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Posted: 4/9/2004 2:39:25 PM EDT
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I have a Bushmaster with a 14 1/2 inh barrel and a Hakko dot sight mounted on top hand guard. At 7 yards it will shoot 2'' low. Perfect zero at 50. If the sight is above the bore line with a close range as 7 yards why does the bullet hit low and not high, like the amount af distance from dot to bore? |
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Your sight plane and your barrel are NOT perfectly parallel. That would work on a laser, but not a gun that shoots a bullet on a parabolic curve. Instead, if the sight plane is level, the barrel will be pointing slightly up, so that the bullet will be climbing as it leaves the barrel. On an AR, the bullet will leave the barrel, and should cross the line of sight, which is about 2.25" higher than the muzzle, at about 50 yards. It will continue to rise until it reaches apogee at around 120 yards, after which it will begin to drop, crossing the line of sight again at around 220 yards. -Troy |
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On another note: I can't help but think another reason for the arc in the bullets trajectory, is the bullet becomes it's own wing, kind of like the dimples in a golf ball. When something is traveling 3000 fps through our atmosphere, I would think that would creat some lift or at least prevent it from dropping rappidly. ^I am way off?^ |
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Brewsky101, No magic wings!!!! Bullets (objects) fall at a constant rate due to gravity. Spin (rate) is just used to stabilize the bullet to keep the front tip forward, and prevent the bullet from tumbling in flight (resistance to surface tension, hence air). If you placed a rifle with it's barrel level to the ground and where to fire a round, once the bullet leaves the barrel, it would drop/fall at the same rate as is you dropped a marble from the same distance as the muzzle/time as the bullet exited the barrel. Trajectory is just the rate at which the bullet free falls in flight, verses the distance that it has traveled during its free fall. Since gravity is a constant, then the speed of the round is the variable, which is a faster round just covers more distance as it's falling. The faster the bullet travels, the less trajectory (drop) it will have (gets to the target sooner, hence less time/fall due to gravity). Aiming the rifle to strike a target is just the amount of hold over that the bullet path must be started (correction for trajectory). Other items can affect trajectory/flight such as air pressure, wind direction, and bullet shape, but lets just let these go for now. In regards to the golf ball with dimples, your correct on wing effect. As the ball spins, the dimples affect the surface tension of the ball. A ball with backspin will climb up do to less tension on the bottom of the ball (ballooning). It goes for sidespin with the effect either being a draw or a fade (slice for you hackers). |
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Quoted: No magic wings!!!! It's not magic, it's physics. Anyway, I was way off so I geuss it realy doesn't matter. What about this statement by rguard? "Mine is very accurate (I can shoot 1 MOA at 100yds) I can hit a deer silhouette at 400M pretty reliably (no idea how much energy it has way out there but it gives a satisfying "thunk") and [b][red]it takes like 2 seconds to get there[/red][/b] and hear the report and you have to correct up like 60-70 inches drop :o At 100 yds this thing is a TANK, it's like throwing bowling balls around (destroyed my 1/3inch thick steel spinner target (spinner actually held up pretty well, but a hit the frame pretty much mangled it beyond usability), at an indoor range the sound pressure transcends into a concussion you can actually feel ;) Ammo feeds fine in any mag (AA says you can bend out the feed lips if you want but I find it is not necessary) Fun is a word that falls short of describing this gun, anyone who has rapid-fired it ends up grinning like an idiot and recoil is really not that bad since I added a "limbsaver" recoil pad and vertical fore-grip. Basically, I LOVE THIS GUN !!!! :) RGUARD" [url=http://www.ar15.com/forums/topic.html?b=3&f=118&t=185947&w=searchPop]link to the above statement[/url] He is refering to his .50 Beowulf upper. It takes less than 2 seconds for a bullet to drop 5 feet when it falls from your hand to the ground. So where is all the extra time coming from? edited to add: I'm not trying to argue that bullets do experience lift after being fired, just trying to understand the whole trajectory process. |
| brewsky101, The extra time is coming from the 60-70 inch holdover. This higher trajectory allows the bullet to trade energy for altitude, thus staying airborne longer. Ever wonder why artillery pieces are pointed way over their targets when they are fired? Same concept. |
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Quoted: It takes less than 2 seconds for a bullet to drop 5 feet when it falls from your hand to the ground. The bullet (path) is arched into the target, or in the case of a bullet traveling less than 200 MPS the bullet is lobbed in. So where is all the extra time coming from? The 50 Beowulf site states that the round has a 548 MPS at muzzle. Depending on the bullet type, the round is still moving well over 304 MPS at impact with the target 400 Meters away. Actual flight time is around one second, but the report must bounce back from the target, and with echoing of the flight report bouncing back from standing objects, his perceived flight time (sound) is longer. Notice that he stated hearing the report back from the target. In order for the bullet to take two seconds to reach the 400-meter target, the muzzle/end flight speed would need to average out to 200 MPS (656 FPS). Maybe with a hand load, but last time I checked, the factory round was loaded at 1800 FPS at muzzle. There are 3.28 feet in a meter. P.S. Since I lack the real ballistics of rounds, and could only find the advertised speed, all my calc’s are just a guess. If someone has the spec's for the round at 437 yards; we can do the correct spec for the actual flight time. |
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Quoted: The 50 Beowulf site states that the round has a 548 MPS at muzzle. Depending on the bullet type, the round is still moving well over 304 MPS at impact with the target 400 Meters away. Actual flight time is around one second, but the report must bounce back from the target, and with echoing of the flight report bouncing back from standing objects, his perceived flight time (sound) is longer. Notice that he stated hearing the report back from the target. In order for the bullet to take two seconds to reach the 400-meter target, the muzzle/end flight speed would need to average out to 200 MPS (656 FPS). Maybe with a hand load, but last time I checked, the factory round was loaded at 1800 FPS at muzzle. That makes sense. Thanks to everyone for straitening me out. From now on I'll keep my delusions of grandeur to myself. |
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