77 Tmk in 1-9 twist

[woods89]

This has been informative.

Is it not the case that more velocity= better stabilization?

 

[amassi]

To a degree yes, some bullets(light jacket) may explode if pushed too fast down too tight(fast) a twist.
Berger has a great stability calculator

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[LaHunter]

What you say could be true, however once the bullet has engaged the rifling, I can't imagine it would spin faster than the rifling. That would tend to obliterate the marks on the bearing surface, and make the rifling grooves on the bullet unreadable. It's possible you are correct.

The bullet does not spin faster than the rifling, it spins faster as it travels down the barrel. RPMs increase as bullet travels down the tube faster. Think of it this way, if it took 1 second for the bullet to travel down the 1:9 twist barrel that was 9" long, it would have a spin of 1 revolution / second. If it takes .1 second to travel down the same barrel, it will be spinning at a rate of 1 revolution / .1 second, or 10 rev / sec.

 

[LaHunter]

This has been informative.

Is it not the case that more velocity= better stabilization?

With more velocity you get higher rpm in a given barrel, so the higher rpm rate is what is stabilizing the bullet better than the slower rpm, up to a certain point for a given caliber and bullet construction

 

[robtattoo]

I don't know if it'll help at all, but this is 10 77tmks out of my 1:9 Axis.
I've only shot steel beyond 100yds with it so I really can't comment on stability, as such, but my 'accuracy cone' remains consistently predictable out to 350 (2"@200, 3"@300, 3.75@350) My next berm is 500 & that's waaaay beyond what i'm happy shooting game at & would require guesstimate holdover or dialing with a non premiere quality scope.
I would think that if I were losing stability that badly, the accuracy come would be inconsistent. Pure speculation on my part, however.

 

[TxxAgg]

Bump

 

[MattB]

What you say could be true, however once the bullet has engaged the rifling, I can't imagine it would spin faster than the rifling. That would tend to obliterate the marks on the bearing surface, and make the rifling grooves on the bullet unreadable. It's possible you are correct.

Can you imagine that a bullet going faster will cover a greater distance in a given period of time than one going slower? It is a simple extension of that concept. The one with the greater velocity will complete more revolutions in a given period of time because it has travelled a greater distance.

 

[Blockcaver]

Think of it this way, if it took 1 second for the bullet to travel down the 1:9 twist barrel that was 9" long, it would have a spin of 9 revolutions / second. If it takes .1 second to travel down the same barrel, it will be spinning at a rate of 9 revolutions / .1 second, or 90 rev / second.

If the twist is 1 in 9, that means 1 revolution of the bullet in 9" of barrel length. So a 9" long barrel in your sample would do 1 revolution / in 1 second. (Just so someone who does not understand this does not get mislead by the example posted).

 

[LaHunter]

If the twist is 1 in 9, that means 1 revolution of the bullet in 9" of barrel length. So a 9" long barrel in your sample would do 1 revolution / in 1 second. (Just so someone who does not understand this does not get mislead by the example posted).

you are correct, i will edit my reply so the numbers are correct

 

[amassi]

If the twist is 1 in 9, that means 1 revolution of the bullet in 9" of barrel length. So a 9" long barrel in your sample would do 1 revolution / in 1 second. (Just so someone who does not understand this does not get mislead by the example posted).

1 revolutions per 9" not per 1 second. In one second that bullet will travel about 3000' and its going to compete more than 1 revolution to get there probably around 4000
3000'x12"/ft =36000" ÷9"=4000 rps, 240,000 rpm.

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[Blockcaver]

amassi, The example given by LaHunter was incorrect. My post is correct based on the example LaHunter gave. See the post above yours where LaHunter corrects his post. We all realize a bullet goes faster/further than 9" in 1 second, but that was the example he used. By quoting my post without the LaHunter post I quoted the context is lost.

 

[LaHunter]

1 revolutions per 9" not per 1 second. In one second that bullet will travel about 3000' and its going to compete more than 1 revolution to get there probably around 4000
3000'x12"/ft =36000" ÷9"=4000 rps, 240,000 rpm.

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The original question was about bullet stability. Bullet stability, at least the way I understand it based on my readings, is primarily influenced by bullet spin, within normal rifle ballistics. I tried to give a very basic illustration to help demonstrate that as the bullet travels faster down a given barrel, the spin rate (rpm) is also faster. I did not try to provide real world mv and rpm rates, just to try to keep it simple. It is this faster spin that will provide better stability, not the faster muzzle velocity. Think of the bullet spin independently from bullet velocity.

 

[grappling_hook]

A couple of years later-- how's it going? Any issues with stability and/or accuracy with longer-range heavies in the 1:9 twist barrels?

 

[Wrench]

Twist rate is a measurement of distance vs rotation. That is a constant (minus drag) the speed of the rotation is a measurement we typically hear expressed as RPM, which is a measurement of time vs rotation.

The Greenhill will help explain.

Greenhill Formula for Optimal Rifling Twist Rate

The Greenhill Rifling Formula is used to calculate the optimal barrel twist rate for stabilizing a bullet's flight given the bullet's diameter(D), length(L), specific gravity (SG) and velocity(V).

www.vcalc.com

 

[woods89]

I never had keyholes or anything like that, but I just couldn't ever get the precision with them that I got with 68-73 gr bullets. I finally decided I was better off just sticking with bullet weights appropriate for the twist rate. I actually shot a doe last year with a 73 gr Berger, which worked fairly well. I have some 69 gr TMK for next year.

Edited to add- Moral of the story, buy 1-8 twist 223s.