What is bc?

H

HBC

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James Mock asked "What is BC?". I responded with an e-mail answer but have since organized the answer a bit better and that revised answer is included as an attachment.

HBC

8/3/09:

Text file changes:
Revised Note 4 due to a slight correction in the exterior ballistics program. The results affected are primarily high angle of fire and the change in the results were minimul.
 

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  • G17.TXT
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This is a great day for us!

Welcome back Henry. You have been missed.

Thanks for your post and sharing your wealth of information.


Jeff in OH
 
Sorry I asked.

Henry is one of the "Mad Scientsts" that try to keep us mere mortals informed about the technical aspects of shooting. Some others include Jerry Hensler and Larry Feusse. I am glad that we have the heavy thinkers in our sport, but they often get over my head.:eek: Good shooting.... James
 
It is a real joy to meet Henry and get him to discussing anything related to bullets and exterior ballistics. There are few shooters with his knowledge and fewer still that can or will put it into words and keep it interesting.
Thanks, Henry, for taking the time to put this dissertation together for us.

Jay McMunn
Moscow, Idaho
 
Thanks!

Thanks Lynn for the invitation, I would love to come out to Sacramento and shoot the Nationals and meet you and your dad. Probably won't make it this year though.

Thanks Jeff, James, JD, Al and Jay for your comments.

Henry
 
Thanks for the great info Henry.

I've always wished bullet makers would produce a multi-cal line to a 'standard' such as you described.
Then, maybe they could have one tested, and provide a drag curve for their standard line.
Then,, maybe they could provide software with which we could plug in their curve.

I know, I'd have to do it myself..
 
Wind Drift added in example of Note 4 in text file.

The program referred to in Note 4 of text file g17.txt above was written primarily to compare wind drift determined by the Lag Rule with wind drift determined from the component of bullet drag force normal to the line of sight but it served as a good example to demonstrate that BC is sectional density.

Wind drift for small arms fire, even to a couple of miles, determined by the two methods agree with high precision but one must use the trajectory arc length, instead of line of sight range, in the Lag Rule calculation for ranges up to a couple of miles. It is in high angle artillery fire where the two methods fail to agree. In high angle artillery fire, air density varies substantially and thus the reason for the failure of the Lag Rule there.

In file attachment g17.txt above a constant 30 mph wind from 1:30 O'clock was added to show the effects of wind on wind drift, range and projectile orientation relative to the line of sight.

A constant wind over a 20 mile range and to altitudes over 40,000 feet is extremely unlikely but the program has no provisions for variable winds. But variable wind inputs can be accommodated by such a program. Numerous wind directions and velocities, including vertical wind components, could be handled by the program.

BTW, the addition of wind drift to g17.txt is the 12 th. revision of the text file.

Henry
 
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Ballisitic coeficient is a measure of how well a bullet penetrates the air and how well it fights velosity degradation.
 
ahhhhh henrys back

well i hope you can make it to iowa,well after a few years of ribbing the 6mm guys and sticking my foot in my mouth i will be shooting a little bitty six mm,i should have done it a long time ago.i remember the group you shot last time in iowa with them itty bitty pills and i hope you can do it again. tim in tx
 
are UU kiddin...

Two Nationals bak to bak......mmmmmm I will be there if at all possible....that wuld be a rootin tootin shootin ,,,knok down drag out ,,good ole" time...that wuld make the trip across the country well worth it....good idea....I wuld have to drive cause I culdnt bring all that ammo on the plane..hahaha...or maby I culd mail it to someone out there...((hint-hint --Lynn..)) uh oh ...I mite have to mark my bullets to make shure a lil' grimlin didnt pull em and put in Luky Charms instead of Rx-15!!!!!....grin...Roger
 
hey lynn

i would love to go ,maybe henry can come by fort worth on the way there and i can hitch a ride as well. tim in tx
 
I found a very slight error in the exterior ballistics program and thus revised the attachment "g17.txt". The error resulted in determining sea level temperature from temperature and elevation at the gun site in order to determine atmospheric pressure at the gun site in pounds per square feet and thus determine air density at the gun site.

In checking for errors in a program one often inputs unusual conditions. Here is one that was a surprise but is correct with bullets in the first few hundreds of yards of flight that have high drag:

For the gun location conditions of 1600 ft. AMSL, 29.92" Hg corrected baro. and 77 F @ 50% R.H.

A 6mm, 68 grain bullet with a muzzle velocity of 3400 f/s and a G1 BC of 0.268 over the first 400 yards will give the following results when shot straight up and straight down 400 yards:

Remaining velocity shooting 400 yards straight up is 2107.5 f/s with a time of flight of 0.447753 seconds.

Remaining velocity shooting 400 yards straight down is 2092.6 f/s with a time of flight of 0.448326 seconds.

The bullet gets to 400 yards shooting straight up quicker because air density is decreasing for the bullet shooting up and air density is increasing for the bullet shooting down.

Deceleration of the bullet at the rifle muzzle due to air drag is 124.6 g's due partially to an air density of 0.06956 lb/ft3. At 400 yards above the muzzle deceleration due to air drag is 62.3 g's due partially to an air density of 0.06735 lb/ft3.

Deceleration of the bullet at 400 yards below the muzzle is 66.1 g's due partially to an air density of 0.07182 lb/ft3.

The above has little to do with precision shooting but is a bit interesting. Of course as the bullet travels much further than 400 yards (comparing up and down), deceleration due to drag becomes much less compared to gravity and the bullet traveling down becomes the faster bullet and reaches ranges much longer than 400 yards quicker.

Henry
 
BUT!!!!!!!!


In reality the bullet fired down will arrive quicker because there ain't no holes in the ground 1/4 mile deep......... so's you gotta' go UP IN THE AIR 400yds to fire the shot back down.


And you get to enjoy the decreased drag right from the muzzle :D

LOL

Fun facts Henry, glad to see you posting.

al
 
Well!!!!!!!!!!!!!!!!!

Well!!!

Al, I agree, it might be difficult to find a location to test the exact situation stated for the 68 grain bullet shooting straight up compared to the same bullet shooting straight down but here is another situation that can actually be tested:

Shooting a 30 cal. 210 grain VLD bullet at a muzzle velocity of 3200 f/s for the conditions of:

30 F @ 10% R.H. & 5000 ft. AMSL (just an example, other situations will work)

Compare shooting up at 15 degrees above horizontal to 1000 yards
to shooting down at 15 degrees below horizontal to 1000 yards

Either by actual test or if some shooters out there will test it with their exterior ballistics program that changes air density as the bullet rises or drops in elevation (I know others have written such programs for small arms) you will find:

The 210 grain VLD shooting up @ 15 deg. will have the highest retained velocity and lowest time of flight to 1000 yards.

The same type 210 grain VLD load shooting horizontal to 1000 yards will have a lower remaining velocity and higher time of flight than the bullet fired up at 15 degrees.

The same type 210 grain VLD load shooting down at 15 degrees to 1000 yards will have the lowest remaining velocity and highest time of flight than the two situations described above.

Actually finding a location to test the above three situations would likely be difficult because shooting 15 degrees is fairly steep over 1000 yards.

In 1984 I killed a 10 point desert Mule deer at 400 yards with a single shot from a 6/284 (100 Sierra SBT @ around 3310 f/s @ muz.) at what I thought was a pretty steep angle (just south of McDonald observatory near Ft. Davis, Texas). Now with Google Earth I was able to go to that location via the internet and find that the angle of my shot was well under 10 degrees.

BTW, I found another error in Note 4 of text file g17.txt attached in post #1 of this thread. The stated sonic velocity of 943 f/s at the apogee of the 175 mm, 148 pound projectile is actually around 971 f/s. It is not an error in the program but a manual calculation error. I divided the projectile ground speed by the Mach number to obtain the 943 f/s result stated in Note 4 but should have divided the projectile air speed (there was a 30 mph wind from 1:30 in the example) by the Mach number to obtain the correct sonic speed.

Henry
 
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