Nine Inch Tumblers

[Hib]

Hoping for feedback from the experience of others.
Reloading for 5.56/223, what bullets have you found to tumble in a 9" twist?
Feedback on tumbles you've seen with any length barrel with 9" twist would be helpful.
Thanks,
Hib

[Bang]

General rule of thumb:

1:9 40,45,50,55,62 gr GTG
1:7 55, 62, 75 77 gr GTG
Note the 55 and 62 gr overlap.

The bigger importance is the length of the bullet. A quick way to get into the ball park is to call the bullet manufacturer and inquire about a specific bullet. CS tech can give good guidance.

BTW...find the actual twist with a ball-bearing cleaning rod and bore brush. Measure the actual distance one turn takes. For example I have a 1:8 that measures 1:8.5. Knowing this can account for some anomalies.

[gghbi]

Interesting graphs on this thread, post #10. Wonder where they are from and what do they look like for lighter bullets.
Calling Bamboomaster for that answer...

http://forum.pafoa.org/showthread.php?t=319399

[ianb1116]

General rule of thumb:

1:9 40,45,50,55,62 gr GTG
1:7 55, 62, 75 77 gr GTG
Note the 55 and 62 gr overlap.

The bigger importance is the length of the bullet. A quick way to get into the ball park is to call the bullet manufacturer and inquire about a specific bullet. CS tech can give good guidance.

BTW...find the actual twist with a ball-bearing cleaning rod and bore brush. Measure the actual distance one turn takes. For example I have a 1:8 that measures 1:8.5. Knowing this can account for some anomalies.

This is good advice. Pretty much hit the nail on the head.

However, I'd be hesitant to shoot a 40 or 45 grainer in a 1:9. Might want to bup up to the grand dad of AR rifling--1:12

[Hib]

If anyone has determined yawing or tumbling in their 9" twist, please give a yell.
A bullet found to have marginal stability in 9"?

[X Hunter]

Bullet stability can be done at the Berger website.
http://www.bergerbullets.com/twist-rate-calculator/
You can "What If" the calculator all you want.

[Hib]

Yep, looked at the Berger calculator a few days ago and it does pretty good on getting you in the ball park on making sure that a bullet you purchase will be stabilized by your piece. However, what I'm after is input from observed experience from folks on what they have seen to manifest instability in their 9" twist pieces (as opposed to what did not).

Got my fingers crossed on maybe getting input.

[ianb1116]

Yep, looked at the Berger calculator a few days ago and it does pretty good on getting you in the ball park on making sure that a bullet you purchase will be stabilized by your piece. However, what I'm after is input from observed experience from folks on what they have seen to manifest instability in their 9" twist pieces (as opposed to what did not).

Got my fingers crossed on maybe getting input.

It may help to explain why you're interested in said tumbling projos.

[Hib]

Cannot imagine how explaining the why of the matter could help seeing as I asked if anyone had experience to offer on which bullets they have seen tumbling or yaw coming out of a 9" twist barrel. However, I am not adverse to describing the experiment that I am engaging in. It involves the use of bullets which are at the very edge of instability in my particular piece. Remember the original design intent of the 5.56 for marginal stability producing tumbling upon impact? That was with short 55 grainers and 14" twist. I am trying to obtain usable accuracy at longer ranges with high BC bullets and marginal stability.
Currently the Sierra 69 grain HPBT's are on the test stand. If they'll shoot straight enough in my particular piece and loads I cobble together then the next step will be impact tests. If they won't settle in and shoot good groups then it's a fail. If they drill straight through then it's a fail.
so that's what I'm up to.

[ianb1116]

Cannot imagine how explaining the why of the matter could help seeing as I asked if anyone had experience to offer on which bullets they have seen tumbling or yaw coming out of a 9" twist barrel. However, I am not adverse to describing the experiment that I am engaging in. It involves the use of bullets which are at the very edge of instability in my particular piece. Remember the original design intent of the 5.56 for marginal stability producing tumbling upon impact? That was with short 55 grainers and 14" twist. I am trying to obtain usable accuracy at longer ranges with high BC bullets and marginal stability.
Currently the Sierra 69 grain HPBT's are on the test stand. If they'll shoot straight enough in my particular piece and loads I cobble together then the next step will be impact tests. If they won't settle in and shoot good groups then it's a fail. If they drill straight through then it's a fail.
so that's what I'm up to.

Why would you want to shoot anything with marginal flight stability?

If you are trying to induce tumbling at terminal/impact conditions, like that of the 5.45x39, you're looking in the wrong place. You will have to drastically change bullet CG such that a smaller, imparted force on the bullet will destabilize and tumble the round. The 5.45 achieved this by using a brass tip, hollow air pocket, and steel core to push the CG as far back on the bullet body as possible during flight. Upon impact the steel core slides forward into the air gap, tumbling the bullet.