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Thread: Cast bullet accuracy and trailing edge failure

  1. #61
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    Guys, I'm typing this on my phone, so bear with me till wednesday when I have a real keyboard.

    IOU... Definition of jargon, rationale of why an engineer does a ROM calculation when he lacks necessary data, and a narrative explaining, simply, what that calc. post was about.

    Thanks 44man for the pictures... Have a couple of thoughts about your picture that I will save till wednesday, but there isn't enough resolution to see much about the base edge of these. A profile picture might also add something...

    Bret, anything simple can be made impossible to understand with an incomplete or poor explanation by the author. I'll try to improve the explanation and then we can go from there.

    Best regards, DrB

    /EDIT: Alright, so I added a lead-in to the calc post on the prior page that should be a good start to satisfying the above. Bret, let me know if it helps/if you have any questions? /EDIT
    Last edited by DrB; 07-07-2011 at 03:35 AM.

  2. #62
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    I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
    The left boolit is from my .45 Vaquero and is engraved perfect.
    The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
    By stopping skid at the base there is no gas cutting or leading in the bore.
    Metal displacement is higher in the GG's then at the base.
    The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.
    Last edited by 44man; 08-17-2011 at 09:02 AM.

  3. #63
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    I wish more fellas would recover boolits and study them. It will tell you your next step for the alloy. The boolit MUST maintain it's original shape.
    Skid has to stop at the base band and not ever get wider then the rifling.
    I would love to see the 16 to 1 boolits shot at 1400 fps. I never could do it because the incantations said over the gun are voodoo and top secret!
    If the base is clean and sharp at the edges, nothing will harm it unless you shoot silly putty. My PB boolits shoot as good as GC ones. Even in my 45-70 BFR and the .454 at max loads. Only base destruction at launch will ruin exit at the muzzle. But then you no longer have the perfect boolit you spent so much time casting, sorting and seating with care. Why some insist on ruining a boolit before it even leaves the brass is a thing I can't comprehend. A cast boolit recovered should have no more damage then a jacketed bullet.

  4. #64
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    Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
    felix

  5. #65
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    Quote Originally Posted by felix View Post
    Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
    OH, OH, water goes down the drain in different directions depending what side of the equator you live in.
    Could we be using the wrong twist directions?

  6. #66
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    Colt likes to think so!!!! ... felix
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  7. #67
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    Quote Originally Posted by felix View Post
    Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
    Enough with the fun.
    That is true and I seem to find that more on a GC but I have not decided if it is caused by the edge of the check being forced back by friction.
    All the rifles I have or had were 25-20, 30-30, .35, .44 and 45-70 so I can't talk real high power rifles with cast.
    I quit hunting with rifles long ago and sold most to buy revolvers.
    I don't see it even up to 1800 fps with my PB but all GC boolits show a center depression.
    This is how I like to ruin a base. Shoot the boolits into one hole at 50 yards and have boolits run into each other in the trap.
    Last edited by 44man; 08-17-2011 at 09:02 AM.

  8. #68
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    I don't see how you can get a better centerline conformance than that! That is very close to a condom configuration. ... felix
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  9. #69
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    Quote Originally Posted by 44man View Post
    I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
    The left boolit is from my .45 Vaquero and is engraved perfect.
    The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
    By stopping skid at the base there is no gas cutting or leading in the bore.
    Metal displacement is higher in the GG's then at the base.
    The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.
    Could there be something else going on here? It looks like the groove on the leading edge (the ogive) is well formed. Can you rotate the boolit so we can have a better look?

    MJ

  10. #70
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    Quote Originally Posted by DrB View Post
    Can't you have pretty stout proof in the form of a recovered bullet with engraving marks intact? As long as there isn't evidence of skidding/stripping, you should be able to calculate nominal spin rate.

    Engraved groove depth as compared to rifling width should even allow you to calculate a maximum error (minimum spin rate) with a few assumptions? Haven't quite thought this through enough to propose a formula when there is widening of the lands evident.

    Nominal spin rate in rotations per minute = (velocity ft/sec)/(twist inch/turn)*(12 inch/ft)*(60 sec/min)

    For experimental measurement you could do high speed photography with a marked bullet, measure total reflected light levels from a scene with a marked bullet vs. time, or you might be able to do it acoustically with a symmetrically spooned nose or such. Probably not necessarily cheap but very doable if you had a budget.

    Well. So I'm back home and I'll try catching up on some thoughts on this thread.

    First, with this one.

    As stated before, knowing the spin rate of a bullet is as simple as knowing the muzzle velocity and barrel twist, as long as you have evidence that your bullet isn't skidding in the bore (as indicated by the engraved lands on a recovered bullet being the same circumferential width as the rifling lands). No fancy smancy instrumentation is necessary beyond a chronograph and a recovered bullet evidencing the engraved lands absent skid. The bullet rpm in this case is calculated as above:

    1) Nominal spin rate in rotations per minute = (velocity ft/sec)/(twist inch/turn)*(12 inch/ft)*(60 sec/min)

    Note also that skid of engraving on a bullets surface may not necessarily mean that the bullet doesn't leave the muzzle at nominal rpm for the muzzle velocity and twist. It is possible for all the evident "skid" of a bore's land engraved on a bullet to occur at some initial or intermediate point in the barrel, after which the rifling holds without further skid. In this case the nominal rpm will be the actual rpm, regardless of stripping.

    One variant of this can be when a bullet first engages the rifling. In this case, the rifling will initially engrave just the initial edge of the bullet diameter and this material may easily provide insufficient strength to impart the necessary torque to the rest of the bullet without enlarging the engraved land on this leading part of the bullet. As more and more of the bullet engraves, more contact area is available to impart torque to the bullet, and absent increasing acceleration, the bullet is likely to evidence less skid the further back on the bullet the rifling engraves. I don't see how this "starting skid" would necessarily impact accuracy as long as the bullets were all of fairly consistent hardness and the engraved rifling at the rear of the bullet wasn't enlarged to the point that gas-leakage occurs. I would though, tend to think that engraving of this sort might be indicative of a borderline load, especially as the widening of the lands approaches the base of the bullet. By "borderline load," I mean one for which if the initial acceleration were slightly increased, accuracy would rapidly deteriorate (due to the enlargement of the engraved groove at the base of the bullet).

    Next, let's discuss calculating a maximum error in estimated rpm due to bullet skid so as to arrive at a minimum possible bullet rpm. Unless you assume something regarding the acceleration profile of the bullet in the bore, you cannot calculate a non-trivial maximum error (to subtract from the calculated nominal bullet spin rate) on the basis of the width of enlarged engraving. For example, assume the (unrealistic) case of a bullet barely moving down the bore (say 1 fps), then slammed in the last inch of travel up to a muzzle velocity of 1500 fps. In this hypothetical case in a 1 turn in 20 inch barrel, the engraved rifling would skid 18 degrees.

    Nominal estimated rpm would be (from 1): (1500 fps)/(20 inch/turn)*(12 inch/ft)*(60 sec/min)=54,000 rpm

    Actual rpm would be (also from 1, but using 1 fps for velocity): (1 fps)/(20 inch/turn)*(12 inch/ft)*(60 sec/min)=36 rpm

    As you can see, if you are unable to make any assumption about the bullet acceleration profile the largest theoretical error is 100% (EDIT: IF THE BULLET IS SKIDDING!)

    <TO BE CONTINUED> (EDIT: I think there are some simple and reasonable assumptions we can make to get an estimate of bullet rpm error in the event of skidding (without stripping)...)
    Last edited by DrB; 07-07-2011 at 02:14 AM.

  11. #71
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    Quote Originally Posted by 44man View Post
    Enough with the fun.
    That is true and I seem to find that more on a GC but I have not decided if it is caused by the edge of the check being forced back by friction.
    All the rifles I have or had were 25-20, 30-30, .35, .44 and 45-70 so I can't talk real high power rifles with cast.
    I quit hunting with rifles long ago and sold most to buy revolvers.
    I don't see it even up to 1800 fps with my PB but all GC boolits show a center depression.
    This is how I like to ruin a base. Shoot the boolits into one hole at 50 yards and have boolits run into each other in the trap.
    Hah! Robinhood, .429 style!

  12. #72
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    Quote Originally Posted by 44man View Post
    I have another picture I took some time ago of boolit skid. Forgive my cheap camera, not enough pixels.
    The left boolit is from my .45 Vaquero and is engraved perfect.
    The right is from my .475 and you can see the skid. It has stopped at the base and this boolit is extremely accurate. By making it just a little harder the skid is reduced. It does not appear to get more accurate by much so the WD, WW metal does good.
    By stopping skid at the base there is no gas cutting or leading in the bore.
    Metal displacement is higher in the GG's then at the base.
    The .45 is shot at 1160 fps and the .475 is 1350 fps. It weighs 420 gr and has more inertia to resist spin.
    Neat...

    So in this picture, the rifling is forcing the top of the bullet to move (rotate) from left to right. The right side of the engraved groove is taking compression, and the rifling is scraping this material to right during skid. Thus the right side of this engraved land looks sharp while the left is pushed down and washed out.

    So I'm thinking that if this is what it appears to be in the photo (wide at nose, narrow at base), then it must be from skidding during the initial engraving. Initially, the rifling has little bullet metal to grab/less contact area, then more and more as the bullet engraves the further it enters the bore (simultaneously spinning up to match the velocity/rifling twist rate rpm), such that before the bullet becomes fully engraved it is spinning at the rifling twist rate.

    That said, it kindof looks like from the photo there may have still been some skid after the base was in the rifling?
    Last edited by DrB; 07-06-2011 at 01:31 AM.

  13. #73
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    The only way to get the RPM is through photography, as was already decided. A projectile with various acceleration points will not be consistent enough to be anywhere close to being accurate enough to be "reliable", either within the barrel or not (negative acceleration for sure). ... felix
    felix

  14. #74
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    Extremely good postulations given.
    I will be bouncing back and forth. I am making beer and sparging, getting ready to boil.
    The 420 gr boolit has base engraving at a perfect fit to the rifling. Had I air cooled, it would be exceeded and create gas channels. Not good!
    True that initial skid means little and the boolit will still spin as required.
    More skid would cause fliers and leading. That would require a harder boolit. To say a softer boolit would correct leading is wrong.
    If I was to shoot this boolit faster, it needs even harder lead.
    This is why it is important to study recovered boolits and not guess. That little piece of lead has a thousand stories.
    I wish I had a dollar for every one that said the boolits need to be softer without knowing all the facts!

  15. #75
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    Quote Originally Posted by felix View Post
    The only way to get the RPM is through photography, as was already decided. A projectile with various acceleration points will not be consistent enough to be anywhere close to being accurate enough to be "reliable", either within the barrel or not (negative acceleration for sure). ... felix
    What?!!

    Who decided that? And did they consult with reality before making that decision?

    I suggested that if you wanted direct measurement, you could use photography to measure bullet rpm. There are other ways, but with photography is way, way within the state of the art. Lord, I think folks have been taking pictures of bullets in flight of one kind or another at least back to the time of Ernst Mach! Well, Mach took pictures of the shock waves around a supersonic bullet (hence "Mach Number" and "Mach Wave")... not sure if he took pictures of the surface of a bullet itself. But heck, 18 years ago as an undergraduate, I did some tests with Johnson's HIRL in which we took stop motion pictures of a hypervelocity impact of a bullet on a target at around ~7km/sec (~23,000 fps).

    In prior posts I also pointed out that if the bullet wasn't skidding (as evidenced by a recovered bullet), it was trivial to get spin using a chrony and barrel twist.

    Absent evidence of skid, rpm can be calculated as the product of muzzle velocity and twist (with appropriate units). Absent skid, the rpm estimate should be as good as the accuracy of the velocity and twist measurements.

    What is "various acceleration points" supposed to mean?

    "Negative acceleration for sure"?

    Felix, I'm not understanding what it is you are trying to say here... all I'm hearing is pops and whistles on this end.
    Last edited by DrB; 07-07-2011 at 02:11 AM.

  16. #76
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    I think bullet base squareness to the bore has been mentioned in the context of accuracy a lot, both in this thread and in others, and I've been thinking about it in the context of this thread.

    I thought I'd do a quick calculation as to what the lateral force is on a bullet due solely to the bullet base angle and base gas pressure. Note that this is entirely aside from the forces resulting from early venting that create pressure on the ventward side of the bullet. As it happens, these two forces should act in concert to give the bullet a sideways shove in the direction the bullet base is angled towards.

    So, definition of variables is as follows:
    A = Bullet base area
    F = Total force on bullet base due to gas pressure
    Fl = Lateral Force component (shoves the bullet sideways)
    C = Caliber (in inches) of the firearm
    P = Bullet base pressure at the muzzle (this can be estimated from any ballistics program that allows you to vary barrel length and see the change in expected muzzle velocity)
    Br = Bullet base angle off normal in radians
    Bo = Bullet base angle off normal in degrees
    pi = the ratio defined by a circles circumference over it's diameter
    ksi = 1000 psi

    A = pi/4 * C^2
    F = A * P = pi/4 * C^2 * P
    Fl = pi/4 * C^2 * P * SIN(Br)

    So, by the small angle approximation,

    Fl = pi/4 * C^2 * P * Br
    Br = Bo * pi/180, so substituting, we have
    Fl = pi^2 / 720 * C^2 * P * Bo

    So, the bottom line is, per degree that the base is off square, and per 1000 psi of bullet base pressure, the lateral force may be calculated as:

    Fl = C^2 * 13.71 lbf / per ksi of bullet base pressure / per deg that bullet is off-square

    So, let's do a sample calculation.

    If you are shooting about a .452 bullet, and base pressure on the bullet at the muzzle is 5,000 psi (= 5ksi), and the bullet base is just 2 degrees out of plumb, then the lateral force on the bullet is:

    Fl = (.452)^2 * 13.71 lbf * (5 ksi) * (2 deg) = 28 lbf

    That's actually a pretty significant force. Consider that in our earlier sample calculation with a bullet going 1500 fps, only .2 fps lateral velocity was needed to open up group size at 100 yards by 1 inch. Also, keep in mind that a bullet of say 300 grains is only .00133 slugs... which means it doesn't take a lot of momentum change to give a 1/1000 of a slug bullet a .2 fps lateral kick.

    So how long would the average base pressure have to stay at 5 ksi (averaged over the time):

    Let's start by finding the acceleration the bullet experiences...
    acceleration = force / mass = 28 lbf / .00133 slugs = 21,053 feet/sec^2 (that's 653 gravities!)

    Next we find the action time required to impart .2 fps...
    time = velocity/acceleration = .2 fps/ 21,053 = 9.4 microseconds

    At 1500 fps, this corresponds to a distance of 14 thousandths of an inch. With a bore full of high pressure gases venting, maintaining the base pressure close to the sealed muzzle value for just 9.4 microseconds and a vent gap of .014 inches shouldn't be hard.


    So, what does all the above mean? Well, it seems pretty clear to me that a VERY slightly off-square base can result in a large degradation in accuracy, solely due to the lateral shove that results from an off-square base. By the time you can feel an off-plumb bullet base, you are probably experiencing some degradation. By the time you can see an off-plumb base, accuracy may well be totally gone. A degree of angle is a very small thing to notice on such a short surface...
    Last edited by DrB; 07-07-2011 at 03:59 AM.

  17. #77
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    Quote Originally Posted by felix View Post
    Shoot them hard enough, Jim, and the bases will go concave. Remember, gas expansion goes along the centerline when the projectile is moving, seeking the least resistance at all times. Picture the DOW bubbles going down the drain. ... felix
    That seems very dubious to me. We're talking about thousands of psi of gas pressure, at high temperatures. The speed of sound in the gas is going to be REALLY high, and the mean free path is going to be REALLY small. The time required to communicate a pressure differential across the base of the bullet (and for the gas to move to compensate) is going to be TINY. I think the best way to give your notion a reality check is probably to calculate the mean velocity of molecules in the gas and the mean free path, which I'm pretty sure would show you that the gas is going to be in quasi-equilibrium across the base of the bullet (base pressure should be relatively uniform). Consider that the gas is able to expand down the bore to maintain relatively high bullet base pressures tens of calibers away (even with the bullet racing away) -- does it really seem likely to you that a significant pressure gradient will be maintained across the base of the bullet (just half a caliber of distance)?

    Have you ever seen a numerical or theoretical calculation that suggests that what you are saying is physically possible, or is this just a hypothesis based on the cupping in a gas check base?

    I've never seen one of these cupped bullet bases, maybe there's something else about them that makes you think it's gas pressure differential across the base of the bullet and not something else?

    As someone else mentioned, if this only happens with gas checked and not PB bullets, in identical loads, then it seems to me that the gas pressure hypotheses would be discredited by that...
    Last edited by DrB; 07-07-2011 at 03:58 AM.

  18. #78
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    Quote Originally Posted by Cap'n Morgan View Post
    Don't know about accuracy, but altering the crown can certainly change the point of impact:

    http://forums.nitroexpress.com/showf...e=0#Post183746

    How's that for a quick 'n dirty double regulation!
    Ah, great!

    So, if one were to accept that a comparable size/geometry of defect on the bullet base edge can have very similar effect as when on a crown, then it becomes obvious that whereas a defect on the crown might change point of impact, defects on the bases of bullets will change accuracy.

    This is because the clocking of the defect on the bullet at departure from the crown is likely to be unbiased to any particular direction. Therefore the lateral shove that results from the gases may occur in any direction, and the bullet drift during flight down range from this shove can be in any direction. In other words, on one shot bullet base edge defects or base edge angle may result in the bullet flying high, the next time right, the next time one way, and then another.

    To maximize accuracy we want everything to be as repeatable as possible. If we do not control, or have no means to control, the clocking of a bullet base edge defect at departure from the crown, then we cannot repeat the resulting shift in point of impact.

    EDIT: I tool another look at the thread referenced on the crown grinding being done on some sabatti rifles. Sure looks like a LOT of material removed. Just to be clear, I kind of doubt anyone here is shooting bullets with comparable defects (or expecting good accuracy if they are).
    Last edited by DrB; 07-07-2011 at 10:38 PM.

  19. #79
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    It is still a habit of many BPCR shooters to chamber rounds in exactly the same position and to seat boolits in some alignment to case orientation. Probably a holdover from when molds were cut by hand in a vise.
    With modern dies and molds it is a waste of time.
    I had a large chunk of aircraft aluminum that I sawed into blocks and milled. I have no way to hold the blocks to the table so I use my vise. The very hardest job is to try and get 90* all around.
    Once the blocks are clamped into my mold vise, the top is milled first so it does not matter if a side is a degree off. Boolits will be 90* to the mold top.
    My molds are not pretty but I have to wonder if that extra step is why my boolits are so accurate???
    There are stories of fellas milling the tops of molds to remove BB's, or change weights. Just how does one align blocks to the center line of the cavities? I am far from being a machinist so I can't understand how it could be done if the blocks are not perfect.
    Take a good machinist square and check the outside of your blocks once. Lee molds are real funny!
    Then a few lap mold tops and sprue plates trying to make them flat. Can't happen!

  20. #80
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    Quote Originally Posted by DrB View Post
    Ah, great!

    So, if one were to accept that a comparable size/geometry of defect on the bullet base edge can have very similar effect as when on a crown, then it becomes obvious that whereas a defect on the crown might change point of impact, defects on the bases of bullets will change accuracy.
    One way to check how much a defect base will interfer with accuracy would be shooting deliberately mangled boolits through a ported barrel where the porting holes could be shut on and off. If your thinking is right (and I believe it is) the deformed base bullets should shoot significantly better when the pressure is bled off while the bullet is still in the barrel.

    I have ported several barrels to reduce recoil. I use several rows of small holes drilled between the lands, and following the twist of the rifling - typically 3, 4 or 6 rows with 5 to 8 holes each - all depending of the number of grooves in the barrel. Not that hard to do if you have a CNC with a controlled 4th axis. It would be a simple matter to make a sleeve to slip over the muzzle to block the holes.

    I'm not sure how the holes would react to high speed cast bullets, though. I have only tried some subsonic flat base in 6.5 x 55 and they didn't cause any problems. But I would expect some minor gas cutting to the base unless gas checks is used.
    Cap'n Morgan

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check