Tikka 223 77 TMK keyhole..

[Wapiti1]

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

Oops. Missed that.

Jeremy

 

[xsn10s]

At your temp and altitude I get you velocity to be 1203 fps at 840 yards according to jbm. Sierra lists the bc to be .380 @ 1750 fps and lower. When I plug in those numbers to Berger's Barrel Twist Calculator here the twist rate I get.

 

[xsn10s]

If I use hereinaz's AB ballistics chart my best guess at 840 yards is a velocity of 1120 fps. And let's say for whatever reason the barrel in question doesn't give a true 1-8" twist. Let's just say that it's twist rate acts more like 1-8.1" twist. His bullet would be unstable at 840 yards. Right around where he would be going transonic.

 

[Harvey_NW]

At your temp and altitude I get you velocity to be 1203 fps at 840 yards according to jbm. Sierra lists the bc to be .380 @ 1750 fps and lower. When I plug in those numbers to Berger's Barrel Twist Calculator here the twist rate I get. View attachment 818670

Interesting, maybe he didn't account for the change in BC. Good catch!

 

[xsn10s]

Interesting, maybe he didn't account for the change in BC. Good catch!

Thanks. Just a suggestion on what might be happening. Hard tellin not knowin.

 

[wind gypsy]

At your temp and altitude I get you velocity to be 1203 fps at 840 yards according to jbm. Sierra lists the bc to be .380 @ 1750 fps and lower. When I plug in those numbers to Berger's Barrel Twist Calculator here the twist rate I get. View attachment 818670

It doesn't work that way. 1203 FPS is not the muzzle velocity. Muzzle velocity is important because that's how you calculate bullet RPM. Bullet RPM decreases much slower than velocity, thus they'll be spinning much faster at 800+ yards than what the scenario you ran indicates.

More RPM = More stability.

 

[xsn10s]

It doesn't work that way. 1203 FPS is not the muzzle velocity. Muzzle velocity is important because that's how you calculate bullet RPM. Bullet RPM decreases much slower than velocity, thus they'll be spinning much faster at 800+ yards than what the scenario you ran indicates.

More RPM = More stability.

However you'd like to explain it. In any case the 77gr TMK has marginal stability according to Berger's calculator.

 

[xsn10s]

According to the above screen shot Berger says to decrease the BC by 4%. If I plug in a 1-8.1" twist it decreases the BC by 5%. Which could put the bullet transonic at 840 yards. 1-8.2" twist drops the BC to 6%.

 

[xsn10s]

Since the barrel was chopped there could be a crown issue too. So that in itself could contribute to the instability. Hard tellin not knowin.

 

[wind gypsy]

According to the above screen shot Berger says to decrease the BC by 4%. If I plug in a 1-8.1" twist it decreases the BC by 5%. Which could put the bullet transonic at 840 yards. 1-8.2" twist drops the BC to 6%.

I think because you’re using the full length including the polymer tip in the Berger calc? Jbm says it’s plenty at standard atmosphere when tip details are entered.

 

[xsn10s]

I think because you’re using the full length including the polymer tip in the Berger calc? Jbm says it’s plenty at standard atmosphere when tip details are entered.

View attachment 818873

Ok well in any case the bullets are keyholing lol.

 

[xsn10s]

According to this article plastic tips elongate the bullets without adding much mass. So they require faster twist. the formula in jbm's site does the opposite. jbm's calculator could be more accurate, but the keyholing suggests otherwise. https://www.xxlreloading.com/post/how-do-you-determine-the-suitable-barrel-twist-rate

 

[riversidejeep]

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.

My .308 is afflicted by this, 168 SMK shoots great out to 800 anything past that it like throwing the dice, probably a 36" "pattern" at 1000 yards. At 800 it a sub MOA.

 

[xsn10s]

My .308 is afflicted by this, 168 SMK shoots great out to 800 anything past that it like throwing the dice, probably a 36" "pattern" at 1000 yards. At 800 it a sub MOA.

My PSS 308 Win is okay out to 1200 yards depending on the day @ 2200 ft elevation. 168gr SMK or CC acts about the same.

 

[solarshooter]

My PSS 308 Win is okay out to 1200 yards depending on the day @ 2200 ft elevation. 168gr SMK or CC acts about the same.

Twist rate? Also what is your definition of "okay"?

Edit: I ask because faster than usual twist (say 1:10 rather than 1:12) and high DA would increase stability such that you might not see this until beyond 1200. Also higher MV. And at 1200 there are enough other large sources of error you might not necessarily notice the effect of the instability.