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Thread: Cast bullet accuracy and trailing edge failure

  1. #41
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    Quote Originally Posted by Bass Ackward View Post
    The real problem with understanding is the magnitude of influences that occur in trying to replicate exactness too understand.

    1. We know that going up through the barrier is destabilizing and coming back through it as well.
    I got that from a 50s TV show on Joe Walker about design problems. (neighbor) And the barrier is broken at the muzzle. You will hear the fracture as you run ladder testing. You go from poom to crack. Not only do you have force, but friction and heat.

    2. Accuracy zones (with velocity) goes up as bore diameter declines regardless of twist rate. 45 caliber is 1300 fps and 22 caliber is where you want it really. Strange thing is that pressure drops (muzzle pressure the least) most in 45 caliber and least in 22. Think about that one as it covers your proposed testing.

    3. If you bring pressure up slowly enough with a PB so as not to ruin a base, then pure lead will fail at one velocity and you can get more velocity as you harden. The narrowest window again is 45 caliber where you have very little advantage to over harden and the widest 22 where hardening pays off big time.

    4. In the same caliber, accuracy points decline (in velocity) as twist rate increases.

    5. I ran penetration testing in gelatin awhile back using pure lead on a 150 gr spitzer design to cut back on the amount of gelatin needed. What I found was that penetration dropped and became erratic as velocity increased beyond the bullets ability to hold during launch. Bullets did not mushroom from the spitzer design really, but I concluded that as velocity increased RPMS were actually dropping, thus the loss of stabilization. The gas checks appeared to hold the rifling, but rotated on the base. (no longer aligned with the bullet) If you picture that in your mind, there is only one way it can happen really.

    That's from too many years of experience.
    1) Ok, so I think when you say barrier you are referring to the sound "barrier?" /EDIT: I think I missed saying something pretty important for the laymen here, about the "sound barrier." The "barrier" is in fact not a barrier at all. Yes, you get an increase in the drag coefficient for a body around Mach 1, but it's not really that much of a "barrier," especially when you're talking about ballistics. Although everyone made a big deal about Yeager breaking the sound barrier, it's not like the engineering community just didn't realize that bullets and other objects travelled faster than the speed of sound (as most everyone here no doubt realizes there are plenty of supersonic blackpowder rifle loads). It was just a little more challenging to push a lightly built self-propelled aircraft faster than Mach 1 than it is a solid bullet shot from a barrel. This was in part because of compressibility impacts on the controllability of the aircraft with the then state of the art, and also with the principle mode of propulsion at the time (the propeller). Props rapidly loose performance and tend to get vibration problems as portions of the blade become supersonic (which inevitably happens as the blade rpm increases and/or aircraft speed increases). /EDIT

    Bullets are not very stable regardless of the sound barrier. The effect of supersonic flight is to make them less aerodynamically stable as the center of pressure moves forward (and you want the center of pressure behind the center of gravity for aerodynamic static stability).

    We spin long bullets really fast for two principal reasons... To keep the launch end forward despite bad aerodynamic stability, and to average out radially asymmetric aerodynamic forces from bullet surface geometry imperfections.

    I think my point regarding the relative magnitude of forces due to ambient gas pressure vs. base pressure still stands. The 64 psi number I stated before was a maximum bound for pressure at the stagnation point on the nose for standard conditions in freeflight. I calculated the "total pressure" as an upper bound, but really it is too high as the total pressure assumes the freestream is isentropically brought to rest, and the gas at the nose of the bullet has experienced shock(s). I'm not even sure though that the nose of a "supersonic" bullet is really moving faster than the freestream until it flies through the first few calibers/gets through some muzzle blast after leaving the bore. You see, the bullet has been pushing air ahead of it down the bore and there is also probably some gas leakage, and so the mouth of the bore is "blowing" prior to the bullets exit (I'm not sure how fast, typically, but it could theoretically be a supersonic jet of gas right before bullet exit /EDIT: Well, actually without leakage gas velocity would have to be as fast as bullet velocity right around exit... with leakage gas velocity blowing out the bore would be higher than bullet velocity immediately prior to bullet exit /EDIT). Then when the main muzzle blast occurs with the uncorking of the bore, the gas around the bullet is traveling faster than it is as the bullet is overtaken by the expanding gases!

    ANYWAY, I'm really pretty confident that with a reasonably sealed bore the freestream forces at bullet nose exit (before the tail of the bullet is released by the bore) are negligible compared to those induced by the escaping gas in the muzzle blast when the bore is uncorked. The freeflight/ambient pressures just shouldn't be of the same order of magnitude as those resulting from the pressures in the barrel.

    Sorry for going on... Did I correctly understand what you meant by "barrier"?

    2) by accuracy zones do you mean the useful velocity range that provides accuracy in a given caliber (and given tailoring of powder to cartridge)?

    3) Do you propose this is true for similar designs of cartridges/firearms/bullets? That is, if I took a 50 bmg rifle (instead of a 45 long colt) and shot it with a similar design bullet do you think I'd have significantly different results in terms of improving accuracy with hardness vs velocity than I would in 22 caliber?

    4) I don't understand what you mean... Please rephrase? What is an "accuracy point"?

    5) neat! This may or may not be the same thing, but this sort of result is VERY well known in the penetrator impact engineering literature. Do the following: calculate the dynamic pressure of your test medium as 1/2 (medium density)*(velocity)^2, for all your test point velocities, and compare the pressures you get with the yield and ultimate stresses (in the same units) for soft lead as you were using. I'll bet your dynamic pressure crossed over the yield and/or ultimate stress for the bullet metal. What happens is you get increasing penetration with increasing velocity up to a point, and then the fluid flow pressures generated by the impacted medium start destroying the penetrator (bullet) which decreases penetration.

    I'm not sure penetration here had anything to do with accuracy or the intactness of the bullet edge? It does sound like you were stripping the rifling, though, which ain't good for shooting groups for a bunch o' different reasons.

    Thanks BA for expanding on your original post.

    Best regards,
    DrB
    Last edited by DrB; 07-08-2011 at 03:51 AM.

  2. #42
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    Quote Originally Posted by 303Guy View Post
    Here is flame cutting that occured quite far up the boolit shank.



    All the leaking gasses got channeled into one jet by the groove in the boolit maybe?

    I've discovered that a light load very fast powder can expand a case neck more than a stiffer charge of slow powder. Primer flattening supports the supposition that pressures were very mild indeed. How do I know the necks were expanding more? Rust damage in the neck area of the neck was gripping those case necks which doesn't happen with more powerful and higher pressure slow powder loads.
    Holy smokes! That's cool. Was that a paper patch (i don't see much engraving on the base)?

    Seems to me that somehow that groove was pressurized by hot gases (an undersize diameter aft of the lube groove? Or if paper patched a tear or chunk out of the patch?). When the hot gases started leaking through some initially smaller channel they started eroding it more and more. But maybe that's making too much soup from the meat here. can't entirely tell texture and depth from the photo, and where gas damage ends and impact damage begins.

    What was it shot into, and what kind of load was it?

    Regarding neck expansion, wouldn't you expect to see neck expansion before primer sign? If you also weren't seeing primer sign on the slower powder load, then that sounds to me like the light fast powder load reached a higher peak chamber pressure (with less total gas volume generated). The heavier slower powder load may have generated more velocity with a higher generated gas volume, but (because of the slower burn) with a lower peak pressure in the chamber?

  3. #43
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    Quote Originally Posted by DrB View Post
    1) I think my point regarding the relative magnitude of forces due to ambient gas pressure vs. base pressure still stands. The 64 psi number I stated before was a maximum bound for pressure at the stagnation point on the nose for standard conditions in freeflight. I calculated the "total pressure" as an upper bound, but really it is too high as the total pressure assumes the freestream is isentropically brought to rest, and the gas at the nose of the bullet has experienced shock(s). I'm not even sure though that the nose of a "supersonic" bullet is really moving faster than the freestream until it flies through the first few calibers/gets through some muzzle blast after leaving the bore. You see, the bullet has been pushing air ahead of it down the bore and there is also probably some gas leakage, and so the mouth of the bore is "blowing" prior to the bullets exit (I'm not sure how fast, typically, but it could theoretically be a supersonic jet of gas right before bullet exit). Then when the main muzzle blast occurs with the uncorking of the bore, the gas around the bullet is traveling faster than it is as the bullet is overtaken by the expanding gases!

    ANYWAY, I'm really pretty confident that with a reasonably sealed bore the freestream forces at bullet nose exit (before the tail of the bullet is released by the bore) are negligible compared to those induced by the escaping gas in the muzzle blast when the bore is uncorked. The freeflight/ambient pressures just shouldn't be of the same order of magnitude as those resulting from the pressures in the barrel.

    Sorry for going on... Did I correctly understand what you meant by "barrier"?

    2) by accuracy zones do you mean the useful velocity range that provides accuracy in a given caliber (and given tailoring of powder to cartridge)?

    3) Do you propose this is true for similar designs of cartridges/firearms/bullets? That is, if I took a 50 bmg rifle (instead of a 45 long colt) and shot it with a similar design bullet do you think I'd have significantly different results in terms of improving accuracy with hardness vs velocity than I would in 22 caliber?

    4) I don't understand what you mean... Please rephrase? What is an "accuracy point"?

    5) neat! This may or may not be the same thing, but this sort of result is VERY well known in the pernetrator impact engineering literature. Do the following: calculate the dynamic pressure of your test medium as 1/2 (medium density)*(velocity)^2, for all your test point velocities, and compare the pressures you get with the yield and ultimate stresses (in the same units) for soft lead as you were using. I'll bet your dynamic pressure crossed over the yield and/or ultimate stress for the bullet metal. What happens is you get increasing penetration with increasing velocity up to a point, and then the fluid flow pressures generated by the impacted medium start destroying the penetrator (bullet) which decreases penetration.

    I'm not sure penetration here had anything to do with accuracy or the intactness of the bullet edge? It does sound like you were stripping the rifling, though, which ain't good for shooting groups for a bunch o' different reasons.

    Thanks BA for expanding on your original post.

    Best regards,
    DrB
    1. Yes the sound barrier. We rotate for stabilization. That is to stabilize in air and through other strike materials as well. As I understand center of pressure, it constantly changes in an accelerating and then decelerating object so it passes the CofB twice. Control of it then is irrational and more or less why bullets find comfort zones. But with increased velocity, we constantly encounter increased headwind and the bullets shape and diameter increases these forces enough that we should have taller rifling at the launch.

    2. Yes 45 caliber is 1100 to 1300 for easiest accuracy, increases to 1600-1800 for 30 cal, etc. The governing factor is control of the launch. Assuming .004 tall rifling as an industry standard, that represents the lowest percentage of bullet diameter in 45 caliber and the most in 22. Control of the launch. Not muzzle pressure at the release. Form a ratio for .004 / 22 caliber and work that out for 45 caliber and the control launch should be comparable. Still the 45 is going to catch more wind as it may in fact need to be taller even though muzzle pressure will be less. (base shape minimized especially if bullet length can aid transition)(most don't want heavy bullets in 45 caliber)

    3. Lost me. What I was getting at is that you can do 1100-1300 in 45 caliber with pure lead or WDWW. With a 22 you can get accuracy from about 800 fps with pure up to 3000 with harder lead. Much wider window. Again because of the rifling height relationship to bore diameter. Not the base edge quality.

    4. Relates to the explanation in 2.

    5. The moral of that story was supposed to be that many people don't think about metal failure. It doesn't just occur at some point of the bore. It starts at the muzzle and the very back or base of the bullet upon exit. As it does you create a new base edge to release gas regardless of perfect the last one was. And that slug will react to the gas causing wobble if you don't have a wad or something behind it to contain it. In other words, the name gas check is totally deceptive and confusing. It should be thought of and called a launch check.
    Last edited by Bass Ackward; 07-02-2011 at 06:56 AM.
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    Quote Originally Posted by DrB View Post
    Bret, I didn't think you were fighting the idea. I do not believe anything about ballistics is "simple" except when a large number of things are already pretty close to perfect.

    I'm not trying to propose a grand unified theory of cast bullet accuracy. As I mentioned, I loved Mollys thread, and not only can the theories she's proposed appeal to me, I can go out and try to test them for myself as she has proposed.

    I've been doing a lot of reading on cast boolits and elsewhere lately, and was wondering about the correlation between increasing velocity and increasing hardness (or checking) with accurate loads. The analog of an imperfect crown with an imperfect bullet base got me wondering as to how much base edge quality at departure might explain.

    And no one has to worry about getting me riled. Reality is. I hope someone (or us collectively) has data they are willing to share that would support or discount this particular notion. There is always something new to learn (or unlearn).

    I have no doubt that an absolutely square, perfect base with a perfectly aligned boolit will outshoot a mediocre example given the other variables are consistent. If I follow your thinking, and I have zero background for this, you're saying it would be better to have a base more resistant to abrasion or gas cutting (harder, stronger, tougher, whatever) as the boolit exits the muzzle. This sort of takes me back to the questions people ask about lead alloys "melting", that is- is there time enough for the damage to occur? At even low speeds of say 1300 fps the boolit is only there for a fraction of a fraction of a second. How much time is needed to make a significant amount of damage occur? I have no clue.

    I do think the little variables add up in this game. I like fat boolits and deep rifling, but common sense tells me the chances for a bunch of little "tags" ( a shepherds term) are more likely to be hanging off the base of a fat boolit where the displaced metal is dragged backwards. I have to wonder what the effect of a whole mess of those little guys would be.

    Keep thinking, this is interesting.

  5. #45
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    I don't know Bret. I just examined a bunch of both PB and GC boolit bases and they look the same. No "chads" hanging, just a little depression in the GC center and some ripples from rifling. The PB actually still have flat bases and show less ripple. There is a darkness to the lead from the powder but even the little sprue lump is still there.
    I can't see any boolit base being damaged at the muzzle. A bad crown can tip a boolit from uneven gas venting but it sure will not melt it or deform it. Not enough heat or pressure left.
    But I refuse to shoot dead soft lead and read stories of ported barrels having the ports full of lead. Just maybe the lead is too soft? Silly Putty shoots funny! Maybe Silly Putty will deform at the muzzle.

  6. #46
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    Those "tags" that Bret is talking about is what i was refering to in my post on driving band width.The wider the bottom band on a pb bullet the more it should increase the "tag" size as would a fatter bullet in theory anyway.

    There was another post where someone was talking about increased accuarcy with bullets having multiple lube groves whether used or not. I theorize that the reason was the groves give a place for the excess lead to flow but i dont know.

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    Those little extrusions at the base mean nothing if they are even all around.
    The problem at the muzzle will still start at the "LAUNCH" as Bass loves to say. He is correct.
    Start a boolit out of line with the bore and it is not aligned at the muzzle.

  8. #48
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    Quote Originally Posted by felix View Post
    Time + Money + an obnoxious attitude for an objective = accurate results. Most folks give up too soon for one reason or another, either good or bad. My experience has been not obtaining external ideas to get over an unexpected hurdle. The real excuse for many of these projects has been no profit potential for the results.

    You are correct in using a "photo" apparatus for measuring the rotation. The boolit would have to be painted with something "hot" that can be read with the existing particle measuring devices. Finding an appropriately short half-life paint would be prohibitive???? Who wants a forever contaminated barrel? ... felix
    Well, I suppose you might do it that way too, but I don't know why you'd use a radio taggant. I think you might just paint one side of the bullet white and one black, or dull one side for contrast. Sure it'll come off in the high points and places blasted by bore gases, but I think you could get it to adhere to the nose. We paint supersonic airplanes all the time, and the paint just has to stick for a few milliseconds on a bullet (2 revolution*14 inch/revolution/(1800 feet/sec*12 inch/foot)=1 msec.

    I'm sure its been done before at the arsenals. I've seen continuous test video of mortar and artillery shells in freeflight (THAT struck me as pretty impressive, but they've been doing that for decades). No reason not to do it with a bullet for a couple of feet (heck of a lot easier, just need a fixed camera and a light source sufficiently bright, like a flashtube, and a "chopper" to pulse the light fast enough to freeze motion).

    But anyone can check to see if there was enlarging of grooves on a bullet, and measure speed with a chrony?

  9. #49
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    I agree 44man that start a bullet out off centered,out of ballance,that there is no way for correction. Not knocking Bass but that is not rocket science figuring.Those "tags" or overruns may or may not cause a problem.Like you have said many times you just dont know. There is a good chance you may very well be right in the fact that they do not matter or it could be that we do not really know.I figure those little overruns have been around for a while and are not only expected but accepted as no problem.

    Now me I am a firm beliver in not fixing whats not broken.I also know that we do not always know whats not broken.

    I would think that until someone designs a pb bullet that will not produce those overuns i not sure that we can be proved either way.
    Without a doubt i have been at this a far shorter time than most but since the first bullet that i recovered and saw those little overruns "tags" that sight has stuck in my mind.I have a design in mind that should ellimnate the overun.Thinking of talking to Accurate molds about cutting the mold and give it a try. Only problem is i am not sure what nose profile to use.

    Being a simpleton i'm not sure why i am in this discussion anyway.Between Felix and DrB i have been left far behind.

    Dwight
    Last edited by PacMan; 07-03-2011 at 01:05 AM.

  10. #50
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    Quote Originally Posted by 44man View Post
    I don't know Bret. I just examined a bunch of both PB and GC boolit bases and they look the same. No "chads" hanging, just a little depression in the GC center and some ripples from rifling. The PB actually still have flat bases and show less ripple. There is a darkness to the lead from the powder but even the little sprue lump is still there.
    I can't see any boolit base being damaged at the muzzle. A bad crown can tip a boolit from uneven gas venting but it sure will not melt it or deform it. Not enough heat or pressure left.
    But I refuse to shoot dead soft lead and read stories of ported barrels having the ports full of lead. Just maybe the lead is too soft? Silly Putty shoots funny! Maybe Silly Putty will deform at the muzzle.


    Hey, can you post before/after pictures and particulars for the load? (load recipe, velocity)
    It would also be interesting to see the same with the load pushed to accuracy failure (if that can safely be done with yours).

    To my way of thinking, if these bullets you looked at were from a "good" load, you shouldn't see: rifling width enlargement, gas cutting, or changes to the plain base edge. The first two could cause variation in muzzle velocity and therefore changes in muzzle location at departure, and all of them could cause asymmetric venting at departure.

  11. #51
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    EDIT/:

    So after the original post, below, there seemed to be a fair bit of confusion as to what the heck I was talking about, and what it might mean. My apologies... I'm not trying to write articles for popular consumption here, or for a professional journal, or anything... rather I'm trying to put some thoughts out for discussion amongst whoever may be able and interested enough to parse them, and get some feedback from the community (there's a huge amount of practical experience, here). That said, the intent of the below was bound to be pretty incomprehensible for almost everyone here without some kind of a lead-in. I'm not trying to write posts that are incomprehensible to everyone and ineffective at eliciting any kind of further discussion.

    So here's a little bit of explanation/lead-in to the original post that hopefully makes the purpose of it a little bit clearer.


    What's an order of magnitude calculation and why would you do one? OK, so sometimes as an engineer (or a scientist) you may know the physical laws a problem has to obey, but not know the precise values of the variables necessary to make an accurate calculation. However, while you may not know the exact value of a variable, that doesn't necessarily mean you know nothing about what values it may take. For example, I may not know the length of your car, but I can say with some confidence that since it's a car, it is of the order of five meters in length. In fact, I would be shocked if your car were less than 1 meter (10^0 m) or more than 10 meters (10^1 m). Note that the exponent (0 or 1) is the order of magnitude of the quantity in parentheses. That's why it's called an order of magnitude calculation... because we estimate the variables based on their typical magnitudes and see what we can learn when we plug numbers of these magnitudes into the physical equations governing the system.

    For most any problem (if the estimator has past experience/familiarity with similar problems!) it's possible to go through all the unknown variables and make order of magnitude estimates for their values. If I make such an estimate at say 1 inch, that doesn't typically mean in this type of calculation that I am confident the correct value isn't 1/2 inch or 3 inches... it means I might be surprised if the quantity were .01 inch or 100 inches (if estimating to within two orders of magnitude) or .1 inch or 10 inches (if estimating to within an order of magnitude).

    I also pay attention to how closely I think I am able to estimate the variables, and consider the possible impact of the uncertainties when interpreting the outcome of the calculation. When estimates of a variable are known to a very close magnitude, then that tends to give much more useful results than when variables (particularly important variables) cannot be estimated closely at all.

    Below I made a very rough calculation of how I thought the accuracy of a bullet might be impacted 100 yards down range given a small base edge defect that resulted in asymmetric venting of gases at bullet departure from the crown. I DO NOT believe that this is a precise calculation by any means... it's not even tied down to a specific load, defect geometry, bullet shape, etc... but I do believe the values I've plugged in are ballpark reasonable.


    So what can you really learn from such a calculation? Well, that depends on how much you believe the estimates for the variables, etc. Sometimes, all you learn is that you cannot discount a hypothesis on the basis of such an approximate calculation... Sometimes you learn a hypothesis is totally absurd. Sometimes it becomes obvious that one thing must be much less important than another, just because of the relative magnitudes of the variables involved and the nature of the equations. Sometimes, you learn something about the relative magnitude of a parameter relative to another that sheds light on the behavior of the system.

    I think the below calculation did this for me with regards to the magnitude of lateral velocity component that's necessary to open up group size at 100 yards, as compared to the bullet down range velocity component. The relative magnitude is (1500 fps ~= to 10^3, and .2 fps ~= 10^-1, so we're talking about a difference in magnitude of 4 (a factor of 10,000))! So it only takes a relatively tiny (10^-4) variation in the lateral velocity component as compared to the overall speed of the bullet to result in groups opening up.

    Other than that, I think the calculation shows that small base edge defects (when determined to be present) are not obviously an unlikely cause for bad accuracy... neither does it prove they are definitely the culprit when present though -- you really need more confidence in the estimates for the variables than I believe I can make without further work.

    You should also realise that you should only have as much confidence in the conclusions of this sort of analysis as you have in the ability of the engineer to reasonably estimate the variables. If the engineer can't get the variable estimates in the same state as the ballpark (to use a metaphor), and furthermore they don't recognise the likely magnitude of their error, then you should be very concerned about the reasonableness of any conclusions they draw.


    Bret, that's a start, let me know if that makes it clearer what I was doing, below, or if I made things worse.
    /EDIT

    Ok, so for a while now I've been thinking maybe I should be doing less hand waving on this venting aspect and at least do an order of magnitude calculation (which still involves a lot of hand waving).

    So here's a first cut, back of the envelope...

    At 100 yards a deflection of .5 inches on a bullet travelling 1500 fps requires a lateral velocity of about (.5 inch)/((100 yard*3 ft/yard)/(1500 ft/sec))=2.5 inches per second

    So asymmetric venting at the base only has to impart a lateral velocity of .2 fps, which is encouraging! This number at least is very close to the right answer and not much of a guess.

    Assume
    the bullet weighs ~250 gr (close estimate),
    distance over which the asymmetric venting acts is .01 inch, (could be half as large or three or four times as large?)
    average pressure is 2000psi (SWAG... has to be much less than base pressure at muzzle due to free expansion over the bullet surface)
    action area of pressure is ~ .25 inches (approx. profile area of a 45 cal bullets... This combined with the averaged pressure are likely the most erroneous? Maybe this should be more like a third of profile area for a SWAG).

    In this case, the action time for the pressure is .56 microseconds. The force is 500 pounds. The mass is (250 gr/ (7000 gr/pound)*1 pound/32.2 slugs) =.00111 slugs

    Therefore acceleration is 500 pounds /(.00111 slugs)=550,550 ft/sec^2

    Velocity is 550,550 ft/sec^2*12 inch/ft*.00000056 sec = 3.7 inch/sec

    This corresponds to about +1.5 inches growth in group size at 100 yards.

    This is in the right ballpark, I guess. So I wouldn't consider gas venting wrecking accuracy to be implausible based on this calculation... But neither does the calc prove anything since it is a rough order of magnitude calculation.

    Of course the bore gas is blasting against the base of the bullet for much longer (~300 times as long?) than the venting would occur, and so the magnitude of the average force acting on the bullet could be this much less. Non uniformity of the bullet base could cause asymmetries in the flow that resulted in the .2fps/inch of lateral velocity needed to increase group size, maybe.

    Ok, so who is going to crank up the CFD to really get a reasonable estimate of imparted velocity from a small base defect?
    Last edited by DrB; 07-07-2011 at 03:47 AM.

  12. #52
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    Quote Originally Posted by 44man View Post
    Those little extrusions at the base mean nothing if they are even all around.
    The problem at the muzzle will still start at the "LAUNCH" as Bass loves to say. He is correct.
    Start a boolit out of line with the bore and it is not aligned at the muzzle.
    They might not mean nothing if they break away or erode asymetrically or aynchronously in the gas blast... Is how big they are and how well they stay attached related to hardness?

    Seems to me that anything asymmetric that can induce that ~.2fps/ (inch group size) lateral velocity can explain accuracy degrading.

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    Quote Originally Posted by DrB View Post
    Ok, so for a while now I've been thinking maybe I should be doing less hand waving on this venting aspect and at least do an order of magnitude calculation (which still involves a lot of hand waving).

    So here's a first cut, back of the envelope...

    At 100 yards a deflection of .5 inches on a bullet travelling 1500 fps requires a lateral velocity of about (.5 inch)/((100 yard*3 ft/yard)/(1500 ft/sec))=2.5 inches per second

    So asymmetric venting at the base only has to impart a lateral velocity of .2 fps, which is encouraging! This number at least is very close to the right answer and not much of a guess.

    Assume
    the bullet weighs ~250 gr (close estimate),
    distance over which the asymmetric venting acts is .01 inch, (could be half as large or three or four times as large?)
    average pressure is 2000psi (SWAG... has to be much less than base pressure at muzzle due to free expansion over the bullet surface)
    action area of pressure is ~ .25 inches (approx. profile area of a 45 cal bullets... This combined with the averaged pressure are likely the most erroneous? Maybe this should be more like a third of profile area for a SWAG).

    In this case, the action time for the pressure is .56 microseconds. The force is 500 pounds. The mass is (250 gr/ (7000 gr/pound)*1 pound/32.2 slugs) =.00111 slugs

    Therefore acceleration is 500 pounds /(.00111 slugs)=550,550 ft/sec^2

    Velocity is 550,550 ft/sec^2*12 inch/ft*.00000056 sec = 3.7 inch/sec

    This corresponds to about +1.5 inches growth in group size at 100 yards.

    This is in the right ballpark, I guess. So I wouldn't consider gas venting wrecking accuracy to be implausible based on this calculation... But neither does the calc prove anything since it is a rough order of magnitude calculation.

    Of course the bore gas is blasting against the base of the bullet for much longer (~300 times as long?) than the venting would occur, and so the magnitude of the average force acting on the bullet could be this much less. Non uniformity of the bullet base could cause asymmetries in the flow that resulted in the .2fps/inch of lateral velocity needed to increase group size, maybe.

    Ok, so who is going to crank up the CFD to really get a reasonable estimate of imparted velocity from a small base defect?
    Can you put that in terms a sheep farmer can grasp? I've read it 4 times and still haven't got a clue.

  14. #54
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    Bret, the good Doc is figuring the size of the retro rocket required to compensate for any directional errors during launch. He is finding out compensation is too minuscule to be cost effective. His disclaimer deals with the obscurity of the actual real-time summation of the erroneous vectors (direction and speed) to work with. His discourse is just thinking out loud. ... felix
    Last edited by felix; 07-03-2011 at 09:57 AM.
    felix

  15. #55
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    After reading the entire thread, I had to go back and read the first post. It seems that all of this assumes a perfect crown on the barrel. Its been my observation that an imperfect (or buggered) crown can be directly blamed for decreased accuracy. This would lead credence to the OP that "Asymmetric venting during departure" would lead to inaccuracy down range due to an uneven "last push". I think most of us deal more with the fact of an imperfect crown than an imperfect boolit base. Of course the effects are additive. I almost said cumulative, but that is not necessarily true. The effects might cancel each other out. I know we are talking small units of measure here, but time and distance down range allow those errors to grow.


    You guys are really pushing the edge of my understanding and I enjoy that. I will continue to follow the thread with interest.


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  16. #56
    Boolit Master bbqncigars's Avatar
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    I know I'm going to re-read this thread a few times to fully appreciate it. I wish Handloader would deal with topics such as this.


    Wayne
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  17. #57
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    Talking

    There is nothing like sharing information especially with mathematical and scientific back grounds from our family here at Cast Boolits.

    Sometimes it is hard to put it in a layman's language so they can understand it, yet not lose the technical background of math and science for the information provided

  18. #58
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    I think you can demonstrate when and to what extent gas jetting at departure can destroy accuracy by baselining an accurate barrel/lead bullet combo for accuracy and then change only the crown at the bore. This would be done by introducing a single small nick at the bore-crown edge that will allow early/asymmetric gas venting during bullet/crown departure.
    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!
    Cap'n Morgan

  19. #59
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    Doc, it's even hard for the scientists, engineers, and supportive technicians to communicate between themselves because of some physical phenomenon being contemplated/analyzed/synthesized is beyond historical recognition, i.e., school books, papers, journals, etc. For example, Newtonian math falls apart at the quantum level and requires a new or modified language. Even then, assurance of a resulting product can be questioned and require the use of actuaries. ... felix
    felix

  20. #60
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    Quote Originally Posted by DrB View Post


    Hey, can you post before/after pictures and particulars for the load? (load recipe, velocity)
    It would also be interesting to see the same with the load pushed to accuracy failure (if that can safely be done with yours).

    To my way of thinking, if these bullets you looked at were from a "good" load, you shouldn't see: rifling width enlargement, gas cutting, or changes to the plain base edge. The first two could cause variation in muzzle velocity and therefore changes in muzzle location at departure, and all of them could cause asymmetric venting at departure.
    Base distortion is minimal from rifling extrusion. I must have melted most of my boolits but found a few. Many of these are fully capable of less then 1" at 100 yards from a revolver.
    A perfect crown is important but it seems as if the boolit base only needs to be even at exit. If you destroy the base at launch, bets are off. Same as an off side launch from a chamber out of line with the bore. The base might be good but the boolit is out of balance from being forced sideways through the bore.
    I actually see more indentations on gas checks then PB.
    Last edited by 44man; 08-17-2011 at 09:02 AM.

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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