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Posted: 10/3/2005 9:24:09 PM EDT
[Last Edit: 10/3/2005 9:25:26 PM EDT by Yammymonkey]
Just to confirm; if my 16" 1 in 9" bbl stabilizes the 75gr rounds I'm trying out at 100yds, they should also be stable past that correct?  Searched the Ammo Oracle, this is what I came up with, just double checking since there seemed to be some contradiction between that and the fact that the 1 in 12" bbls won't stabilize the SS109 past 100yds.

Thanks.
Link Posted: 10/3/2005 9:38:24 PM EDT
think of the bullet like a top. As velocity (spin) drops off so does the stability. At what point it will happen I can't say for sure but it will happen at some point.
Link Posted: 10/3/2005 9:49:46 PM EDT
Link Posted: 10/3/2005 10:05:12 PM EDT
It's the standard BHA 75gr loading, looks like 2750 fps, but I have no idea what the length is, if someone else can chime in.  If not, I'll try to give them a call tomorrow.
Link Posted: 10/4/2005 3:44:16 AM EDT
I believe these are Hornady 75 g HPBT which are .99" in length. If the bullet becomes unstable at greater distances you will see oblong holes in the target or keyholing accompanied by a loss of accuracy.
Link Posted: 10/4/2005 12:09:59 PM EDT
Link Posted: 10/4/2005 1:32:21 PM EDT
If it is stable out of the barrel it will be for its entire flight.  Forward velocity slows MUCH faster than spin.
Link Posted: 10/4/2005 2:00:33 PM EDT
[Last Edit: 10/4/2005 2:01:09 PM EDT by Zak-Smith]

Originally Posted By Austrian:
If it is stable out of the barrel it will be for its entire flight.  Forward velocity slows MUCH faster than spin.


This is halfway true.

Forward velocity drops off dramatically due to drag, but spin does not slow down as fast.  Thus, as a bullet travels downrange, it spins more revolutions per incremental yard than it did at the muzzle.  You could think of it as its effective "twist rate" is increased as it moves downrange.

The other half of the problem is that rotational inerta resists the transverse torque about the bullet's center of mass, applied by the drag on the tip.  The longer the flight distance, the more the bullet's axis orientation differs from its direction of flight (precession).   This acts to increase the transverse force on the bullet tip, which decreases its stability.

-z
Link Posted: 10/4/2005 6:23:33 PM EDT
Shoot it at the longest range that you will use it during high winds(20+mph).  If you see round holes you are cool.  I prefer this method.  Less math and more trigger.

BTW, I was pushing some 75OTMs around 2600 fps out of a 16" 1/9 twist barrel and they were good to 600 yds with a 8 mph steady wind.  Your results may vary.
Link Posted: 10/4/2005 7:30:45 PM EDT
I tried 77 grain Black Hills with Sierra Matchkings from my 16" chrome-lined 1/9 twist barrel.  I was really impressed with 3/4 inch groups at 100 yards.  However, at 200 yards only a fraction hit the paper, and at 300 yards none of them made it on the paper (and it was not a zeroing issue).  
Link Posted: 10/5/2005 11:54:33 AM EDT
I'll try them out at 100 for now and go from there if I can get some outdoor time at the 400yd range.

Thanks guys.
Link Posted: 10/5/2005 8:27:18 PM EDT

Originally Posted By Zak-Smith:

Originally Posted By Austrian:
If it is stable out of the barrel it will be for its entire flight.  Forward velocity slows MUCH faster than spin.


This is halfway true.

Forward velocity drops off dramatically due to drag, but spin does not slow down as fast.  Thus, as a bullet travels downrange, it spins more revolutions per incremental yard than it did at the muzzle.  You could think of it as its effective "twist rate" is increased as it moves downrange.

The other half of the problem is that rotational inerta resists the transverse torque about the bullet's center of mass, applied by the drag on the tip.  The longer the flight distance, the more the bullet's axis orientation differs from its direction of flight (precession).   This acts to increase the transverse force on the bullet tip, which decreases its stability.

-z



In which case you started off overstabilized (excessive static stability), not gyroscopically stabilized.

Gyroscopic stability is a necessary but not sufficient condition to stable flight.  We aren't talking about dynamic stability or tractability yet but we are WAY beyond the original question at this point.

For the purpose of the original question, if the round left the barrel stable it will be stable until it stops moving forward.  The only time traversal movement would be a factor is with twists around 1:4 or so in .223.
Link Posted: 10/5/2005 9:03:50 PM EDT
I don't think any of my statements were false, though I can't say what you read into them.  Please correct me if I'm wrong.
Link Posted: 10/13/2005 9:58:35 PM EDT
If anyone cares, I got out to the range last weekend and they were stable out of my gun from contact distance out to 300yds.  The holes in the targets were all perfectly round with the exception of the 200yd target where they looked almost like two rounds had passed through the same hole.  It was probably just a function of the target not being held tight to the plastic board backing though.

Thanks for the input, even though I didn't understand some of the physics stuff that was up there.
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