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Thread: Cast bullet accuracy and trailing edge failure

  1. #81
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    A very cool idea, Cap'n. ... felix
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  2. #82
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    Quote Originally Posted by Cap'n Morgan View Post
    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.
    CM, one good experiment is often worth a pile of calculations, and years of lip flapping. Reality is. That's one of the reasons I think Ackley is still worthy of respect today. He did a number of experiments with firearms to demonstrate the failings of various lines of thinking at the time.

    Yes, I had nearly the same thought in one of the earlier posts, but I thought to do the accuracy test before and after porting (and deburring of the barrel). It may be more repeatable your way, don't know. You've certainly got the opportunity to go back and forth your way. There's the potential for a lot of reaction force on a sleeve if it vents more one way than another, and you might rupture it if it is too thin. I would think though you'd be fine if you made it relatively thick, with a tight id to barrel od tolerance, and pin, bolted or threaded it. And lots of grease between the barrel and sleeve plus a flat or groove to help you drift it back off if it fouls to the barrel.

    I don't know what the holes will do to the bullet. On the one hand I think folks shoot gas operated guns with well fitted boolits with negligible leading. On the other hand I've come across suppresor manufacturers saying they avoid small holes in integrally suppressed guns due to excessive fouling with boolits (large holes supposedly didn't foul so bad).

    If you ever decide to undertake such an experiment, let me know and i'll pitch in (if it would be of help to you) for one of those green mountain gunsmithing special barrels.

    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.
    Last edited by DrB; 07-08-2011 at 09:25 PM.

  3. #83
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    Quote Originally Posted by 44man View Post
    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.
    So the two thoughts I mentioned regarding your pictures were that first of all, it's interesting how well the check conforms to the bullet base. It seems like you can see every bit of the sprue impression through the copper check. Given the pressure with which the check is swaged to the base that isn't all that surprising, though.

    Second, the edge of the gas check appears to have been both stiff enough and supported enough that it doesn't appear to have deformed much at the edge from base pressure, but has kept what looks like a nice even radius. This would be a good thing from a venting at departure standpoint. I'll bet you could file a noticeable notch in the base edge of a cast bullet, seat a gc, shoot, and recover it, and the gc would maintain a pretty good edge in spite of the underlying defect. If you then shot some modified this way with and without gas checks i'll bet it would show the majority of impact to accuracy from the removed material was from gas flow, not unbalancing of the bullet (the gas check would shield the defect from the flow).

    It also makes me wonder if a short ring gas check (with no base) would stay on the bullet without the huge base gas pressure force which tends to hold on a standard gas check. I had wondered earlier about testing the base edge venting effect of a short copper ring, but I wonder now if it would be able to stay on or if it would leak between the check and bullet base.
    Last edited by DrB; 07-08-2011 at 01:52 PM.

  4. #84
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    Naked based condoms will stick to the bore during some instances of firing, allowing the lead to squirt through as a naked condom. Happens when 30-06 ammo is loaded full tilt with a 150-152 grainer military bullet sawed off about 1/8 inch and then hollow pointed. Might not happen if a drilled out gas check was severely crimped on. ... felix
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    Quote Originally Posted by felix View Post
    Naked based condoms will stick to the bore during some instances of firing, allowing the lead to squirt through as a naked condom. Happens when 30-06 ammo is loaded full tilt with a 150-152 grainer military bullet sawed off about 1/8 inch and then hollow pointed. Might not happen if a drilled out gas check was severely crimped on. ... felix
    Darn, I just printed a bunch and a thunderstorm shut the juice off for a second.
    It could be a disaster if the ring is pulled off and sticks in the bore. I still think the edge of a gas check is pulled back from friction and gives the appearance of center pressure. That would pull a ring of copper.
    The sprue mark shown means the copper just formed around it and did not flatten it.
    Think of the thin skinned bullets shot out of the .357 max and the .454 that the cores were driven out of leaving the jackets in the bore. That is why they made magnum bullets. Copper has more friction then lead.

  6. #86
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    Quote Originally Posted by 44man View Post
    I still think the edge of a gas check is pulled back from friction and gives the appearance of center pressure.
    Yes, that too. The hollow base appears with no gas check as well. I first saw it first shooting 41 maggies quite hard. ... felix
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  7. #87
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    I'd REALLY like to see some checked and pb examples of base cupping. Reality is, and I've never held one of these in my hand and seen what it looks like.

    Felix (or anyone), could you please recommend a bullet hardness/weight/powder load in 357 that you have seen produce base cupping, if you have for that cartridge? I'm better equipped for experimenting in 357 than I am 44, and I have no 45 lc in the stable right now.

    Best regards,
    DrB



    EDIT 7/12/2011: No one has a load they can suggest that they think will produce cupped base bullets with and without gaschecks? Or just not in .357 Magnum?
    Last edited by DrB; 07-12-2011 at 05:19 PM.

  8. #88
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    Bullet gas check angle estimation...

    In a previous post in this thread I derived an estimate for the lateral shove given by a non-flat gas check, as a function of the angle of the gas check (or pb) from normal. The problem with relating this to the gas checked bullets sitting on yer bench is that it's not so easy to measure angle directly... it's much easier for most of us to measure length.

    I was having some difficulty with some poorly fitting gas checks last night, so I thought I'd do a quick calculation to estimate the angle a gas check was off flat based on a bullet length measurement from the shortest to the longest lengths of the bullet from nose to tail (easiest to measure with a wide meplat or cyclindrical bullet, probably).

    Symbols:
    Bo = angle off normal in degrees (zero for a perfectly normal base)
    Brad = " " in radians
    C = Caliber of the bullet
    l2 = longest length, nose-to-base, of the bullet
    l1 = shortest length, nose-to-base, of the bullet
    pi = the ratio of a circles circumference to its diameter

    (l2-l1) = C*sin(Brad) ~= C*Brad = C* Bo * pi/180

    So, therefore:
    Bo = (l2-l1)*180/(C*pi)

    Sample calculation:
    Let's say we have a .225 diameter bullet, and prior to seating the gas check the nose to length measurement was uniform to within a thousandths, but after seating the gas check there was .005" difference.

    This would give us:

    Bo = (.475-.470)*180/(.225*3.14159) = 1.27 degrees


    Best regards,
    DrB
    Last edited by DrB; 07-12-2011 at 06:03 PM.

  9. #89
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    DrB,

    Kudos to you for applying the knowledge you have and sharing it with us! If you wrote articles for one of the gun magazines with the in-depth writing you have done in this thread, I would subscribe (dropped my subscriptions 20 years ago). This is a very intriguing topic to say the least even though I forgot most of my high school Physics and Geometry from 35 years ago. I can grasp the concept but it is still mind boggling for me and I would also assume for others too as some of the subject matter is very “heavy”.

    My question is not directly related to the trailing edge and I don’t want to hijack this thread, but I would like to hear your and others opinions with regards 44Man’s reference to both BPCR shooters chambering rounds in the exact same position and molds where the sprue plate does not cut the boolit base perpendicular to centerline. As much as I would like to test this my self (not equipped at this time), would a batch cartridges with boolits of identical exaggerated out-of-square bases where the base was oriented the same on firing for each cartridge, hit the same POI or would the group just open up? Maybe this was already mentioned in this or another thread or someone elsewhere has already tested this and I just haven’t found reference to it yet? My ‘imagination’ says that a person could test this with a little time and effort if there were some tools available. The test, if it already has not been done, could be accomplished via a “control load” and a load where a boolit’s base angle has been purposely modified.

    Instead of trimming the case mouth with a case trimming die and a file, make (with a lathe) a couple devises similar to them except the dies purpose would be for trimming the base of a boolit. The boolit would be inserted nose first in to an adjustable depth hole (tap threads in bottom of the die for bolt) where the boolit can just be inserted via very light finger pressure. One die’s top would have the “exaggerated” angle (grind or cut the angle) and the other die’s top would be perpendicular to the hole the boolit is inserted into. For the “variable” batch of boolits, lightly file each base to the angle (how much angle?) of the angled top die (leaving a little bit of the base not filed for orientation use). Then to make the other batch of “control” boolits the same weight as the angled base boolits, weight a couple of the angled base boolits for an average weight and then trim off the appropriate weight for the quasi-control boolits so that the only “variable” to the loaded cartridges are the boolit bases. Then load and shoot for groups using the BPCR orientation technique to see what comes of it.

    With what I’ve garnered so far in this thread, my assumption (everybody knows what assuming does…) is that that even though the batch of cartridges with the angled boolit bases are oriented the same upon firing, the group will open up instead of shift POI. Then again, there is the article Cap’n Morgan referenced about the "fine tune regulated" Sabatti rifles where the rifling in the crown was removed to ‘steer’ the bullets. Angling the base of a boolit is kinda-sorta-similar to a "fine tune regulated” crown?

  10. #90
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    Doc, use WW or softer (air dried) with nothing slower than 231. I use 700X because my lot is even faster than my lot of BE. The wholesaler sold it to me cheap because he knew the lot was faster than spec somehow, someway. Use FULL load, 35K CUP or thereabouts. If the boolit does not fit correctly, expect tons of leading. Have a film of oil in the cylinder and barrel before shooting. Have some steel or copper wool handy. Steel wool is soft enough because it has little or no carbon content compared to most gun metal. Make sure bases are flat before firing. Finger rub test is good enough, before and after firing. ... felix
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    Quote Originally Posted by DrB View Post

    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...
    So, being rather the dullard mathematically, did you add alignment issues into the mix? To my mind there is very little difference in the end result between a poorly aligned boolit and an unsquare base or crown. In fact, I suppose it's possible they'd occasionally cancel each other out. Variables add up, that I don't think we can fight.

    Thought's?

  12. #92
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    That, Bret, causes more havoc than anything. Can't tell what's going on. Once detected, it time to relegate that ammo to charging beer cans. Make the ammo mo'betta' next time. Been there, done that. ... felix
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    Thanks Uncle Felix, at least I don't feel like a complete 'tard for asking!

  14. #94
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    Quote Originally Posted by jandbn View Post
    Kudos to you for applying the knowledge you have and sharing it with us! If you wrote articles for one of the gun magazines with the in-depth writing you have done in this thread, I would subscribe (dropped my subscriptions 20 years ago). This is a very intriguing topic to say the least even though I forgot most of my high school Physics and Geometry from 35 years ago. I can grasp the concept but it is still mind boggling for me and I would also assume for others too as some of the subject matter is very “heavy”.
    Thanks, jandbn. It's kind of you to say that...

    OTOH, the great thing about this site is the wealth of observations that all the participants have accumulated over the years. We can all benefit by sharing the results of our occasional experiment/range trial. The forum of today actually has a great deal in common with the early scientific journals (where letters amongst interested individuals were exchanged and published). On the other hand, the latest engineering computational tools are out of reach of the typical reloader/enthusiast. It'd be nice if we could get some basic results updated... there are a lot of informative articles that could be published.

    Quote Originally Posted by jandbn View Post
    My question is not directly related to the trailing edge and I don’t want to hijack this thread, but I would like to hear your and others opinions with regards 44Man’s reference to both BPCR shooters chambering rounds in the exact same position and molds where the sprue plate does not cut the boolit base perpendicular to centerline. As much as I would like to test this my self (not equipped at this time), would a batch cartridges with boolits of identical exaggerated out-of-square bases where the base was oriented the same on firing for each cartridge, hit the same POI or would the group just open up? Maybe this was already mentioned in this or another thread or someone elsewhere has already tested this and I just haven’t found reference to it yet? My ‘imagination’ says that a person could test this with a little time and effort if there were some tools available. The test, if it already has not been done, could be accomplished via a “control load” and a load where a boolit’s base angle has been purposely modified.
    As long as the bullets maintain their clocking on exit from the muzzle (no skid, or consistent skid of the bullet in the rifling), then yes, you'd expect the group to be deflected in the direction the base was pointed towards upon exit from the muzzle. With an indexed ramrod and tight fitting patch, I'd suppose you should even be able to predict what direction that should be by checking rotation from chamber to crown.

    Now, you would have to remember that group size is due to a number of different sources and not just the lateral shove a bullet may get upon departure from the crown... so if you had a sufficient enough angle on the base you would expect the group to be steered. It probably would be most observable for a given group of off-flat projectiles if you ran some clocked one way and another batch grouped 180 degrees opposite, shooting into two different targets.

    Quote Originally Posted by jandbn View Post
    Instead of trimming the case mouth with a case trimming die and a file, make (with a lathe) a couple devises similar to them except the dies purpose would be for trimming the base of a boolit. The boolit would be inserted nose first in to an adjustable depth hole (tap threads in bottom of the die for bolt) where the boolit can just be inserted via very light finger pressure. One die’s top would have the “exaggerated” angle (grind or cut the angle) and the other die’s top would be perpendicular to the hole the boolit is inserted into. For the “variable” batch of boolits, lightly file each base to the angle (how much angle?) of the angled top die (leaving a little bit of the base not filed for orientation use). Then to make the other batch of “control” boolits the same weight as the angled base boolits, weight a couple of the angled base boolits for an average weight and then trim off the appropriate weight for the quasi-control boolits so that the only “variable” to the loaded cartridges are the boolit bases. Then load and shoot for groups using the BPCR orientation technique to see what comes of it.
    Another way to do this would be to take a naturally varying population (collection) of bullets and characterize their out-of-flatness, clock each one to a known orientation, and shoot each one into it's own target. You'd want to do this at relatively close range to reduce the impact of wind, and you'd want to randomize or distribute the unflatness in the order of shots so as to average out the effect of barrel heating, fouling, etc. In this study it probably would also make sense to clock half of the bullets 180 degrees in opposition to the rest to maximize the round-to-round Point of Impact (POI) variability due to base non-flatness. You then would measure the POI on each target for each bullet, and using the POI with the measured base angle you could do a regression analysis (least squares fit) and see how good a correlation there was.

    Quote Originally Posted by jandbn View Post
    With what I’ve garnered so far in this thread, my assumption (everybody knows what assuming does…) is that that even though the batch of cartridges with the angled boolit bases are oriented the same upon firing, the group will open up instead of shift POI. Then again, there is the article Cap’n Morgan referenced about the "fine tune regulated" Sabatti rifles where the rifling in the crown was removed to ‘steer’ the bullets. Angling the base of a boolit is kinda-sorta-similar to a "fine tune regulated” crown?
    I would not expect the group to necessarily open up if the base were angled the same each time. I think that usually this sort of effect is not controlled for by most (we don't mark and "clock" our ammo) and thus the angle of clocking of the cartridge in the chamber is randomized, and hence the effect on target is randomized such that the group opens up instead of shifting.

    RE filling of a crown vs. bullet base: Yes... in that both should result in a lateral shove to the bullet. I would speculate that if you had an angled based boolit in a a load that didn't skid them down the bore, that you should be able to "regulate" a rifle by controlling the clocking of the bullet base at muzzle departure (by clocking the bullet on insertion to the chamber).


    Actually, I was thinking of doing the same experiment (sort of) as the one you've suggested, but with some poorly fitting gas checks on some 22 caliber boolits. I suspected that they were sitting to varying degrees of angle as the gas check was too narrow to fully accept the boolit shank, and marks on the bullet seemed to indicate they were not seating concentrically. So I had high plans for measuring a bunch of them, marking them individually, and then clocking and labelling each of the loaded rounds. Before starting I measured a few of the unchecked boolits and round that while the bases appeared to be nicely flat, the mold halves were apparently about .002-.003" offset. Gas-checked brethren came in with about the same variation in OAL of the boolit, so as near as I can tell the checks are ending up flat.

    So for me, it looks like that experiment will have to wait.

    Something I'm curious about that maybe one of our machinists here could help with is better ideas for measuring the angle of a boolit base. Absent making a mold, slicing it, and putting it on a comparator, or some pretty expensive CNC digital measuring equipment I don't know how exactly you'd go about doing it (unless the boolit started out consistent in nose flatness/OAL and you could measure the base to nose variation in OAL and get an angle thereby).

    Best regards,
    DrB
    Last edited by DrB; 07-15-2011 at 03:39 PM.

  15. #95
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    Quote Originally Posted by felix View Post
    Doc, use WW or softer (air dried) with nothing slower than 231. I use 700X because my lot is even faster than my lot of BE. The wholesaler sold it to me cheap because he knew the lot was faster than spec somehow, someway. Use FULL load, 35K CUP or thereabouts. If the boolit does not fit correctly, expect tons of leading. Have a film of oil in the cylinder and barrel before shooting. Have some steel or copper wool handy. Steel wool is soft enough because it has little or no carbon content compared to most gun metal. Make sure bases are flat before firing. Finger rub test is good enough, before and after firing. ... felix
    Thanks, felix. Any suggested weights for boolits (would you expect this to be more noticeable with a light for caliber bullet where in bore velocity/acceleration was higher)?

  16. #96
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    Quote Originally Posted by Bret4207 View Post
    So, being rather the dullard mathematically, did you add alignment issues into the mix? To my mind there is very little difference in the end result between a poorly aligned boolit and an unsquare base or crown. In fact, I suppose it's possible they'd occasionally cancel each other out. Variables add up, that I don't think we can fight.

    Thought's?
    No, Bret -- to my way of thinking, all that matters is the geometry of the base at departure (as far as the lateral shove a bullet gets from the escaping bore gases).

    If the base is off flat because of how the boolit entered the forcing cone/rifling, that should cause just as much of a shove per degree of off flatness as a bullet base that starts off flat (assuming that the base of a bullet askew from a forcing cone entry is flat and not some other geometry -- seems reasonable enough to me as a first cut).

    EDIT/: Now, one thing I'm not sure of is what happens with a gas-check that's not flat, initially. There's some serious pressure that will tend to flatten it against a bullets base, but I don't know if the thicker material of the edge of the check will fully level to the bullet base, and if it will do so all the time/uniformly. Folks have talked about on this forum before that bullets with initially unflat checks tend to be poorly accurate, and that makes sense to me. But I do wonder how much the edge gets ironed down vs. the flat of the check...
    Last edited by DrB; 07-15-2011 at 11:11 PM.

  17. #97
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    Heavier boolits work mo'betta', Doc, for the experiments you are contemplating. ... felix
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    Thanks, Felix.

  19. #99
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    I've had plenty examples of cupped bases (I use a 'test tube' in my shed). I put it down to extrusion of the boolit as it enters the bore after being 'swaged' down through the tapered throat and leade. The cupping would be caused by the pressure exerted on the base preventing even rearward metal displacement. I take the view that if there is lead displacement causing cupping then there is a chance of that cupping being uneven. For that reason I make my boolits with a small rebate.

    I have a suspician that a damaged or uneven boolit base is also going to have an effect boolit flight as in causing some degree of yaw.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

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  20. #100
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    Quote Originally Posted by DrB View Post
    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. <snip>
    Ok, so I was looking at one of 303Guy's pictures he posted last night of recovered patched and unpatched loads showing base cupping, and had an idea on the subject of base cupping. Other than cream of wheat filler in the cupped bullet base, as I understand it, these loads were identical paper patched bullets loads. In the first picture on the left, the bullet on the left shows cupping with a roughened peened base, the bullet on the right does not. The picture on the right, below shows a closer/better contrast picture of the cupped base bullet from the first picture.




    Felix, et al, I stand by what I said about the mean free path in the gas being tiny, and unlikely to effect base cupping due to pressure... but what I wasn't considering is that the flow at the base of the bullet is generally NOT JUST A GAS. Most loads are not equivalent to a "pre-burned propellant" gun (loads like light bullseye behind a very heavy bullet probably come closest to a PBP gun). That is, the flow in the barrel is a mixed phase flow comprised of both gas and solid particles. In 303Guy's example, much of the solid component is comprised of cream of wheat. In other examples, it could be comprised of burning powder particles.

    I suspect that what these larger particles effectively do, when high in concentration in the flow, is decrease the effective mean random velocity of the flow and increase the mean free path. I would expect there to be a ring vortex of particles following the base of the bullet, with the particles travelling fastest up the center of the bore, impinging on the bullet base, travelling outward towards the rifling, and then slowing down and travelling backwards relative to the base of the bullet, then turning back inward under the influence of the burning gases expanding down the bore and the centerline vacuum created by the vortex.

    So the idea is that because of the influence of the LARGE particles in the flow of gas, and the boundary condition at the bore/bullet trailing edge (which is travelling quickly backwards relative to the bullet fixed frame of reference), the effective pressure on the edge of the bullet is substantially reduced relative to that at the centerline of the bullet. The effect is not due to "gas pressure" so much as the reaction pressure on the bullet base of the solid component of the mixed phase flow. Gas pressure is reduced at this boundary, but it is because of the effect of entrainment of the gas by the larger particles, which are in turn greatly retarded by the barrel/bore boundary condition.

    In 303Guy's picture, I suspect that the effect of the cream of wheat was to effectively peen the base of the bullet out to the edge, where the peening pressure was least due to the vortex turning and reversing towards the breech along the bore, and the bullet metal/paper peened together at the trailing edge.

    If this bullet base cupping is due to the mixed phase flow (and not bullet base gas pressure) it should be more observable in cream of wheat type loads, and high case capacity loads of slower burning powders. It should be not be observed in loads using very high combustion rate powders, irrespective of peak or high average pressures. Examination of the bullet base, particularly with softer bullets, may also indicate a correlation between the roughness of the bullet base and cupping of the base due to impingement of the solid phase of the flow (this doesn't necessarily have to be the case as it depends on the relative hardness of the solid phase and the bullet base, and the velocity of impingement).

    This also gets me to wondering about the folks who have reported shooting unlubed bullets with a cream of wheat filler. If the above ring vortex theory is true, I suspect that you might be able to do the same thing with a slow burning charge of powder. Even though the powder particles were burning (and thus base temperature/gases were just as hot), the influence of the mixed-phase flow vortex at the base of the bullet would reduce the gas pressure and leakage at the bullet/bore junction and reduce gas cutting. You could experiment with this by either using a large load of slower burning powder, or a duplex load of relatively slow burning over faster powder. In the latter case the idea would be that as long as the slow burning powder were ignited and burning, the gas temperature at the base of the bullet should be close to the adiabatic flame temperature of the propellant, demonstrating that the reduction in leading was due to the reduction in pressure at the bore/base edge and not related to 1) any reduction in gas temperature by the "filler", or 2) any mechanical plugging of leak paths by the solid particles.

    Best regards,
    DrB
    Last edited by DrB; 08-02-2011 at 04:23 PM.

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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