jeremy

 

[solarshooter]

I think twist rate definitely needs to be verified. The 8 twist barrels are not stamped with twist rate, correct? It might be hard to tell the 1:8 vs 1:10 twist barrels using the cleaning rod method, that's a pretty small difference.

Another thing I can think of is that perhaps the barrel has damage or wear in the rifling/lands such that the surface of the bullet is getting torn up in the barrel. This can change the aerodynamics of the bullet significantly and might explain why he is seeing this and others are not. How many rounds on it? What's the max COAL of the chamber? Do you have a borescope?

 

[Dos Perros]

He said in the first post he verified the twist with a cleaning rod.

 

[Reburn]

I think twist rate definitely needs to be verified. The 8 twist barrels are not stamped with twist rate, correct? It might be hard to tell the 1:8 vs 1:10 twist barrels using the cleaning rod method, that's a pretty small difference.

Another thing I can think of is that perhaps the barrel has damage or wear in the rifling/lands such that the surface of the bullet is getting torn up in the barrel. This can change the aerodynamics of the bullet significantly and might explain why he is seeing this and others are not. How many rounds on it? What's the max COAL of the chamber? Do you have a borescope?

What makes you say 2" is a small difference to measure?

 

[solarshooter]

What makes you say 2" is a small difference to measure?

Well the cleaning rod method (at least as I do it) relies on counting 1 revolution of an indicator mark or a tape flag and then measuring the distance traveled. If you had a 10 twist barrel, and your assessment of indicator angle was off by ~30deg at either end, you could mistake that for an 8 twist (ie measure 8" from "vertical" to "vertical" for instance). Now 30deg is a lot and your friend is probably not making that mistake but it is definitely one of the critical inputs to verify.

 

[Reburn]

Well the cleaning rod method (at least as I do it) relies on counting 1 revolution of an indicator mark or a tape flag and then measuring the distance traveled. If you had a 10 twist barrel, and your assessment of indicator angle was off by ~30deg at either end, you could mistake that for an 8 twist (ie measure 8" from "vertical" to "vertical" for instance). Now 30deg is a lot and your friend is probably not making that mistake but it is definitely one of the critical inputs to verify.

You like to use alot of words.
He isnt my friend.
I do agree he should double check his twist.

However. On a 16" barrel its NOT hard at all to measure 8" or 10". You can be well away from the throat or muzzle.
You said it was a small difference when in fact 2" and 30 degrees is not a small difference.

Words mean things. You are the one always harping on being accurate. Lets be accurate with our words as well.