Bullets and BC's... thought experiment

Discussion in 'Practical Precision--PRS, NRL, ELR' started by SAPenguin, Jun 10, 2019.

  1. Ccrider

    Ccrider Gold $$ Contributor

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    Man it’s a bunch of hyphaluted conversation going on here. Why don’t some of you professors drop 80 bucks and go out and shoot these bullets and tell us what they really do?;)
    That’d be a lots cheaper than the cost of all the professor time theorizing. And it dang shore would be more instructive.
     
  2. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    This is a shooting forum...theorizing is allowed, besides which, it's fun. The fact of the matter is, these values have clearly already been measured by companies like Berger, and probably some individuals as well. There is no reason to do additional testing if they are willing to share some of their information, such as J Peelen did with his reference above (thanks for that, by the way!).
     
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  3. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    We'd still be in the stone ages shooting unstable black powder if trial and error were good enough.
     
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  4. Longrange57

    Longrange57

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    Thank you Ned Ludd...aptly put! At 68, I've shot no less than half million rounds the last 55 years. Every minute has been fun. I'm sure I'm the least amongst us and a far cry from being a professor. However, my son worked for NASA for 14 years and we have had some great conversations about a number of these topics. The original question here is a great one. On a really humorous note....by the way I want some response to this. I saw a ways back where our scientist claim we have the ability to put a person in space with a gun, line up the trajectory, fire it...it goes into orbit...and hits the shooter in the back. I said, WOW!...you better check that out because many folks will dispute that this is possible with a 168 SMK. LOL... He said, actually that is the perfect bullet and we would slow the twist down to about 16 or 18. I said, OMG, don't tell me twist comes into play in space. He said, well not so much for the bullet as the shooter! You can't shoot from the shoulder or it will spin you and throw you off course. Oh no...NASA is going to have ballistics for the shooter now! You mean you will assign a BC to short, fat, heavy ones and also long, skinny, lightweight ones?? What if they have the same BC? Which one would you pick? He said, probably the heavy! But thats not our main concern! Our program can't distinguish between a male/female shooter and form factor has become our main concern. Being that you have to shoot center mass, if the male shooter shoots from the groin area, he will tumble! If the female shoots from around the chest, she will surely yaw! So if you guys and gals on earth have any suggestions, they will greatly be appreciated! Laurie, Ned, 284winner, Ballisticboy, I know darn well that we have some answers! Or do we? It's still lots of fun to shoot!
     
    Last edited: Jun 20, 2019
  5. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    There is no BC in space because there is no drag in space, so no twist required. But, it would be near impossible to make a bullet fly nose forward in space because there is no way to stabilize it. (Spacecraft are stabilized by small jets and carefully controlled gyroscopes). Orbital mechanics wasn't my thing (I also worked at NASA, but on other things), but I'm pretty sure there's a non-zero chance at hitting space junk before it makes it around the planet! There's a lot of space junk floating around up there. There are also a lot of micrometeors whizzing about that are could be a whole lot worse than getting shot by a bullet.
     
    Last edited: Jun 20, 2019
  6. Longrange57

    Longrange57

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  7. Longrange57

    Longrange57

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    You sound like my son! This old fart is still trying to wrap my mind around the universe expanding faster than a bullet can travel! I think I read that somewhere???
     
  8. Ccrider

    Ccrider Gold $$ Contributor

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    Though it may have sounded like it, I wasn’t being facetious when I said “professors”. Some of you guys clearly have the credentials or knowledge. And, I appreciate the value of theorizing.

    My point is, rubber got to hit the road at some point. And no, none of us are going to believe the line the manufacturer is selling regarding their test results. We know they have an angle.

    So, are these bullets better for their intended purpose than what is out there?Theorize all day but you don’t know the real answer till you put some powder and primers behind them.
     
  9. fyrewall

    fyrewall

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    Just struggling to keep up with this stuff.

    The Berger manual simply describes SD, FF, & BC. The other bullet manufacturers manuals seem to boost BC values to market bullets - customers scanning lists of bullets looking for those with highest BC values; not Berger.

    Looking at the time of flight for a bullet to reach a target, (at least at the distances where most people shoot, 1000 & under, all super sonic bullets) the spin rate decay would be next to zippo and of no practical consequence. But then again some target might just be several miles away and the spin rate decay would be a factor - I have no plans to get into that stuff.

    Should there be something to shoot at on the moon, with all those craters, I would go for a round ball to achieve the best practical cost effective uniform results - nice round holes in targets.

    Looking at previous posts, I noted that, upon occasion, bullets having lower BC values, out performed bullets having higher BC values. In one comparison G1 BC values were used to describe both bullets, apparently some 6.5 and a 210 gr. Berger .308. The G1 BC's for the 210 Berger bullets are .626 & .631 (as per circa 2013). The highest G1 BC for any 6.5 target bullet that Berger makes is .618 (as per circa 2013). Was an assigned, apparent G1 BC value of .670, correct for the 6.5 bullet, if it was a Berger bullet? If the G1 BC was .670, might the bullet be some real long heavy 6.5 having a slower MV than the .308 210?

    Am I right with this?

    The SD or sectional density, SD = (bullet weight in grains/7000) / (caliber ^2); G7 BC = SD / FF; G7 FF = SD / G7 BC

    The form factor being a measurement of drag as compared to a standard G7 value of 1

    Plugging in some numbers of various calculations, using one of my favorite bullets, 95 6mm Berger VLD, as a standard :

    G7 BC = SD/FF = ((95/7000)/.243**2) / .923 = (.0136/.0590) / .923 = .248, this agrees with table page 169, Berger manual

    Another BC calculation in this thread:
    This matches

    What's "lag time"?
     
    Last edited: Jun 20, 2019
  10. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Lag time is the difference between actual time of flight and the time or flight you’d see in a vacuum. It turns out to be proportional to wind deflection, but is generally useless to shooters, since calculating time of fight already gives you wind deflection, and measuring it requires equipment most of us don’t have.
     
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  11. fyrewall

    fyrewall

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    Thank you - seeing one of my favorite bullets in the green column makes me happy
     
    Last edited: Jun 20, 2019
  12. Ballisticboy

    Ballisticboy

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    I have worked on a few space based projectiles in the past. Keeping them pointed in the same direction is usually accomplished by using a small amount of spin. These were all short range projectiles usually to be fired at asteroids etc. from a mother ship.

    I also worked on guns capable of launching satellites for a while. We looked at all types including gas and EM guns but concluded that the most likely to work was a large conventional gun firing rockets just like the Gerry Bull gun systems. It makes the heat problems much simpler.

    On lag time, it is important as, along with wind speed, it is one of the most important factors in wind deflection. It also tells us that, if there is zero lag time, then no matter how long the time of flight may be, there is zero wind deflection or, with negative lag time, the projectile will drift upwind. These are both effects seen in rocket boosted systems. For most people the only way to get lag time is to measure it or model it using BC and available commercial programs.

    Spin damping coefficients on smooth bodies are closely linked to skin friction drag coefficients, a method which I have used in the past to estimate spin damping. There is no linear relationship between velocity loss and spin loss, though for supersonic speeds it is fairly close. At transonic and subsonic speeds any supersonic ratio between the two will not apply as the drag coefficient undergoes large sudden changes which the spin damping will not.
     
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  13. fyrewall

    fyrewall

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    Apparently in outer space there are enough external forces exerted on a projectile to cause some deflection, thus "a small amount of spin". I take it "negative lag time" would mean the actual time of flight would be shorter than that occurring in a vacuum - would this mean that some external force was pushing or pulling (suction?) the bullet - gravitational attraction?

    A long time ago I had some talking with some Boeing engineers and I got the impression that skin friction was of some benefit - I could never understand how this could be.
     
  14. Longrange57

    Longrange57

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  15. Longrange57

    Longrange57

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    .670 G1 BC is measured for 147 ELDM, and yes a 210 Berger has shot inside me...even when shooters swapped guns. Not by much but it was impressive. Don't get me wrong I like both bullets and I'm a huge Berger fan. 147 ELDM is a fantastic bullet...but with big wind switches the 284's and 30's whip up on us.
     
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  16. Laurie

    Laurie

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    One interesting thing that comes out of lag time IIRC is that it 'proves' that a standard velocity (ie subsonic) 40gn LRN .22LR match cartridge moves less in a crosswind than a supersonic (at the muzzle anyway) equivalent 40gn LRN (same BC bullet) HV that starts out around 200 or 250 fps faster. Most people won't believe it and it of course it disagrees with the common adage that movement from wind is solely determined by BC in combination with MV. I can't remember now how the 'proof' works but I've read it a few times and apparently it is backed up by actual range testing. (Or more likely, people noticed empirically the SV variety did better then looked to see why in ballistics math terms?)

    I have the Gerry Bull biography somewhere, a fascinating driven man and fascinating work on the satellite launchers. Has anybody ever made and used such launchers since his experimental work? We in the UK of my generation have a great deal of peripheral interest in Bull's later work in that the sections of the Iraqi 'Supergun', (or is that Superguns as I believe there were two under construction of different calbres?), by a specialised forging outfit in Coventry, England name of Matrix Churchill. These had been ordered as bespoke oil pipeline parts, but lots of people knew or guessed what they actually were. Just before completion and shipping Matrix Churchill was raided by HM Customs and Excise, the directors arrested and charged with illegal arms exports, many other companies and suppliers similarly treated and this massively publicized as a great coup for government forces of law and order. Years later when it finally came to trial and several people and outfits had been comprehensively ruined, the trial collapsed after a former government minister called to give evidence publicly stated that he knew what the parts were, several other ministries likewise, and SIS did too (and so did the Israeli Mossad) and had secretly given Matrix Churchill the OK as they wanted to see what Saddam Hussein was up to. It was a great miscarriage of justice and had a lot of echoes about Bull's complaints about the US Government that got him a year in prison, only in this case the dirt came out in public. Today, I'd think it would bring a UK government down.

    https://www.theguardian.com/world/defence-and-security-blog/2012/nov/09/arms-iraq-saddam-hussein

    The southern US F-Class shooters who have adopted the 300WSM and mostly shoot 210s, a few the 215 Hybrid, often make this claim and say they've proved it many times in actual match conditions against sevens that should be theoretically better. I'm sure that if the simple BC allied to MV is a factors A to B relationship that there is a hidden factor C somewhere that says that A and B alone sometimes produce the sole relationship, but not always in all conditions. Don't ask me for any scientific justification but a lot (and apparently growing number) or very experienced F-Class competitors now think there is something going on. Incidentally, I shot in club matches all last year usually pairing a very competent shooter who won several of our mid to long-range matches with a 6.5 Creedmoor and the 147 ELD at some quite scary MVs reserving his 7mm Shehane for 900 and 1,000 yards. One helluva combination, but the barrel pegged out after one season.
     
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  17. Longrange57

    Longrange57

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  18. Longrange57

    Longrange57

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    One of our shooters also shoots a really fast 6mm, which on paper should compete well at 5-600 yards. The wind kicks his tail. I love to run the numbers and look the situation over, but more often than not, we see some anomalies with the heavies that sure make you want to move that way! My problem is neck and shoulder....6.5 guy forever I guess! I do shoot a few 7mm's...just not long strings!
     
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  19. 284winner

    284winner Gold $$ Contributor

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    That's saying how it is. The high 6 & 6.5 BCs are very impressive and as impressive at very long ranges BUT when wind is in the game, the bigger 7 & 30 caliber bullets need to come out and play.
     
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  20. Longrange57

    Longrange57

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    We have beat this deal up pretty good! One more comment....a number of years back (10-12 yrs) German Salazar had an article about heavy for caliber bullets. He shot 30-06, 308, and 6XC I believe. He pretty much used the heavies! He did do very well shooting all of these calibers, but I remember something about the 30-06 and 210 Berger seemed to be his favorite in terms of wind. All I can go by is my own experiences!
     
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