Question about BC.

[Captcurt]

Ran across something that has me scratching my head and maybe you can shed some light on it. We went to the range to do some semi-long range shooting and I had a new load for my 6.5 CR using the Hornady 143 gr. ELD-X with a BC of 625. On our first trip I shot the Nosler 129gr Accubond LR having a BC of 561. I was cranking it around 2800 fps. My Vortex retical was dead on out to 500 yards. All shots were in a 6 inch Bull except one and the wind had drifted it out about an inch. Elevation was perfect.

When I went to the Hornady load I was good to 300 yards. At 400 yards I was 5 inches high and at 500 yards I was 8 inches high. My question is: Will the higher BC account for a flatter trajectory even at a reduced velocity of 200 fps? The load chrono'd at 2625fps.

 

[jmr40]

Go to one of the ballistics calculators and run the numbers. I use the one on Hornady's website, but there are others as good or better.

I think there are at least 3 things going on.

#1 As a rule velocity trumps BC out to several hundred yards as to trajectory. At extreme ranges BC becomes more important and 400-500 yards is about the point where BC starts to matter. As a hunter BC matters even at much closer ranges. A bullet with poor BC's may start out 300 fps faster than an equal weight bullet with great BC's. But at as little as 100 yards the high BC bullet will impact at the same speed. At 300-400 yards a high BC bullet will impact considerably faster and with more energy even though it may have started considerably slower. In a nutshell a 30-06 firing a high BC bullet will impact game with more speed and energy at 300-400 yards than a 300 WM firing a bullet with poor a BC. BUT, the 300 WM will still shoot flatter out to at least 500-600 yards. The extra velocity for the 1st 100 yards takes a while to overcome.

#2 Often heavier bullets impact higher than lighter bullets. The rifle begins to recoil before the bullet exits the muzzle. Heavier bullets will cause the barrel to rise ever so slightly more than lighter bullets due to greater recoil. You only need a couple of 1/1000's of an inch for the bullet to impact several inches higher at longer ranges.

#3. The BC numbers listed by many manufacturers are optimistic. There was an independent test done some months back and the results posted online. Some manufacturers listed BC's that were very accurate, some were off considerably. It did vary by the individual bullet, but as a manufacturer Nosler was the least accurate. Hornady the most accurate. Particularly the ELD bullets.

I'm guessing a combination of #2 and #3 is what is causing this. The true BC of the Nosler bullets is probably less than listed and the heavier bullet is just impacting higher. When I ran the numbers the 143 gr bullet should have been 5" lower at 500 yards with a 100 yard zero

 

[Captcurt]

Go to one of the ballistics calculators and run the numbers. I use the one on Hornady's website, but there are others as good or better.

I think there are at least 3 things going on.

#1 As a rule velocity trumps BC out to several hundred yards as to trajectory. At extreme ranges BC becomes more important and 400-500 yards is about the point where BC starts to matter. As a hunter BC matters even at much closer ranges. A bullet with poor BC's may start out 300 fps faster than an equal weight bullet with great BC's. But at as little as 100 yards the high BC bullet will impact at the same speed. At 300-400 yards a high BC bullet will impact considerably faster and with more energy even though it may have started considerably slower. In a nutshell a 30-06 firing a high BC bullet will impact game with more speed and energy at 300-400 yards than a 300 WM firing a bullet with poor a BC. BUT, the 300 WM will still shoot flatter out to at least 500-600 yards. The extra velocity for the 1st 100 yards takes a while to overcome.

#2 Often heavier bullets impact higher than lighter bullets. The rifle begins to recoil before the bullet exits the muzzle. Heavier bullets will cause the barrel to rise ever so slightly more than lighter bullets due to greater recoil. You only need a couple of 1/1000's of an inch for the bullet to impact several inches higher at longer ranges.

#3. The BC numbers listed by many manufacturers are optimistic. There was an independent test done some months back and the results posted online. Some manufacturers listed BC's that were very accurate, some were off considerably. It did vary by the individual bullet, but as a manufacturer Nosler was the least accurate. Hornady the most accurate. Particularly the ELD bullets.

I'm guessing a combination of #2 and #3 is what is causing this. The true BC of the Nosler bullets is probably less than listed and the heavier bullet is just impacting higher. When I ran the numbers the 143 gr bullet should have been 5" lower at 500 yards with a 100 yard zero

Thanks. It all makes sense now. I should have considered the recoil factor. I ran across the same thing when shooting IDPA with my 40 S&W. I could fine tune my load by changing bullet weight and velocity.

 

[Sunray]

"...Will the higher BC account for..." Nope. Ballistic Coefficients have nothing to do with trajectory. The reduced velocity does though. The whole thing just means your rifle prefers light bullets.

 

[RecoilRob]

Remember also that BC is constantly varying as the velocity drops. The advertised numbers are only valid within one narrow velocity window and as it decays the BC will change. See: http://www.exteriorballistics.com/ebexplained/5th/24.cfm