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Thread: Chronograph Question

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    Member bigswede358's Avatar
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    Default Chronograph Question

    I'm hoping there is an easy answer to this question. How much if any velocity do I add, by having my chronograph 10 feet out from my muzzle?
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    The bullet will generally continue to accelerate for about 15 feet from the muzzle. It won't be much. I saw a figure years ago but cannot remember it. It is less the 30fps though.
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    Member bigswede358's Avatar
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    It doesn't sound like its really enough to worry about. Thanks Nitroman
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    Quote Originally Posted by Nitroman View Post
    The bullet will generally continue to accelerate for about 15 feet from the muzzle. It won't be much. I saw a figure years ago but cannot remember it. It is less the 30fps though.
    I'm having a hard time accepting a bullet that continues to accelerate after it leaves the barrel. Out of the barrel it has no pressure pushing it, only gravity and atmospheric drag acting on it. I've seen ballistic calculations that correct for chronograph distance but I don't recall it adding velocity. I'm going to need to do a little research and figure this out.

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    Quote Originally Posted by marshall View Post
    I'm having a hard time accepting a bullet that continues to accelerate after it leaves the barrel. Out of the barrel it has no pressure pushing it, only gravity and atmospheric drag acting on it. I've seen ballistic calculations that correct for chronograph distance but I don't recall it adding velocity. I'm going to need to do a little research and figure this out.
    I'm with you on this one. Physics will dictate that a bullet will begin decelerating almost immediately upon exiting the muzzle. Therefore, any ballistics equation that calculates MV from the speed measured at a point 12 or 15 feet in front of the muzzle will by necessity be adding velocity to the measured speed.
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    The bullet is accelerating towards the muzzle because of the pressure of the gas behind it. The gas pressure is high enough to overcome the friction of the bullet against the bore. At the muzzle this friction is suddenly removed, but the gas pressure is not, it continues to push the bullet, depending on the cartridge barrel length combination that pressure could be 8k to 15k psi. At the same time, the gas released is moving at ~ 5,200 fps around the bullet, removing any "air friction" as the bullet is in a high-speed gas envelope removing that friction.
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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by Nitroman View Post
    The bullet is accelerating towards the muzzle because of the pressure of the gas behind it. The gas pressure is high enough to overcome the friction of the bullet against the bore. At the muzzle this friction is suddenly removed, but the gas pressure is not, it continues to push the bullet, depending on the cartridge barrel length combination that pressure could be 8k to 15k psi. At the same time, the gas released is moving at ~ 5,200 fps around the bullet, removing any "air friction" as the bullet is in a high-speed gas envelope removing that friction.
    Agreed, but I think any case this effect does not persist very far beyond the muzzle, perhaps a foot or so depending on the gun, as can be seen in the attached photo. Once the bullet is clear of the gas turbulence associated with the "muzzle blast", it must start decelerating.
    Attachment 66853
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    Member GD Yankee's Avatar
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    Affirmative.

    Nitroman's idea about the gas envelope probably lasts all of 2 inches from the muzzle. BigSwede, either way, I would imagine most industry testing has the chronograph some set distance from the muzzle (to avoid muzzle blast and debris), so any velocity loss from the position of your chronograph compared to the industry published numbers is minimal.

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    I asked the question to Bryan Litz. Bryan founded Applied Ballistics LLC in 2007. In 2009, Bryan authored and published his first book Applied Ballistics For Long Range Shooting. In just two years the Second edition went to print. In 2011 Bryan founded his second company, Applied Ballistics Munitions, which specializes in the precision manufacturing of ammunition.

    Bryan earned his Aerospace Engineering degree from Pennsylvania State University in 2002. For the next 6 years, he worked for the US Air Force on Air-to-air missile design, modeling and simulation. In November 2008, he became the Chief Ballistician for Berger Bullets. He competes on a national level in long range matches and has won a few championships.

    His response to the question follows:

    The bullet may accelerate slightly or maintain speed for about 1/2" or so after exiting the muzzle due to the high pressure muzzle blast, but beyond that, the aerodynamic drag overcomes and begins slowing the bullet. You can use a conventional ballistics program, set up with 1 yard increments to see how much velocity is lost in the first few yards. Typically you loose 10-15 fps from the muzzle to the chrono, depending on the cartridge, bullet BC, etc...

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    I am just wondering because I am going to try out the Leupold CDS system and when I provide Leupold with my data for my turrets I want it to be right on the money. I just didn't know if having my chronograph 10 feet in front of the muzzle would make me need to add X amount of fps to the readings I got on my chronograph.
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    You need a Magnetospeed Chrono!!
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    Quote Originally Posted by bigswede358 View Post
    I am just wondering because I am going to try out the Leupold CDS system and when I provide Leupold with my data for my turrets I want it to be right on the money. I just didn't know if having my chronograph 10 feet in front of the muzzle would make me need to add X amount of fps to the readings I got on my chronograph.
    Howdy Bigswede,

    I put a Leupold turret on my .308Win setup a couple of years ago. I provided them my load data based on the chrony speed at 10 feet along with bullet weight and BC. I gave them an altitude of 2000 feet and a temp of 80 degrees because of my shooting area. When I go up to my summer camping spot at 7000 feet I gain 5000 feet of altitude but I don't lose 50 degrees of temperature. Rules of thumb dictate a 1 moa shift for each change of 10 degrees of temperature change or 1000 feet of altitude change, density altitude is the issue here. I've found it to be a little less than that but it does make a large difference when you shoot beyond 300 yards.

    With the Leupold turret I shot five shots into one target at 100, 200, 300, 400 and 500 yards to verify their calibration. I didn't make any change in hold for wind. All shots were aimed at the bulls eye, a slight right to left wind is apparent with the drifting of most shots left of center. The object was to check the trajectory and validate the system for that. It is quite good as seen below. All five shots are within 2.8 inches of each other.

    Since you're in Alaska I would order your cap with the average of your hunting altitude and temperature. I would also shoot a target at 200 yards and get the bulls eye nailed at that range. Then put the cap on to indicate 200 yards. By doing the above recommendation you set up your scope for averages which will keep your point of impact at a minimum variation under most shooting conditions. If the cap is slightly wrong a 100 yard zero would give more error at 400 than a 200 yard zero.

    For quick hunting adjustments the cap is a good system but being so worried about the 10 fps of velocity is not necessary. The difference in shooting elevation and temperature is more of a factor than the slight velocity error you have mentioned.

    In a perfect world the standard moa cap and a hand held ballistic calculator would make for better long range, (greater than 500 yards) accuracy. Current altitude, temperature and pressure effect density altitude calculations. None of this makes a huge difference inside of 300 yards and the cap is perfect for that under a wide range of conditions.

    DSC01391.JPG DSC01388.JPG

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    Member bigswede358's Avatar
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    Thanks for the info Marshall. Looks like the turrets work pretty well on your 308. Here in IDAHO, the temp ranges from 10 to probably 45 or so un the autumn months. I think I will have Leupold put 30 degrees down for average temp. And I usually hunt between 3 and 4 thousand feet elevation. Those must be Target Turrets on your scope?
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    Quote Originally Posted by bigswede358 View Post
    Thanks for the info Marshall. Looks like the turrets work pretty well on your 308. Here in IDAHO, the temp ranges from 10 to probably 45 or so un the autumn months. I think I will have Leupold put 30 degrees down for average temp. And I usually hunt between 3 and 4 thousand feet elevation. Those must be Target Turrets on your scope?
    Yes they are target turrets. This particular scope is the 6.5-20X50 LR target. The turrets will get me on steel at 800 yards but they will only be correct if the temp and altitude are as inscribed when shooting beyond 400.

    If your target shooting sending a sighter shot down range to see where it hits followed by a little adjustment works well. For hunting I wouldn't have any concerns inside of 500 with the cap as long as the shooting position was stable.

    I recently ordered a cap for the CDS scope on my daughters 243. The cap on that scope has a lower profile. Her shooting is limited to targets at this time.

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    What Marshal said... and I wouldn't worry about the temp too much, it will have a minimal effect to 500 yds. Just pick a good average. Your pressure altitude could have an effect, but it should be a whole lot. By pressure altitude, I mean the combined actual elevation and barometric pressure. Even though your actual elevation may only change by 1000' your pressure altitude may change by several thousand feet. Also, if you're hunting in steep country, The angle could have some effect on the drop. For steep angles, up or down, you will have a little less drop. So I would dial a little less yardage.

    If you want to check the actual ballistics of your bullet and load you might play with this a while. Click on trajectory...

    http://www.jbmballistics.com/ballist...culators.shtml

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    Member bigswede358's Avatar
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    Quote Originally Posted by MontanaRifleman View Post
    What Marshal said... and I wouldn't worry about the temp too much, it will have a minimal effect to 500 yds. Just pick a good average. Your pressure altitude could have an effect, but it should be a whole lot. By pressure altitude, I mean the combined actual elevation and barometric pressure. Even though your actual elevation may only change by 1000' your pressure altitude may change by several thousand feet. Also, if you're hunting in steep country, The angle could have some effect on the drop. For steep angles, up or down, you will have a little less drop. So I would dial a little less yardage.

    If you want to check the actual ballistics of your bullet and load you might play with this a while. Click on trajectory...

    http://www.jbmballistics.com/ballist...culators.shtml
    Hornady's Ballistics calculator is pretty good also. I've compared it to this one and others, it always comes out really close if not right on with the others.
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