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Old December 12, 2018, 07:20 PM   #1
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7,000 fps?

Random question. My chronograph is advertised to be able to clock projectiles up to 7,000 fps. I shrugged it off as an advertising thing, but i noticed some others can do 10,000 fps? I know there is no commercial load that fast, but are there any handloads or wildcats that will do even half that? Or are people putting something really fast over the chronograph other than bullets? Can't imagine what... Or what is the advantage of having a chronograph with those capabilities? Thanks in advance for entertaining my curiosity.
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Old December 12, 2018, 07:39 PM   #2
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The only thing I can think of that will travel close to 7000fps is a Rail Gun. I dont' think they go that fast.
It has to be marketing hype.
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Old December 12, 2018, 07:58 PM   #3
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yea, probably marketing hype for dummies like me.

Like; "wow, if it can read/measure 7,000 fps, then it must be really, really accurate when measuring my 2,000 fps loads!"
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Old December 12, 2018, 08:17 PM   #4
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There is a thread on that. I believe 4K+ is achieved with some common cartridges (barrels go fast!). 5k I think is a normal upper limit.

One aspect of the high value is the lower readings might get more accurate.
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Old December 12, 2018, 09:17 PM   #5
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The 220 swift was advertised at 5200 when it certainly out.
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Old December 12, 2018, 10:50 PM   #6
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The only thing you would need a chronograph that measured over 4500 for would be if you wanted to measure the speed of the gas.
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Old December 12, 2018, 10:59 PM   #7
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Lots of instruments are most accurate near the middle of their operating range.
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Old December 13, 2018, 07:39 AM   #8
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There are a few cartridges that go over 4000. My understanding is the realistic velocity is limited to the rate that burning powder can expand. I know some smooth bore tank rounds go about 5000 FPS, but you aren't going to fire that over a regular chrono.
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Old December 13, 2018, 03:04 PM   #9
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Wow, i must get all kinds of error at my 250 fps bow...
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Old December 13, 2018, 03:25 PM   #10
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Half of 7,000 is 3,500. Lots of rifle loads in that area. Doesn't really matter what the chronograph will do though.
"...smooth bore tank rounds..." The depleted uranium projectile of an APFSDS will go that fast, but not regular rounds.
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Old December 13, 2018, 09:37 PM   #11
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7000 fps is used for timing a politician grabbing a dollar.
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Old December 13, 2018, 10:01 PM   #12
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The 220 swift was advertised at 5200 when it certainly out.
Uh, no. The original .220 Swift factory loading was 4110 fps with a 48-grain bullet.

The reason few bullets exceed 4000 fps is due to the velocity of the gas pushing them. Obviously the bullet can’t go faster than the gas, which various sources list as exiting the muzzle around 4500 fps depending on the chamber pressure of the particular cartridge. Bullets have been clocked faster, but only at extremely high pressures.

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Old December 13, 2018, 10:12 PM   #13
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It think the reason was hit upon in that a Chrono that can see a projectile at 10,000 fps will be very accurate at speeds 1/2 that and below. For instance, if you Chrono is accurate to 3500 fps and you routinely shoot at 3300 fps then the margin of error is probably pretty high where as a chrono that could do 10,000 fps might not ever have an error. It's the only thing that makes sense to me. Most of bullets I shoot would turn to dust at 5000 fps.
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Old December 14, 2018, 02:50 AM   #14
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Just because the chronograph can measure it doesn't mean you will ever get anything to go that fast. My wife's Subaru's speedometer goes to 120, but I hope she never tests it out.
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Old December 14, 2018, 07:37 AM   #15
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Actually most measuring instruments are accurate to some percentage of full scale. So their accuracy, percentage wise, will be better closer to full scale. Think of a yard stick that can measure down to 1/32 of an inch. If you were measuring something 32 inches long the error could only be as high a tenth of a percent (1/32 out of 32 inches) but if you measured something a quarter inch long the same 1/32 inch error could be 25 percent.
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Old December 14, 2018, 05:11 PM   #16
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A chronograph's resolution is limited by its clock speed. One cycle of the clock is a larger percentage of the transit time of a fast projectile than it is for a slow one, so, no, the percent resolution is not constant with a chronograph.

The velocity limits are imposed by the speed gas can get up to. The gas behind a bullet only goes as fast as the bullet does until the bullet clears the muzzle. At that point, it is accelerated to escape the barrel, but that short time and limited gas supply means it tends to be some amount over the bullet speed. That's not the same limit it will have in a longer barrel, for example.

IIRC, the military did a bunch of experiments necking down 20 mm or even 37 mm cases to 30 calibers to see how fast they could get bullets to go, and I think that even when they had half a pound of powder behind it, they could only get to about 6,000 fps. So that's probably closer to a real limit.

Rail guns can, in theory, get to 10,000 fps. I can't recall the velocities for the ones I've seen YouTube videos of.

I did once get a close look at a gun in a lab that fired a 1/4" aluminum ball at 17,000 fps. The way it was done was through a 40 ft long barrel that had two parts that came apart in the middle. The first 20 foot section was chambered for a 40 mm casing that held a pound of powder. The projectile it fired was a disposable plastic piston. At the second section, the bore narrowed from 40 mm to 50 caliber. That portion was loaded with a sabot that held the aluminum ball and a 60,000 psi burst disc was placed behind it.

To fire the gun, both sections were loaded, and the bore of the first section was evacuated and then back-filled with helium. Under equal pressure as air, helium can move about three times faster. Firing the pound of powder pushed the plastic piston (use-once, throw-away) down the first section of the barrel, compressing the helium rapidly. The burst disc would let go, and the 60,000 psi helium drove the Nylon sabot down the second 20 feet, and that's how they got the velocity. The plastic piston extruded into the 50 cal portion of the bore about half way before the pressure behind it bled off. The Nylon sabot had two halves that separated at the muzzle and struck some armor plate that had a hole in the middle for the ball to pass through. That plate was heavily dinged by the sabot halves hitting it with that much energy. The ball went through the hole and only traveled a few feet to its target. There was no rifling involved. All smoothbore.
Powder gases would be much too heavy to get to 17,000 fps. The gun was in a lab that tested experimental satellite armor against small meteorite impact, as represented by the little aluminum ball, taking super high speed X-ray images to study its interaction with the armor.
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