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Bullet coefficiency question

This is a discussion on Bullet coefficiency question within the Ruger Semi-Auto forums, part of the Rifle & Shotgun Forum category; I have a Ruger SR 5.56 with a Nikon scope with a custom turret for 55gr. 3240 FPS, Im shooting Winchester 55gr. Ballistic tips which ...

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Old March 25th, 2020, 08:36 PM   #1
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Bullet coefficiency question

I have a Ruger SR 5.56 with a Nikon scope with a custom turret for 55gr. 3240 FPS, Im shooting Winchester 55gr. Ballistic tips which the box says 3240FPS! I shot through the chronograph today from 10 away with my average being 2970FPS! Wondering if I order a new custom turret with 2970FPS DOES MY BULLET COEFFICIENCY CHANGE FROM THE (Stated 3240 0.267) on the box Shooting 2970 FPS?? If so what would the new coefficiency be?? Thanks in advance!!

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Old March 26th, 2020, 05:12 AM   #2
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The bullet coefficient is based on the shape of the bullet and it s diameter. What bullet is being used by WInchester ? The coefficent of the bullet and the velocity then determine the trajectory.

You should be fine keeping the coefficient the same and getting the turrents for the observed velocity and not over think it. That would likely get you fairly close. Now will you shoot that load all the time ?
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Old March 26th, 2020, 06:10 AM   #3
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The ballistic coefficient of a projectile is a measure of the ability of that projectile to maintain velocity through air of a given density. Projectiles with high BCs experience less aerodynamic drag than those with lower BCs and therefore lose velocity more slowly.

If you have two projectiles that leave the muzzle at the same velocity but have different BCs, the one with the higher BC will cover any given range faster and therefore will have less drop due to gravity over that range. Lonestar driver was nearly correct in that the BC of a bullet is dependent on its shape and diameter, but it is also dependent on the projectile's mass. Bullets of a given caliber with greater mass have a higher BC than those of lower mass, given exactly the same shape. Conversely, bullets of greater diameter experience more aerodynamic drag given the same shape and mass as those of lesser diameter and therefore have lower BCs.

The concept of a static ballistic coefficient has some fundamental flaws. It is based on a "fixed drag" model and, unfortunately, the ballistic coefficient varies with velocity. But it works well enough to be useful although some accommodations need to be made when shooting at very long range. Another problem with the ballistic coefficient is that in addition to projectile diameter (caliber) and mass it is dependent on a "form factor". That is where the shape of the bullet enters in. The form factor is the ratio of the aerodynamic drag coefficient of the bullet you are shooting to that of a standardized "test bullet" of a specific shape. If the shape of the bullet you are shooting matches that of the test bullet closely you can assume that the curves of the aerodynamic drag coefficients of the two bullets remain proportional (parallel) over the entire range of bullet velocities.

Unfortunately, that is often not the case. The BC of 0.267 that you cited is based on the G1 drag coefficient. That uses a rather antiquated standard projectile shape that is more like a modern pistol projectile than a modern rifle projectile. So as projectile velocity drops, the aerodynamic drag coefficient curves of the bullet you are shooting and that of the G1 standard projectile start to diverge. Still, the G1 ballistic coefficient is what most projectile manufacturers use and it is still widely used by most ballistic computational software.

Another issue is that aerodynamic drag is not a constant for any bullet of fixed mass, diameter, and shape traveling at any given velocity. It is dependent on air density. Air density is determined by air temperature and local atmospheric pressure, and to a much lesser extent, by relative humidity. Unless you are shooting at very long distances, the relative humidity won't make a difference, but air temperature and barometric pressure will. Ballistic coefficients are based on "standard atmospheric conditions". The standards most often used by bullet makers and ballistic software programs assumes a local barometric pressure of 29.92 inches mercury at mean sea level, a temperature of 15 degrees centigrade (59 degrees Fahrenheit) and a relative humidity of 0%. But if the air temperature when you are shooting is different or the local barometric pressure is different, the air density will be greater or less than standard and the observed ballistic coefficient of your projectile will not correspond to the stated BC.

So ballistic drop compensation reticles, sights, and turrets are really only accurate for a specific cartridge at a specific muzzle velocity, and with atmospheric conditions that conform to the standard. Local barometric pressure varies quite predictably with altitude. The average local barometric pressure drops by approximately one inch of mercury with each 1000 foot gain in elevation. So the standard atmospheric pressure is 29.92 inches of mercury at mean sea level, but only 24.89 in Hg at 5000 feet. That difference will result in a big drop in air density and corresponding increase in observed ballistic coefficient. Each one percent change in atmospheric pressure will result in a corresponding one percent change in observed ballistic coefficient. If you know the elevation at which you will be shooting and can estimate the local average temperature of the season you will be doing most of your shooting, you can use a ballistics program to tailor your bullet's BC accordingly. Tailoring the BC is what ballistic software programs do to adjust bullet drop and bullet velocity for local air density.

So whether or not it is worth changing the turrets on your scope will depend on a lot of factors. First, is your observed muzzle velocity accurate? If you can I would confirm it with another chronograph. Second, will you be shooting in conditions that depart very significantly from "standard" conditions? If you live at significant elevation and plan to do much shooting in weather warmer than 59 degrees F, the air density will be significantly less than the "standard" air density that the BC is based on. High levels of relative humidity also decrease air density a little bit, especially at very warm temperatures. If your actual muzzle velocity is less than what your scope manufacturer based the turret adjustments on, but the air density is significantly less than the standard that the bullet maker based the BC on, the two may cancel each other out to some extent.

Lastly, what ranges do you expect to be shooting at? Small departures from standard muzzle velocity and air density might not make much difference shooting at 200 yards, but make a big difference if you are planning to shoot out beyond 600 yards. But if you are planning to shoot much beyond 400-500 yards you probably won't be shooting 55 grain .223 Remington.

Last edited by pblanc; March 26th, 2020 at 06:18 AM.
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Old March 26th, 2020, 06:52 AM   #4
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FishSqueezer, Good information from pblanc but BC is not your problem …. it's barrel length. The factory rating of 3240 fps assumes a 24" barrel. Your SR556 has a 16" barrel so it doesn't develop near as much velocity as the advertised rating. A slower velocity means it takes longer for a bullet to reach the target so gravity has a longer period of time to pull the bullet down.

A 16" barrel is not intended for long range shooting …. probably more like 100~150 yards. I don't have a SR556 but I do have a Ruger AR556 with a 16" barrel and have tried several types of ammo/bullets. My best accuracy results were with Ballistic Tip 55 gr and 26.5gr of Varget powder. This load produces nearly identical velocity as factory loads. In my bolt action Rem 700, chambered in 223 Rem and with a 24" barrel, this load chronographs very close to 3240 fps but with my AR556, velocity dropped to 2950 fps …. not much different than your SR556. Accuracy at 100 yards was decent (about an inch sized group) but at 200 yards, groups opened up quite a bit …. about 6". I have a Nikon 3X scope on my AR556, which is a great match for the rifle's capability.
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Old March 26th, 2020, 09:00 AM   #5
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FishSqueezer, I have zero experience with Nikon’s custom turrets, but I know a little about Leupold’s CDS. My shooting buddy had a similar situation with his 6.5 Creedmoor but the velocities went in the other direction. His rifle was brand new when he requested his CDS turret from Leupold calibrated for a MV of 2650 and a .625 BC. I told him he should wait until he had at least 150 rounds down the tube to see where the MV was, 150 rounds is about the number in my experience when the MV flattens out and remains constant.

He was on a trip in Iowa when he calls and says his rifle is printing high at 300yds and that he had to dial the scope down to 275yds for his 300yd target. His round count at the time was 160. The fix for the Leupold was simple, re-zero the rifle at 200yds, (that was his requested zero) pull the turret cap and realign the number “2” on the cap with the hash mark on the tube. It now shoots the same as it did with the slower MV.

I wouldn’t put to much emphasis on BC unless you’re running out past 600yds, at 300 and less it really a non factor. I’d call Nikon and run your situation by them, the “fix” may be as simple as it is for a Leupold. Before I bought a new turret I’d come up with my own firing solution though.
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Old March 26th, 2020, 02:42 PM   #6
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Muzzle velocity and twist rate do affect ballistic coefficient. Most typically, for long range precision shooting, we will use a known quality chronograph to establish the muzzle velocity, then shoot at ~80% of the estimated supersonic range to determine actual drop data, which is then used to “true” the ballistic solution for the actual ballistic coefficient for that particular load, barrel, and bullet.

Unfortunately, or maybe fortunately for you, trajectory calibrated knobs are only accurate at one specific density altitude, a very few number of temperature, humidity, elevation, and barometric pressure combinations. This might mean a difference of a couple clicks between morning and noon, and a couple more before sunset, let alone gross variances from month to month, season to season. Let alone when we talk about trajectory changes due to velocity shifts caused by temperature sensitivity of your powder. So unfortunately, the dials will only be correct for that very specific combination of parameters - muzzle velocity, temp, ballistic coefficient, humidity, altitude, and barometric pressure, and most of your time, they won’t match up to the exact click. So FORTUNATELY, you’ll have plenty of chances to correct the dial for various conditions, and your condition data will shift your impacts as much or more than being off a hundredth or so on your ballistic coefficient.

Personally, I’d save myself any headache and simply use a ballistic app and dial to the output number rather than dial to a range, then have to look up how far to correct that number based on the current environment.
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