Bullets and BC's... thought experiment

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

  1. 284winner

    284winner Gold $$ Contributor

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    We'll, No, you sound like your going by your own as well. We all have experiences and many times not all are equal. One thing I have discovered is that all things being equal except bullet weight, the heavy bullets seem to do best in funny or heavier winds gassing at unpredictable times. I've done damn good with heavy 6.5 bullets as well. Not a great deal of difference but one all the same.
     
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  2. fyrewall

    fyrewall

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    I am not a competitive shooter but think of my self as a student of this stuff before getting more into the actual event(s).

    A given - with standard .22 LR ammo @ 100 yards, seen & done it - standard vel. is all I buy except for 50 yard gophers.

    Could that Iraqi rail gun to be built or actually built, on some hill side, impart spinning to a projectile, (projectile thru rifled barrel)? I heard that one of the people involved with the project had his succession interrupted.

    Looking at the local 1000 BR shoot a common winner & high place showings is the 6mm Dasher with 105's, probably Bergers @ about 3,000. Western Montana is a windy place, lots of storms, especially in spring. Like 1/2 of the rifles on the line were 6mm Dashers for one match - again I am only a casual shooter. I did NRA hi power a long time ago before being drawn away by job stuff.

    Some Hornady G7 BC & FF info - real high values 6.5 147 ELDM & 7mm 180 ELDM

    6.5 147 ELDM, FF = SD / G7 BC = .301/.351 = .856 ; 7mm 180 ELDM, FF = SD / G7 BC = .318 / .401 = .796

    Berger 6mm bullet info;

    105 match hybrid = FF = SD / G7 BC = .254 / .278 = .913

    This sort of makes the 6mm bullet look like a BB compared to the bigger 6.5 & 7 mm bullets - am I missing something? How were the G7 BC values for all bullets determined?
     
  3. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    This does indeed show up in lag time, which is pretty cool. It works for any bullet, not just rimfire. You can run a bullet through any calculator right below Mach 1 and right above Mach 1, and you will see the effect. The massive increase in drag that occurs just over Mach 1 causes a large lateral force early in the trajectory, which “sticks” with it, while the subsonic’s relatively small drag force does not cause as much trouble. The extra velocity, which isn’t much, is not enough to make up for it. This isn’t a problem with the modeling so much as a quirk or drag curves, which spike dramatically near the speed of sound.

    Bob McCoy did a study of rimfire match ammo and determined that something like 900 fps (don’t quote me on the exact number, but it was subsonic) was the best velocity as far as wind goes (for the ammo he tested). It’s on the web somewhere.

    Another interesting thing is subsonic heavy bullets in .300 blackout (like the 220 SMKs). They basically do not lose velocity. They drop like a stone, but when they hit the ground they haven’t lost much forward velocity. There just isn’t much drag on them below Mach 1.
     
    Last edited: Jun 20, 2019
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  4. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    Both Berger and Hornady measure their BCs. I believe they do it with fixed head radars, but that may not be universally true.

    What you are missing is that you’re too focused on form factor. Form factor in itself is basically a measure of ballistic efficiency, not a measure of ballistic performance. BC is the number that will tell you about performance. Form factor by itself is one of those things that is more useful to a bullet designer than a shooter. It’s not totally worthless to a shooter, but you need to keep it in context.

    And yes, the 6mm is not going to hang with the 6.5mm as far as BC goes. But load a 6.5 with enough powder to match the 6mm’s velocity, and you might prefer the 6mm after all. Certainly, you can push the 105s at a petty high velocity while paying little price in recoil. Not so with the 147s, which will kick you pretty hard at 3100 fps. But If you don’t mind that, the 6.5mm will win out all day long. (Assuming accurate BCs and supersonic flight. I have no experience or knowledge of how these bullets fare in the transonic/subsonic range.)
     
  5. Laurie

    Laurie

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    Thanks Damon. Yes, I expected the high drag 1.3 to 0.9 MACH section of the flight to be the culprit here. Interesting on the subsonic Blackout and similar - never had much to do with subsonic loads / cartridges other than 22 rimfire. I seem to remember Bryan LItz says somewhere in one of his books that it's tricky to calculate the ultimate range of bullets from centrefire rifles because of the three stages in the drag curve as the bullet travels supersonically, then through the high drag zone, then finally very low drag subsonic flight, also dynamic stability issues coming into play in ELR flights.
     
  6. Laurie

    Laurie

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    It's a useful way of doing a quick and dirty comparison between different weight bullets in a single calibre as long as one remembers that two different weight bullets with identical form factors in a calibre will see the heavier model perform better in the wind at equivalent MVs. ('Equivalent' being defined as bullet weight + MV producing the same ME for the pair.)
     
  7. fyrewall

    fyrewall

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    Thanks all
     
  8. 284winner

    284winner Gold $$ Contributor

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    I suppose "pretty hard" is a relative term vs the 168/180s from say even a 284. My 6.5s are very manageable while shooting the 140 class up to the 147s. Saying that, the rifles weight plays a large part in that comfort zone in recoil. Point taken tho, the 6mm with 100-115s is a kitten vs many of the 6.5-7s. Someone dealing with unpredictable winds will find that 147s a much better option at the cost of a little more recoil. Heavier bullets and higher BCs go well together in these conditions.
     
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  9. Longrange57

    Longrange57

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    Absolutely agree! The 6mm I was referring to is 115 DTAC, 6XC or 243 Imp...shooting these at 3100-3200 in calm winds is deadly and a comparable BC to 140 gr 6.5
     
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  10. Longrange57

    Longrange57

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    Keep in mind I'm talking about 5-600 yds....not 1,000. However, I have seen some good results up to 1,000 in calm conditions!
     
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  11. Ballisticboy

    Ballisticboy

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    The Iraqi gun was not a rail gun; it used a conventional charge system, a very large charge system. It fired gun launched rockets hence a rifled barrel was not used. With the right rocket it was capable of putting a large payload into orbit, the advantage over a conventional rocket being that the gun launched rocket would be much smaller for the same payload. I had to analyse all this for the enquiry after the export of the barrel sections had been stopped.

    Gerry Bull was assassinated by person or persons unknown.
     
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  12. fyrewall

    fyrewall

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    BC - derived from actual testing ?
    SD - simple math, diameter & weight
    FF = SD/BC
    BC = SD/FF

    lower FF, less drag, more efficiency?
    higher BC, higher performance?

    In my comparisons of the 6mm 105 Berger, 6.5mm 147 Hornady ELDM, and 7mm 180 Hornady ELDM, I used manufacturers supplied G7 BC's and SD (weight & caliber). There was a big difference in the FF's produced using the provided BC's that measure "performance". My intent was to get some idea or focus of how these bullets related to drag - if the BC's are correct the bigger bullets would appear to slide thru air better (more efficient).

    "But load a 6.5 with enough powder to match the 6mm’s velocity, and you might prefer the 6mm after all" Should that be "prefer the 6.5mm after all"

    Lapua has all sorts of ballistic resources and I have noted that they came up with a 6.5 mm bullet vs. a 6mm for the 6.5X47 Lapua.

    A 147 6.5 @ 3100 would be some big rifle.

    The question has been asked - what would be preferred given two bullets each having equal BC's but different weights, if the lighter bullet could be pushed faster? Then, should BC's be the primary criteria for bullet selection?

    I did not see BC's in Hornady's 4DOF calculator but BC's are included in bullet specs (marketing?)

    All I know about Iraqi weapony is what I see on TV and Internet news and I really appreciate being tuned into the situation more.
     
  13. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    You must take BC and velocity into account. Form factor, caliber, and weight are already accounted for in BC. You can’t pick a bullet in a vacuum. You have to have a cartridge (and rifle) in mind.
     
  14. Laurie

    Laurie

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    The 6.5X47L was designed specifically for one discipline, northern European 300 metre ISSF / CISM 3-position shooting. 300M ISSF is very big in Scandinavia and some continental European countries attracting big money (by Europe's standards) sponsorship and gets lot of media and shooter attention. (Nearly all of their non-military ranges are 300M too, have concrete floor / covered firing lines, and have been e-target equipped for many years now, so ideal for this discipline.)

    When Lapua designed its new cartridge, the 6 Norma BR dominated ISSF. Lapua wanted very badly indeed to have a cartridge bearing its name seize that crown! In 2019, 6mm Norma BR dominates ISSF! Fortunately for Lapua, its cartridge does other things at longer distances rather well.

    ISSF's needs set the specification for the 6.5X47 - 6.5mm to match 105gn 6mm external ballistics at modest pressures and velocities, but provide a large increase in barrel life over the 6 BR; precision that at least matches the BR, ideally improves on it; as small a case / charge as possible in order to again maximise barrel life and also to miminise recoil which would nevertheless inevitably exceed that of the BR.

    The reason for the barrel life priority is that ISSF is an extremely demanding discipline which requires constant and frequent practice sessions in each position to get to the top and stay there, and its courses of fire see a lot of rounds downrange, so a big tournament will see hefty ammunition consumption.

    The very top competitors are factory sponsored and get factory match cartridges and no doubt new barrels too so the barrel life issue isn't an incentive to change for them and I imagine that on balance the 6.5's recoil increase, even if modest, tipped the balance away from the Lapua cartridge. With very small 10 and X rings, the kneeling and standing stages are very, very demanding and overall shooter fitness and avoidance of fatigue essential.

    An interesting issue and story, but not one to use in an argument for 6.5mm v 6mm calibres per se.
     

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