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October 17, 2008, 12:12 PM | #1 |
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Bullet stabilization question
If a bullet seems to be stable at 100 yds. (not keyholeing)
is it safe to assume it will still be stable at longer ranges or can it become unstable as the bullet slows? |
October 17, 2008, 12:19 PM | #2 |
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Good question. I don't know, but I'm going to hypothesize that it will not keyhole because it will maintain a parabolic trajectory straight into the ground and because velocity decreases much more quickly than spin rate (gyroscopic stability). I hope somebody has a definitive answer.
For example, if you shot a bullet dead horizontally and dropped a bullet from barrel height at the exact moment the gun was shot, both bullets would hit the ground at the same time, assuming uniform ground elevation throughout the shot bullet's flight (horizontal component of motion is independent of the vertical component). The bullet out of the gun is simply shot at such a speed as to arrive at a target before gravity makes any appreciable effect on the elevation, unless you're a sniper (in which case you need to do some basic trigonometry in your head before you shoot). I'd imagine it would take some sort of disruption to cause the bullet to tumble (hitting something, or being shot straight up, stopping, and beginning to fall at terminal velocity). It has been a couple years since those college physics and calc classes. |
October 17, 2008, 07:49 PM | #3 |
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It's my understanding that supersonic bullets are generally pretty stable (assuming twist rate is proper for bullet length) as long as their velocity remains above the speed of sound. At the 1000 yard target line you can really tell when the loads are going subsonic. Gives new meaning to the term "flyer".
Even a high power rifle has to be fired upward to hit level with the sight plane at even 100 yards. The bullet will cross the sighting plane at 25-40 yds, depending on sight height, before crossing back through the plane at 100 yards.
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October 17, 2008, 08:02 PM | #4 |
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Yes, but I still think (generally speaking) if a bullet departs properly it will maintain gyroscopic stability throughout the flight because the velocity will drop off faster. Are there any true physicists out there to set us armchair scientists straight?
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October 17, 2008, 08:58 PM | #5 |
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Depending on the balance of the bullet, it may or may not tumble or yaw at longer ranges. Elmer Keith designed his bullets to shoot well at unheard of ranges. The long nose helps balance the bullet versus the shank length. Shorter bullets may well tumble because the balance point gets obscured as velocity drops and the twist slows over distance. The only sure way to find out is to shoot at longer range at white paper and measure the holes to see if the holes elongate or widen.
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October 17, 2008, 09:47 PM | #6 |
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So am I to assume that a longer length for caliber bullet should be better
than a shorter length bullet ? |
October 17, 2008, 11:29 PM | #7 |
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October 18, 2008, 12:22 AM | #8 |
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Depends on the calibre. A .308" bullet doesn't completely stabilise, as in stop wobbling, and give its best penetration until it has gone about 300 yards. That doesn't mean it'll keyhole though. Key holing is usually caused by undersized bullets. Not enough bullet diameter for the barrel diameter. A .308" bullet out of a .303 British barrel, for example.
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October 18, 2008, 02:25 AM | #9 |
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I cannot answer your question, but can give you an example of what I think is a stabilization thing: I have a old ruger 77 varmint in 22-250 thats shoots consistantly 1 1/4 inchs groups at 100 yards. At 200 yards it opens to 2 inches. At 300 yards it shoots the same as the 100 yard 1 1/4 inches.
It has a 1-12 twist and I shoot 55 grainers in it. My assumption is that it doesn't stabilize untill it gets a ways downrange. I may be wrong thinking its a stabilization thing. Does anyone have a theory on this? |
October 18, 2008, 08:34 AM | #10 |
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The cartridge in question is a 250-3000 Savage in a 1 in 14" twist. I've read
that the 100 grn. bullet may or may not stabilize with this twist. Where I normally go to shoot Is only 100 yds. I want to be able to shoot deer out to 200 yds. which got me to thinking of the original question. I guess shooting it at the different ranges is only way to know for sure. I was just wondering if there was an exact mathematical or physical answer to the question. Thanks for all the replies and I 'll welcome any other Ideas or experience with this cartridge. |
October 19, 2008, 11:07 AM | #11 |
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da troot
Bullets can stabilize or destabilze at any point along their path.
Crap, huh? Set up targets at 25 yd increments to witness this phenomenon yerself.....(make 'em thin so they don't affect stabilization; high-speed photography works best).
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October 19, 2008, 11:10 AM | #12 |
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easier
Call the Sierra bullet-tech line....
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October 20, 2008, 02:35 PM | #13 |
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When I went out to Gunsite for the PR1 class in 2000, I and most others there purchased the PMC ammo loaded with the 168 grain Sierra MatchKing. All went fine until we got to shoot at a popper at a lasered 748 yards across a small valley. Nobody could stay on it. Hits to the right, hits to the left, above, below and all around the clock. It's was like the guns were throwing curve-balls and sinkers at it. The problem was put down to gusting of the 20 mph crosswind we were shooting through, and we only shot at it that one day, so we shrugged it off.
Later that year I attended the Long Range Firing School at Camp Perry. Our first target was at 800 yards. All kinds of guns were there, including a lot of .30 cals. At the sight-in, all sorts of cursing and yowling went on up and down the line. All from people shooting 168 grain MatchKings or their clones. Report from the pits included full sideways hits and a lot of keyholes—when the holes were on the paper at all. Lots of dust kicked up, too, and the target frames took a beating. Did I mention whizzing sounds over the heads of the target pullers? As it happened, Sierra ballistic technician, Kevin Thomas was attending the school that year. After that first morning of 800 yard shooting going so badly, we asked him about it. Well, it turns out the 168 is a 1959 bullet design for 300 meter International Match shooting. That it works well for 600 yard service rifle shooting is just a bonus. But from the .30-06 and .308's, it starts to drop into the transonic range (about 1050-1350 fps at sea level under standard conditions) at around 700 yards. In the transonic range, drag shoots up. Parts of the bullet have air going over it that exceed the speed of sound (around the ogive, for example) and parts have air going over it that is subsonic (off the back, for example), supersonic shock waves are collapsing and all sorts of drag-related disruption of the bullet's flight goes on. The 168 was never shaped to avoid any of that. Happily, the 175 grain SMK, with its long secant ogive is able to withstand transonic disturbances much better. Everyone at the school raced over to Commercial Row during the lunch break and bought enough commercially loaded ammo with the 175 grains bullets to finish the school. After lunch, no more keyholes. Even at 1000 yards, no keyholes. So, the answer to the original question is that stable bullets can be destabilized by falling into the transonic range where drag increases rather dramatically. The necessary spin rate for bullets to remain stable goes up with it. For some shapes the required spin rate just gets faster than the bullet is actually spinning.
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October 20, 2008, 03:31 PM | #14 |
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Well, there you have it.
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October 20, 2008, 08:16 PM | #15 |
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I've always said that a 30/06 loaded with Sierra 180 grain Matchkings and 47.0 to 48.0 of IMR 4064 is D*** hard to beat. See above.
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