Let's argue about BC's

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by WildcatB, Feb 15, 2011.

  1. WildcatB

    WildcatB Well-Known Member

    Messages:
    461
    Joined:
    Jan 27, 2008
    Let’s argue about BC’s.

    I sent some of our 7mm 200 grain ULD RBBT bullets to Bryan Litz for BC testing. He got .74 – this is a huge difference between the .9 many people use to get their drops to match. For the longest time I’ve been trying to figure out why such a big difference.

    At first I thought it might be because Bryan tested these bullets with a 284 at about 2500 fps and a 9 twist barrel possibly causing the bullet to not stabilize for a while which would lower the measured BC. Bryan said that they were stabilizing just fine though.

    When I first got my 7mm AM, I zeroed it in at 100, running them at 3250 fps, I plugged in .9 and hit my 1000 yard target about 5” low 3 times. This seemed to confirm the .9 bc. Not long after, I shot at a coyote at 2002 yards and missed left by six inches. Drop looked dead on. I used .9 to get the drop.

    When people would email me asking for the BC of this bullet, I’d say “Myself along with many of my customers plug in .9 to get the drops to match. However, Bryan Litz from Berger and Applied Ballistics tested them and got .74 – I’m not sure why such a big difference…” or something like that.

    I’ve never had anyone email back and say .9 didn’t work… and no one said .74 worked either. The difference still bugged me (still bugs me). I think there is an honest explanation.

    Trying to figure out why, I checked and re-checked everything. I noticed that I left the default scope height of 1.5” on the ballistic calculator (jbm’s) My actual scope height is 2.375 measured from centerline of bore to centerline of scope. After I made that adjustment…. the numbers all changed. I haven’t gone out and tested long range since but the numbers seemed to match with the 5” low hits I was getting at 1000.

    Last weekend I went out and shot my most consistent load over a chrony at 10 feet and 888 feet. 3 shots each. The ES of this load runs about 9.

    Here’s the data (averages of 3 shots)
    Near Velocity: 3300
    Far Velocity: 2956
    Chrono Separation: 878 ft
    Temp: 48
    Humidity 28.5 %
    Pressure 25.08 (uncorrected)
    Altitude 5127

    I plugged it into JBM’s “Ballistic Coefficient – Velocity” calculator and got a g7 bc of .740

    Why are some people using .9 and hitting targets long range and the measured BC is .74 as per Bryan Litz and myself?I finally think I have a fairly good explanation and I’ll try to explain this later after I’ve shot 1000+ yards with my new numbers.

    In the mean time,…. does anyone have any input on this? Why such a big difference? Byan, Kirby, Dick, Len, James, Don, ????? anyone?
     
  2. groper

    groper Well-Known Member

    Messages:
    327
    Joined:
    Sep 2, 2008
    Im not having a dig at you, but i have never beleived the stated BC of these bullets to be anywhere near 0.9. There is no such thing as magic bullets...

    The reason BC numbers often get mixed up, is that very small details such as zeroing error, aiming error, environmental errors, small velocity errors, and distance errors etc... can very quickly add up to HUGE differences in derived BCs based on drop tests ESPECIALLY at anything less than 1000yds on medium calibers. The way Bryan Litz tests the bullets, using multiple time of flight measurements rather than error prone drop tests, he gets rid of just about all this error and he has the experience to conduct the tests accurately.

    most people also beleive that 1000yds is a long way... for a serious long range caliber, it actually isnt far enough. If you want to complete a more accurate drop test for a derived REAL WORLD BC, you really should shoot your bullets out to its maximum supersonic range which can be up near 2000yds if you have a good jacketed bullet in a powerful caliber, or even +2500yds with solids and the cheytac derivative cartridges.

    Have you shot to 1 mile with your bullets? I can guarantee if you put 0.9 into your ballistic calculator for 1760yds and your velocity, you will need ALOT more elevation than your calc predicts - provided your bullet is stable thru the transonic region aswell...
     
    Last edited: Feb 15, 2011

  3. Long Time Long Ranger

    Long Time Long Ranger Well-Known Member

    Messages:
    2,612
    Joined:
    Mar 12, 2002
    Paul, I am working a similar issue with the new 338 caliber 185 grain Barnes TTSX. Barnes states a bc of .432 but I am having to plug in closer to .52 to get my drops to match up out to 800 yards. There is two feet of snow at my long range shooting spot and can't work on it until it melts. I am calibrating two chrono's and doing that test when the snow melts. I had my scope height right so it is something else. Let us know what you find out. When I can get on the range and figure mine out I will let you know.
     
  4. Jon A

    Jon A Well-Known Member

    Messages:
    1,092
    Joined:
    Dec 28, 2001
    At 3300 FPS the bullets will have a higher G1 BC than they will at 284 velocities. So it isn’t surprising the AM users experience an observed G1 BC higher than the average Bryan got, especially only out to 1000 or so. Had he measured it out of an AM it likely would have gotten a higher G1 number (assuming he measured to the same distances) but the G7 number likely would have been very similar. This is one of the big reason to use G7 BC’s as they should be more nearly constant for all MV’s for bullets of this shape.

    I used the JBM calculator with the data you showed and got BC’s of .358 G7 or a .741 G1 which is right in line with what Bryan got. I think you double did the elevation/pressure somehow.

    FWIW, don’t feel bad, it’s still an outstanding bullet. It’s really not surprising to me it didn’t quite make an honest .9 as I always kind of thought it wouldn’t. It only has the equivalent SD of a 235 grain 30 cal. It still kicks but on anything else widely available in that caliber (or 30 caliber at the moment for that matter) by a long ways.
     
  5. groper

    groper Well-Known Member

    Messages:
    327
    Joined:
    Sep 2, 2008
    If you want to do a more accurate real world BC calculation, rather than do a drop test, provided you own a laptop computer

    try this --> http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

    Follow the theory outlined in this paper, we are going to use the same theory for BC calculation rather than velocity calculation. We will shoot thru the chrony for the velocity.

    Remember to allow 0.001 seconds for every foot your microphone is from the muzzle AND gong.

    Make sure you allow for wind speed in your speed of sound, or simply wait for a no wind day.

    Make sure you get the air temperature exact.

    Shoot a gong at as much range as possible, the mic of your laptop will pic up the sound of a gong @ 1000yds.

    Record each shot velocity and time of flight seperately. Once you have enough data, run several calculations to figure BC, and use the median result for your actual real world BC.

    You can thank me with beer :D
     
  6. Buffalobob

    Buffalobob Writers Guild

    Messages:
    5,085
    Joined:
    Jun 12, 2001
    I use a G7 BC of 0.370 with JBM and multiple G1s in Exbal (older version) which run from about 0.8 - 0.76 and drop my input velocity. This begins to match my results in the 1800+ yard range.

    My feeling is that somehwere in the altitude / calculate pressure input is where the problems arise.
     
  7. Jon A

    Jon A Well-Known Member

    Messages:
    1,092
    Joined:
    Dec 28, 2001
    That's genius! Why didn't I think of that? That'll be much easier than the dual chronograph method I've always used. Probably more accurate as well since I can safely do it at a much longer distance.
     
  8. MontanaRifleman

    MontanaRifleman Well-Known Member

    Messages:
    6,068
    Joined:
    May 21, 2008
    It's refreshing to see a bullet manufacturer be so transparent. Good job Paul!
     
  9. eddybo

    eddybo Well-Known Member

    Messages:
    1,651
    Joined:
    Jan 5, 2007
    I have scratched my head over this very issue many times. I use .9 for a different bullet. Bryan Computer modeled the same bullet at .86 IIRC. ( I have the PM somewhere but it has been archived in TXT format) He shot the bullets and found them to be something much lower. I dunno. All I know is what I plug into my exbal to make bullets hit targets out to 2100. The variances just blow my mind.

    I did a whole bunch of playing with my equipment to figure where I was wrong. It was apparently wasted time as I could never find what caused the difference.
     
  10. elkaholic

    elkaholic Well-Known Member

    Messages:
    5,341
    Joined:
    Dec 4, 2008
    Groper, I need you to do my ballistic stuff while I make bullets......Rich:D
     
  11. Long Time Long Ranger

    Long Time Long Ranger Well-Known Member

    Messages:
    2,612
    Joined:
    Mar 12, 2002
    I was looking beyond the obvious things like bc's changing with velocity and atmospheric conditions which do affect the bullet but not as great as the discrepancies Paul and I are getting. JWP416 was in areospace missile work like me I think, maybe he has some ideas.

    Here is another one that involves the most difficult part of long range shooting which is reading the wind. Wind effects are the primary reason I spend so much time testing a bullet when I change from one to another. BC's are calculated for frontal wind resistance and that bc is applied to wind from other angles (wind drift) which is inaccurate other than for the time of flight portion of the equation. How are the side profiles of the projectile and mass considered with the wind vectoring in at different angles and different speeds. You can guestimate the wind vector fairly close with new stuff out there but the frontal bc of the bullet is still used and not the actual realistic side profile and mass of the bullet. That is why it is so hard. Wind effects on the side profile are also different with all bullets and that may or may not correlate to the frontal BC. So how exactly is wind drift calculated by most ballistics programs. One of my degrees is in applied mathematics if anyone knows where I can get the formulas to go over them.

    I have found that the wind portion of the ballistics tables are off the most with different bullets and you just have to know your particular bullet and have experience with it. I have found that for the highest bc bullets the tables are closer which makes sense but for some of the mid range bc bullets they can be off quite a ways. It seems to me to be right on with accuracy you would need a side profile wind variable to plug into the equation for each bullet just like the frontal bc that predicts how well the bullet resists atmospheric conditons from the front. With aircraft it is easy since we have rudders to vector the aircraft to a point in space. But with bullets we must predict the wind effect and change our sight point to where we think the bullet is going.

    Experts care to shed some light on this. Or did I take one of my missiles off in space somewhere.
     
  12. groper

    groper Well-Known Member

    Messages:
    327
    Joined:
    Sep 2, 2008
    I dont really see the point in making very small gains in accuracy with regard to wind drift, when the wind itself is impossible to quantify over the entire trajectory.

    How do you know if the drift error was from the calculation, or the wind velocity itself different from what you input into the calculator? Even if you shot in perfectly consistent wind tunnels, how will this help in the real world variable winds?
     
  13. Long Time Long Ranger

    Long Time Long Ranger Well-Known Member

    Messages:
    2,612
    Joined:
    Mar 12, 2002
    I was just wondering the equation of how they calculated wind drift. I agree it is impossible to judge the wind. I was just speaking in general terms and not a specific incident. Does the equation take into effect bullets don't drift in the wind according to their frontal bc. I have fired two or more rifles simultaneously side by side at my range so they experience as close to the same wind as possible. Drifts are different with same bc from heavy large calibers to lighter smaller calibers. Has anyone else ever done this and noticed the effect.

    At a 1000+ yards I have seen significant differences and not just small gains in a strong wind. I don't want to hijack Paul's thread so I will stop. I was just wondering.
     
  14. backwoods83

    backwoods83 Well-Known Member

    Messages:
    2,206
    Joined:
    Jan 8, 2011
    Using the G1 drag force it is possible that you have a .9 bc at 3200 but depending on atmospheric conditions it may be as low as a .6 at 1800fps. Time of flight is always the better way to check, when your doing drop test think about it like this, if you are shooting 1500yrds your muzzle angle is pretty high and peak elevation before its starts leveling its self out could be 8-900yrds so for that distance launching vld or uld bullets at those muzzle velocities the bullets are literally surfing on the air so that tends to make them less predictable until there leveled out.