Real world difference between ballistic coefficient


PDA






Eb1
March 2, 2009, 11:46 AM
What is the real world difference between a .305 BC and .362 BC?

Is the .057 making an extreme difference in mid-range shooting?

Thank you for your replies.

If you enjoyed reading about "Real world difference between ballistic coefficient" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
everallm
March 2, 2009, 11:59 AM
In general, the further the range and the more chaotic the conditions (wind gusting, heat, humidity etc) the less troubled a higher BC will be.

In general only and if it's in the 100-300 meter ranges probably little if any noticeable difference. With the two BC's you've mentioned you may get the same or greater variation between the same BC's shot from the same caliber from different manufacturers.

mstirton
March 2, 2009, 12:07 PM
I have no experience in this matter, but I'm sure it would only make a difference to long range shooters. I never shot anything over 400m so it wouldn't matter to me. The guy trying to hit a body at over 1000m will care to have all the advantage he can get I'm sure.
On that note, anyone know the BC difference between most 50bmg and the 408 CheyTac or 416 Barrett?

Eb1
March 2, 2009, 12:14 PM
I mentioned mid-range to imply 600 yard HighPower shooting. I have recently developed a good loading for my Colt 1:9 upper. I am using the 69 grain SMK at 2842 avg fps. This gives me a .305 to .301 BC. Looking at the 77 grain SMK it shows .362 for its BC.

I am just mucking around trying to learn as much as possible, and get different opinions. I will be shooting what I have for now. I cannot afford to buy a new upper.

30Cal
March 2, 2009, 12:23 PM
Not very much for Highpower.

I think it's laughable that they publish BC's to three digits; implying a high degree of accuracy in the number. Ha!

everallm
March 2, 2009, 12:42 PM
EB1

With a 1:9 barrel you're pretty much at or edging past the stabilization limit with the 69gr round, the 77gr is definitely past the usually acceptable range.

Sam Adams
March 2, 2009, 01:32 PM
With a 1:9 barrel you're pretty much at or edging past the stabilization limit with the 69gr round, the 77gr is definitely past the usually acceptable range.

I'd say edging past - I've never been able to get 69s to be too accurate even at 100 yards in my 1:9 with a 20 inch barrel - and I've tried a variety of different powders and weights for each; 52s are tack drivers for me.

The shorter the range and the calmer the conditions, the less BC matters.

Eb1
March 2, 2009, 01:49 PM
I can put 20 rounds in a 1/2 @ 100 yards with my 1:9. I guess it likes them fine. This is with A2 sights off a bench.
They are moving at an average of 2842 fps. I have not had a problem getting them to group.
I find it hard to take the blanket statements that I am past what "my" gun will handle.
So you guys had a hard time getting the 69 gr SMK to produce groups in you guns?

Now the 75 grain AMAX.. That bullet is bad out of my gun.

1858
March 2, 2009, 02:23 PM
I think it's laughable that they publish BC's to three digits; implying a high degree of accuracy in the number. Ha!

I don't. BC can be derived, or determined empirically, or both.

BC = M/(d^2 * i) where M is the mass of the bullet in lb or kg, d is the diameter of the bullet in inches or meters and i is the drag coefficient or form factor.

A 168gr bullet weighs 0.0240 lbs and a 208gr bullet weighs 0.0297 lb and if you convert many common bullet weights to pounds, you'll realize that at least four decimal places is required to distinguish bullets that are close in weight.

Diameter in inches is typically given to three decimal places such as .233, .308, .357. Even if a caliber is listed to two decimal places, you'll notice that bullets are listed to three decimal places such as .452 for .45 Colt.

i is often listed with at least three decimal places in engineering literature.

In the BC formula, the least number of decimal places is three (bullet diameter). BC is used in ballistic calculators to calculate bullet drop and/or wind drift so why would you want to enter a value with only two or less decimal places?

As for the OP's original question, I know a bunch of people that shoot 80gr SMK HPBT (.223) bullets at 600 yards but shoot 77gr SMK HPBT at 200 and 300 yards for the specific reason that the 80gr bullets perform better at 600 yards i.e. more accurate, more consistent, more predictable, therefore higher scores. The 77gr has a BC of .362 and the 80gr has a BC of .420, a difference of 0.058 which is virtually identical to the difference mentioned by the OP when comparing the 69gr to the 77gr bullet.

:)

TimRB
March 2, 2009, 02:42 PM
I just did a check using the PCB freeware ballistic program using SMK 168 at 2560fps muzzle velocity, 600 yard zero, nominal BC 0.447.

The drop (drop of bullet from bore axis) nominally is 134.5 inches. Changing to BC=.440 gives a drop of 135.3 inches, and BC=.449 gives a drop of 134.3 inches.

135.3 - 134.3 = 1 inch. One inch out of 134 is a small enough difference that I'm not going to sweat it. YMMV

Tim

SlamFire1
March 2, 2009, 02:44 PM
mentioned mid-range to imply 600 yard HighPower shooting. I have recently developed a good loading for my Colt 1:9 upper. I am using the 69 grain SMK at 2842 avg fps. This gives me a .305 to .301 BC. Looking at the 77 grain SMK it shows .362 for its BC
How much wind?

One highpower range that I shoot at, you can shoot 69's out to 500 yards (the limit of the range) and shoot near perfect scores. About the most wind I ever put on at that range, during normal weather, was half MOA.

Another range that I shoot at, locally, the wind conditions are much worse.

At that range, at 600 yards, I was shooting 69's at 600 yards to get a good rattle battle zero. I was hammering them in the X ring, it was unreal how accurate those 69's are. And then a light puff of wind. The round went out in the eight ring.

At Camp Perry, one service rifle champ told me of the day he won the 1000 yard service rifle match with his M14. He had full right windage on his rear sight (either 33 MOA or 15.5 MOA) and he was aiming three target frames over to the right. That’s a lot of wind! Lucky he was not on the last target to the right, his aimpoint would have Canada!


Out to 300 yards, I use 69's (Sierra, Nosler and Hornday 68's) all the time. These bullets are wonderfully accurate. The wind sensitivity at 300 yards is there, but is manageable. Just look to see if the mirage has changed before you shoot the first round rapid fire.

At 600 yards, the .223, even with 80 grain bullets, is highly wind sensitive. If your rifle can shoot 77’s at 600 yards, I would use these before I used 69’s.

Eb1
March 2, 2009, 03:08 PM
I have shot the 75 AMAX, and that is a bust. Maybe the length was a little long. Having a 1:9 twist Colt HBAR I feel I am doing good to get such a nice grouping at 100 yards with the 69 grain SMK.

I will will buy a box of 100 77 grain SMK's go shoot. I have heard some good and some bad about the ability of the 1:9 to shoot this bullet. It cannot do any worse than the Hornady AMAX did which by the way is marked 1:9 twist on the box :cuss:

Maybe the lenght of the 77 grain SMK being able to be mag feed will be okay. We will just have to see now won't we? :D

Eb1
March 2, 2009, 03:10 PM
At Camp Perry, one service rifle champ told me of the day he won the 1000 yard service rifle match with his M14. He had full right windage on his rear sight (either 33 MOA or 15.5 MOA) and he was aiming three target frames over to the right. Thatís a lot of wind! Lucky he was not on the last target to the right, his aimpoint would have Canada!



That makes me wonder why they let the shooters shoot in such conditions, and also why he only had bullets on his target from his rifle. It would seem that the person who's target he was using to aim would be hitting his target as well.

1858
March 2, 2009, 03:32 PM
I just did a check using the PCB freeware ballistic program using SMK 168 at 2560fps muzzle velocity, 600 yard zero, nominal BC 0.447.

The drop (drop of bullet from bore axis) nominally is 134.5 inches. Changing to BC=.440 gives a drop of 135.3 inches, and BC=.449 gives a drop of 134.3 inches.

135.3 - 134.3 = 1 inch. One inch out of 134 is a small enough difference that I'm not going to sweat it.

Tim, what atmospheric conditions did you select for that example? Is PCB a good free program?

One of the issues I've noticed when shooting at 600 yards is that my scope has 0.25MOA adjustments which equals 1.5". I have to round the comeups to the nearest 0.25MOA which introduces error. Couple that with the .25MOA error (or more) introduced by a BC with only two decimal places and that might make the difference between an X and 10, or 10 and 9 or worse.

I'll readily admit that I have no idea as to how many significant figures are generated by the BC calculation method used by each bullet manufacturer whether it's derived or measured empirically or some combination of both.

:)

TimRB
March 2, 2009, 03:52 PM
"Tim, what atmospheric conditions did you select for that example? Is PCB a good free program?"

I used 75 degrees F, 0 feet altitude, 0 crosswind. These are the only environmental variables available.

I have found PCB very helpful, though I will admit that I sometimes have trouble shifting gears when using it, since it is DOS-based and does not have a GUI. I guess I am an old enough computer geek. PCB and other programs are available for download here:

http://www.snipercountry.com/ballistics/index.html

Tim

1858
March 2, 2009, 04:06 PM
I used 75 degrees F, 0 feet altitude, 0 crosswind. These are the only environmental variables available.

Hmmm ... so no option to change the humidity? That's one of the issues I've noticed using Hornady's free ballistic calculator on their website ... no matter what the temperature or atmospheric pressure, the humidity is the same at 78%. :confused: I've been reading about altitude density recently but I don't know if the free calculators are using that or not. All very confusing. :confused: Anyway, it'd be interesting to see how small BC changes affect bullet drop and wind drift for a range of BC's and atmospheric conditions.

:)

TimRB
March 2, 2009, 04:52 PM
Here's an article that addresses the humidity issue:

http://www.riflebarrels.com/bcchange.htm

Personally, I'm going to spend as much time worrying about humidity as I do the third digit in published BCs.

Edit: From another article at the same site:

"Measurements of air density are important, but not as critical as velocity errors. Relative humidity has the least effect of any of the measurements. A total change in humidity from 1 percent to 100 percent changes the ballistic coefficient by only about 1 percent."

Tim

1858
March 2, 2009, 10:25 PM
Tim,
Thanks for the link ... I had no idea that humidity has such a minimal effect on bullet drop or wind drift (due to the minimal change in BC). So elevation and temperature (and of course BC and velocity) are the only important variables in calculating bullet drop. No wonder the Hornady ballistic calculator uses the standard humidity value of 78%.

Thanks.

:)

Eb1
March 3, 2009, 06:57 AM
Deleted.

30Cal
March 3, 2009, 01:24 PM
BC = M/(d^2 * i) where M is the mass of the bullet in lb or kg, d is the diameter of the bullet in inches or meters and i is the drag coefficient or form factor.

A 168gr bullet weighs 0.0240 lbs and a 208gr bullet weighs 0.0297 lb and if you convert many common bullet weights to pounds, you'll realize that at least four decimal places is required to distinguish bullets that are close in weight.

Diameter in inches is typically given to three decimal places such as .233, .308, .357. Even if a caliber is listed to two decimal places, you'll notice that bullets are listed to three decimal places such as .452 for .45 Colt.

i is often listed with at least three decimal places in engineering literature.

The drag coefficient isn't directly measured. It's value is chosen to make the model function similar to the real world.

Look at Berger. Last year, they published their 80gr BC at .472. Today, it's .444. With three significant digits, that's a pretty massive correction.

Take any BC number, drop the third digit, and the predicted bullet path remains virtually unchanged.

Zak Smith
March 3, 2009, 02:54 PM
Here's how:

_Bullet_ _BC_ _MV_ 0 200 400 600 800 | YARDS
0.305 2500 > 0.00 5.15 23.19 58.36 111.77 | wind (inches)
0.362 2500 > 0.00 4.26 18.80 46.75 90.41 | wind (inches)
0.305 2750 > 0.00 4.49 20.12 51.07 100.53 | wind (inches)
0.362 2750 > 0.00 3.72 16.33 40.67 79.76 | wind (inches)
0.305 3000 > 0.00 3.96 17.66 44.84 89.62 | wind (inches)
0.362 3000 > 0.00 3.28 14.36 35.65 70.26 | wind (inches)

0.305 2500 > -0.00 2.51 10.69 22.85 40.21 | drop (moa)
0.362 2500 > -0.00 2.38 9.89 20.39 34.75 | drop (moa)
0.305 2750 > -0.00 1.90 8.45 18.18 32.32 | drop (moa)
0.362 2750 > -0.00 1.81 7.83 16.25 27.79 | drop (moa)
0.305 3000 > -0.00 1.45 6.79 14.69 26.21 | drop (moa)
0.362 3000 > -0.00 1.37 6.29 13.16 22.53 | drop (moa)





Three different starting velocities for each, 2500, 2750, and 3000, which should cover the range of most typical rifle velocities.

Eb1
March 3, 2009, 05:00 PM
Thank you, Zack for the chart. It looks like a couple were duplicated, and cannot be compared.

Zak Smith
March 3, 2009, 05:01 PM
Woops. Lemme got back and fix that.

Howard Roark
March 3, 2009, 07:11 PM
Here is some interesting reading on BCs from Berger. (http://02b0516.netsolhost.com/blog1/?p=12)

This article explains why Berger lowered their published BC.

gvnwst
March 3, 2009, 07:25 PM
Here is some interesting reading on BCs from Berger.

This article explains why Berger lowered their published BC.

I was wondering why they did that, some of the BCs i knew had changed all of a sudden...Thanks!

If you enjoyed reading about "Real world difference between ballistic coefficient" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!