Bullets and BC's... thought experiment

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

  1. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Generally speaking, the higher BC will win out (ballistically) when fired from the same rifle. There are limits to this simplistic thinking, but it’s a good rule of thumb. The extra muzzle velocity bleeds off quickly and the lighter bullet’s BC can’t keep up for the long haul.
     
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  2. fyrewall

    fyrewall

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    Quote Berger Bullets, reloading manual, 1st edition, page 167-168 'If you've ever heard somebody make a comment to the effect of "it's a high BC bullet for its caliber and weight", what they are essentially saying is "that bullet has low drag, and good (low) form factor". These are the bullets you want to identify because they will give the best ballistic performance, regardless of what weight or caliber the bullet is, and what MV you can achieve with it.'

    Page 168, "However, the form factor is unrelated to the caliber and weight, so it clearly indicates the merit of a bullets profile, as it relates to low drag and ballistic performance"

    BC includes weight & caliber of bullet. Performance potential would include retention of velocity and resistance to wind drift.

    Sometimes but with noticeable exceptions.

    Expanding the FF stuff, I take it that the BC calculation combines mass and drag? If bullets have equal BC's would a bullet having low drag but less weight be preferred over a bullet having more drag and more weight. Does FF = SD/ G7 BC reflect this? If this is correct would the lighter bullet (same BC as the heavier one) be preferred because it could be driven faster? Comparing the 6mm, 95 grain Berger, BC = .249 with the 6mm 105 grain Berger BTHP, BC = .253 (higher than the 95 grain bullet), I think under this reasoning the 95 would be the better choice - slightly inferior BC, but using the calculation FF = SD/ G7 BC, or .230 / .249 = .923, as opposed to FF .254/.253 = 1.004 - the 95 has a higher form factor. In this case the bullet with the higher BC has the lower FF.

    The 95 VLD (hunting & target) is a wonderful bullet, when shot out of a common ordinary .243 is gives terrific hits on rodents way out there, except it does not blow them up good. For shooting deers I would go for a 105 grain 6mm bullet at reasonable ranges.
     
    Last edited: Jun 11, 2019
  3. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    The BC incorporates mass, frontal area, and shape (drag coefficient), and relates it to a standard drag function to incorporate dependence of drag on mach number. Form factor is just the part of BC attributable to the shape of the projectile. It's a fudge factor once you've accounted for the easy stuff.

    So yes, if you have two bullets with the same bc, but one is lighter, and your'e shooting them out of the same rifle, and internal ballistics are close enough to the same the lighter bullet will have better ballistic performance because it will have a higher velocity, and it will shed it no differently than the heavier bullet. This lighter bullet will have a lower form factor - a lower contribution of the shape to drag (otherwise known as Cd - the drag coefficient). So the lighter bullet in this case can be thought of as a more efficient design.

    I've made bullets with the same design, varying only the a mount of lead core. Over a range of 180-195 grains, ballistics were nearly identical out of the same rifle. What was gained in BC by adding weight was lost by losing muzzle velocity. This is not exact, but it shows you that there's no free lunch - no easy way to improve performance over what we have available to us today. There are optimizations for specific applications that can be had, but leaps in performance aren't going to happen in any given well developed weight class (105 gr 6mm, 200 gr .308, 140 gr 6.5, etc.).

    Berger's manual is a little fixated on BC, if you ask me. I love what Bryan Litz has done in spreading accessible information about ballistics and pushing forward the market for low drag bullet design, but I think that we're getting a little tunnel vision here. There is more to good bullet design than raw ballistic performance. You need to be able to stabilize it at a reasonable twist rate, keep dynamic stability in check, minimize long range yaw, minimize sensitivity to aerodynamic jump, achieve consistent ballistic performance, etc. Not all of these things are conducive to a high BC. I'd argue that more than a couple manufacturers have pushed the limits too far and found that the market for high BC has it's limits. There's always a trade-off.
     
  4. Laurie

    Laurie

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    Hear! Hear! However, [high] BC has always sold bullets and if anything appears to have an even greater pull on the punters today.

    I have a bunch of friends in a North West of England club, the 101 RC based on the UK MoD reserve military facility at Altcar near Liverpool. They have managed to obtain sponsorship from Nammo Lapua for an F/TR team with the proviso they use Lapua metallic components and Viht powders. In practice that means 308 'Palma' brass and the 175gn Scenar-L bullet, a very well made but relatively high form-factor (1.067) / low BC (0.247) design. So far, they are doing much better than many would have believed possible in top-end UK National competition at long ranges, other top F/TR shooters nearly all shooting the Berger 200.20X model nowadays.

    https://www.lapua.com/match-report-...rb1VTWyUlouguQ3SBDLh_TE4EwE0bTcnpUy0HnX8OHaKA
     
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  5. Longrange57

    Longrange57

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    I'm in the camp that the heavies win out with BC being equal simply because I've seen it with my own eyes a number of times. In my mind and looking at the ballistics as others have pointed out...all should be equal. We have had 25 shooters at the line and about half shooting small caliber (6mm/6.5mm) with ballistics that show the same drift as the other half shooting 7mm/30 cal. and upon wind switches at 600 yards virtually every small cal group would be moved over 1-2 inches further than the heavies. We have actually experimented with this and seen it a number of times. I do not have the answers but many of our shooters have witnessed this phenomenon. This thread specifically stated 1,000 yards plus! Again, as has been indicated we have many changing conditions and it becomes very hard to determine if some of this is BC or a couple of dozen other factors. I have asked the question on a couple of other threads about spin decay rates and not gotten responses. If we place any faith in the Kolbey spin decay rate formula, there are questions that come to mind. First off, solving for target spin rate (regardless of range), we only have 3 parameters to input, and that is bullet rpm at the muzzle, flight time, and bullet diameter. As an example if you calculate for 3 different calibers, .308, .338, .375 at 3,000 muzzle velocity which gives 240,000 rpm, we will see a pretty good difference in spin decay by bullet diameter. At 1800 yds the 308 loses around 27%, the 338 24%, and the 375 21%. As the distance goes further the decay gets worse but not near as bad as velocity. Regardless of the accuracy of this formula, is it possible that the decay rate of spin (whatever that is) and velocity at long ranges causes unusually long skinny bullets to become unstable although our calculator shows it to be stable. We have solid bullets with super high BC's and low weights but will not hang with the heavies of the same length at really long ranges (1-2 miles). Although a few 338's have been successful in the KO2M events, more often than not the heavies will be used at much lower velocities. I don't have any answers, just asking the question....could spin decay have anything to do with the original question?
     
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  6. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    That’s an impressive gap to make up between a 175 Scenar L and a 200.20x. On a calm-ish day, though, a slow twist and those 175s ought to shoot very well.
     
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  7. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    I don’t have a good answer to that question - it’s beyond my expertise/experience. I would say, however, that once you hit the transonic zone, you’re not really comparing bullets of equal BC anymore. You’re getting into factors that a BC just isn’t sophisticated enough to capture.

    In supersonic flight, on the other hand, it’s safe to say that all that matters is BC and muzzle velocity. For most of us, that’s all we do.
     
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  8. Longrange57

    Longrange57

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    I gotcha! I've definitely been guilty of chasing BC, however the distances I shoot at...accuracy has been the new rule for a number of years! Thx
     
  9. Ballisticboy

    Ballisticboy

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    I agree with damoncali here, just because two bullets are sold as having the same BC does not mean they will be the same at long range. I have tracked bullets out to anything between 2.5 to 5 km depending on the bullet calibre and muzzle velocity and once the bullet speed drops below Mach 1 all bets are off. While the supersonic drag may be similar between types some bullets display large drag increases almost immediately after Mach 1 while others will hang on to much lower Mach numbers. As soon as the drag increases the wind response will also increase. The key is dynamic stability hence short fat bullets will tend to do better than long thin ones. Apart from its effects on dynamic stability which can be significant, spin does not really come into it.
     
    Last edited: Jun 12, 2019
  10. Laurie

    Laurie

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    The military had an obsession with ultra long-range ballistics from rifle calibre machine-guns starting in late WW1 and developed in the Western Front static trench warfare conditions. Massed MG fire on fixed lines would shoot over the support trench networks, light rail and road communications facilities far behind the front lines at ranges that reportedly reached 5,000 yards on occasions. During that period and for some years after the war there was a lot of bullet development that produced relatively heavy FMJBT designs, the 197gn German 8mm sS (schwerer Spitzgeschoss = heavy pointed bullet); the famous Lapua D-series some of which are still available and so on. (Sometime around the 1930s they mostly lost interest although the Germans made their hot 197gn sS round the common MG and rifle cartridge in 1932 and on some of the more open WW2 eastern front battlefields, tripod-mounted MG34s and 42s inflicted carnage on Red Army formations at some very long distances.)

    These bullet designs were all relatively short and blunt compared to an equivalent weight and successful modern match bullet such as the 30-cal 185 Juggernaut never mind Sierra's new 28-cal radius nose MatchKings, RDFs and others. The designers came up with these forms the hard way - going out and shooting things far, far away and seeing what did or didn't stay stable enough to hit targets at a mile or two miles. I was always amazed on reading of the now almost forgotten ultra long-range matches run in Scotland, some until after WW2, using the common or garden 0.303 round. Although there were some long-range match 'streamlined' (heavier and boat-tailed) 303 bullets developed, AFAIK much of this super-shooting used selected lots of standard 174gn FMJ ammunition, a flat-based bullet design.

    So, it's obvious that at some stage in the bullet's flight / retained speed new factors above and beyond supersonic flight BC come into play and the ideal 3,000 yard bullet may or more likely may not correspond to an ideal 1,000 yard design. There is more to this though - there are now just too many reports around from very sound citizens / accomplished L-R match shooters that some of the latest generation of very high BC designs are proving too finicky to 'tune', and even when this is apparently achieved, they don't always perform consistently enough year round in all temperature and wind conditions. I've met more than a few top F/TR shooters who raved about a particular bullet, won a major match with it, then after getting nowhere in the next round of a championship series, go very quiet and if pressed admit they're back to shooting the 200.20X. Berger seems to have managed (generally, not always) to combine high production consistency, a good BC, and good manners / flexibility. I won't say the more radical design challengers don't, but so far none appears to have convinced large numbers of top, experienced people to switch permanently.
     
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  11. Longrange57

    Longrange57

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    Good analogies by all! Due to some bullets key-holing at relatively short ranges when the calculator shows it to be stable and other bullets shooting well and we would expect them not to because calculator shows them to be very unstable, I'm convinced that spin rate decay and whatever is going on with spin rate and possibly yaw past 1,000-1,200 yards starts to affect BC. No scientific data...just observation. It's a given that once they go subsonic all bets are off, but appears we have things going on prior to that velocity.
     
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  12. DocUSMCRetired

    DocUSMCRetired

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    While you guys ponder on this, remember a couple of things.

    1) BC in and of itself is calculated in part using weight. BC = (weight in grains / 7000) / ( caliber ^2 * form factor).
    2) Lag Time is important.
    3) Since lag time is important, Muzzle velocity is also very important. Bullet weights have an impact on our ability to obtain the same muzzle velocity depending on your cartridge selection. Also remember a BC is true at a given velocity.
    4) Bullets are not "pushed" by the wind, they are actually pulled by drag. Wind changes the orientation of the gyroscopically stabilized projectile, and a small amount of drag is shifted pulling the bullet off course.
     
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  13. DocUSMCRetired

    DocUSMCRetired

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    The further you separate the CG from the CP the faster you need to spin them to be stable, aka the more inherently unstable the bullet is.

    Spin rate decay is almost a non issue from everything I have seen with modern bullets. Velocity decay happens much faster, which naturally increases stability as spin rate decay is much slower than velocity decay. If that makes sense. As long as the bullet starts out with good stability.
     
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  14. Laurie

    Laurie

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    This seems to be at least in part at the heart of many problems with recent designs. I was therefore intrigued by Hornady claiming that its new A-Tips have their design / construction 'tuned' by machining the tip such that its effect on bullet OAL will cause the CG and CP to perfectly coincide.

    So .... now ALL we have to do is wait and see if enough serious precision shooters buy these expensive projectiles and whether they actually deliver claimed stability / precision in Bench-Rest, F and ELR competition in various calibres fired from barrels with various rifling pitches at various different MVs and over various distances. o_O :(

    (As a budget-conscious retired and certified old f*rt, I'll only add it won't be me finding this out. :) )
     
  15. Ballisticboy

    Ballisticboy

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    Having the CG and the CP coincide is the absolute worst configuration you can have as your gyroscopic stability factor will be infinite and the bullet will be unable to correct any yaw or yaw rates. At least with an unstable bullet it will just tumble and not go very far but a neutrally stable bullet as proposed can literally go anywhere.

    It is worrying if a major manufacturer is not aware of this.
     
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  16. Ballisticboy

    Ballisticboy

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    Agree with this, the only proviso I would make is that the relative increase in spin relative to velocity can lead to dynamic stability problems if dynamic stability is already marginal and the ratio becomes too great.
     
  17. Laurie

    Laurie

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    Wow! :eek:

    Ah well, it'll be even more interesting than usually applies to (manufacturer claimed) radical new designs to see how the A-Tip actually performs.
     
  18. Longrange57

    Longrange57

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    Spin relative to velocity is the big question. For the most part my answer would be the same Doc gave....that is, "spin rate decay is almost a non issue." However, this patented answer is because we do not really know what the spin rate is at the target (1,000, 1,500, or 3,500). The guesstimates are 25-60% ranges depending upon bullet diameter. Are we missing something here? Is there an actual way to determine if this is something to worry about? Probably never have to be concerned with the issue except for ELR, but would be nice to know.
     
  19. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    I think the reality is that the a tip is just the same idea as the plastic tip bullets in that they’re attempting to shorten the effective length of the bullet for stability and accuracy without giving up BC. Hornady seems to be saying that they’re aluminum because they couldn’t make the plastic ones work because injection molding chunky parts is difficult. Plastic would be a better choice if it can be done. Hornady’s marketing guys get a little carried away at times. I doubt very much that the cp and cg coincide.
     
  20. Laurie

    Laurie

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    I'm sure you're right. It'll be interesting in any event as with any new premium and claimed high-precision design to see how they work out. The one thing we can be sure of is that the 'innovation' will add another few dollars / 100 for you and pounds sterling for us over here. (Not exactly innovative anyway - I have a box of small-scale production alloy-tip bullets pressed on me to try out maybe 20 years ago and I'd imagine they weren't the first of this type by any means - I never actually shot any of them as it turned out.)
     

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