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Bullet stabilization, strictly RPM?

 
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Old 10-04-2007, 08:54 PM
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Bullet stabilization, strictly RPM?

Let's say that you have two different cartridges like the 6.5-284 and the 6.5 STW. Also assume that the same bullet is being used like the Berger VLD 140. Lastly, assume that the rifling twist rate on the two barrels is such that they both spin the bullet at the same RPM--faster twist on the 6.5-284.

Is the bullet equally stabilized by each cartridge due to the same RPM spin even though it is most likely traveling at 300 or more fps faster out of the 6.5 STW? Will either bullet lose its rate of spin faster than the other in flight? I realize that the bullet from the 6.5-284 will slow to transsonic first, I'm talking strictly supersonic.
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Old 10-04-2007, 11:40 PM
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No two barrels can be counted on to equally stabilize bullets by simply matching their RPM's alone. Even with proper twists for the bullet at hand, one barrel will treat the bullet different than the other even if the RPM's are the same.

Stabilization is more than just cranking up RPM's to a certain number. It matters more that the yaw is reduced as much as possible as fast as possible and that comes from twist rate or distance covered by one revolution and also a host of other things. RPM's of course are intertwined in this equation but are not the direct comparison.

In other words, you could get a Berger 140 6.5 mm bullet to stabilize in a 10 twist by pushing up the speed to attain a specific RPM and even fire it in a thin atmosphere and it would work, but it would be better served to just use the recommended twist of 9and keep the bullet in it's factory design specifications. I have always seen this to give more accurate groups.
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Old 10-05-2007, 07:13 AM
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Quote:
Originally Posted by goodgrouper View Post
It matters more that the yaw is reduced as much as possible as fast as possible and that comes from twist rate or distance covered by one revolution and also a host of other things.
This is true.
Stabilization follows displacement per turn, NOT turns per time.
In your example above I can demonstrate why anyone should forget RPMs -forever.

140vld at 3000fps from a 8.5:1 barrel would have an Sg~1.40 under ICAO conditions. This happens to be 254Krpms.

Same bullet at 3300fps would be spinning 254Krpms from a 9.4:1 barrel. But under same conditions this twist rate would produce an Sg of only ~1.15

At 3600fps, 254Krpm would come from a 10.2:1 barrel. this is the point where the bullet performance completely sucks though, as Sg would be ~1.00 at ICAO.


So stability here is greatly affected regardless of rpms..
It is DISPLACEMENT PER TURN that has by far the largest influence on bullet stability.
Also, another factor not accounted for from an rpm perspective, is air density. This can wildly affect stability as well. It is the second largest influence, followed by muzzle release quality, and then dynamic design issues.

Any connection with RPMs is pure coincidental.

Last edited by Mikecr; 10-05-2007 at 07:39 AM. Reason: spelling
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Old 10-05-2007, 08:52 AM
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Sg

I happen to use a Sg of 1.35 to consider a bullet fully stabilized for target purposes..Military is using much lower value for their 6.8 trials expecting that the bullet may tumble upon impact thereby going back to the people popper abilities we had early in Nam until the REMf's decided to classify helmets as enemies---. Overbore
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Old 10-05-2007, 04:32 PM
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Makes good sense. I hadn't thought about it enough to consider distance traveled per revolution based on velocity. A clean bullet release is also obviously very important. A buddy of mine found this out when he took it upon himself to shorten the barrel on his Ruger Super Redhawk without recrowning it and then couldn't figure out why his groups went to crap.

I have a 700 7mm RUM that shoots nice small groups around 0.5" or less at 100 yds with 150 gr BT's. I believe the factory twist is 1-10". I am thinking about putting a brake on it and making it my long range deer/prairie goat/elk rig. Sounds like it probably would not stabilize some of the heavier pills in the 180+ gr range. I would think the 160 gr AB would stabilize nicely, maybe the 168 gr VLD.

As an aside, does a longer more aerodynamic bullet require a faster rifling twist on average? Comparing say that same 6.5 mm 140 gr VLD with its listed BC of 0.64 and the 140 gr Sierra spitzer boat tail with a listed BC of around 0.49. Does it go back to the issue of the longer bullet being more prone to yaw?

Thanks for the excellent feedback, guys!

Sam
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Old 10-05-2007, 07:37 PM
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As an aside, I think a lot of people get confused because a top spinning on the ground will stabalize with no forward velocity, IE based solely on rpm, and think that a bullet in flight will also ...
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Old 10-05-2007, 10:40 PM
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Quote:
Originally Posted by sambo3006 View Post

As an aside, does a longer more aerodynamic bullet require a faster rifling twist on average? Comparing say that same 6.5 mm 140 gr VLD with its listed BC of 0.64 and the 140 gr Sierra spitzer boat tail with a listed BC of around 0.49. Does it go back to the issue of the longer bullet being more prone to yaw?

Thanks for the excellent feedback, guys!

Sam
The longer the bullet, the faster it must turn in displacement to stabilize it. Typically, the longer a bullet is, the more aerodynamic it will be if shapes are same but if you change the shape to say a flat base round nose bullet the aerodynamics will obviously suffer.
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