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Thread: Velocity threshold for cast rifle boolits.

  1. #81
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    Heck, You fellas need someplace to shoot, I gots THOUSANDS and thousands of acres of BLM land just minutes from my house. I have Wall tents, and there's plenty of room for campers and such. We REALLY need another Wyoming get together. How about next summer? We start planning now, and by next June or July, this could happen. I would love to see a bunch of you guys getting together and putting these theories to the test on steel, paper, ballistic gelatin, and various other targets..
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  2. #82
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    Tim, the precision only makes things easier. Within reason, most any rifle can be made to shoot IF, and that's a pretty big IF, one knows what they're doing at the loading bench. Slowing the twist rate is a way of getting around the ill effects of "standard" loading techniques, casting techniques, alloys, and tooling, not necessarily poor riflesmithing. You'd be astonished at what can be done with "janky" rifles.

    Gear
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  3. #83
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    Mike

    My bad, received a PM suggesting I misread your posts. Sure enough I put the wrong slant on them. ....my apologies. Looking forward to getting into your neck of the woods sometime and would enjoy shooting with you. No need of the couch, I've a decent 5th whl that follows me.

    Larry Gibson

  4. #84
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    Dan

    That sound s like a plan to me. I plan on being in northern Wyoming next June for PD shooting. When hace you got in mind?

    Larry Gibson

  5. #85
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    goodsteel

    ..... I think that the reason the RPM theory exists, is because all the standard rifles out there, with a very few exceptions, damage a soft lead boolit before it gets out of the barrel. ..........At the pressures and speeds we are shooting, lead begins to act like silly putty, and any small misalignment can really have a detrimental effect on it as it is squirted out of the barrel. ie, what get's to the target is not the nice concentric boolit that dropped on the bench.

    You are for the most part quite correct. The problem is one of casting a balanced (concentric certainly is a large part of that) bullet, keeping balanced through the sizing/seating GC, keeping the bullet balanced during loading and keeping it balances during shooting i.e. shooting or the internal phase of ballistics. If the bullet is not balanced on muzzle exit the physics of the external ballistic phase take over. At some point of RPM the centrifugal force makes the bullet depart from the line of flight on a tangent or it makes the bullet (the most often occurrence to happen first) begin a helical arc around the line of flight which slowly increases non linearly in group dispersion as the range increases. That is the reason why most find the accuracy of their cast bullet loads decrease at some point.

    That point is not a limit as some pundits keep wanting to believe. I have pointed out many times that the RPM threshold can be pushed upwards by controlling the amount of unbalancing that occurs to the bullet during acceleration (shooting). We can control that by the design of the cast bullet, the alloy used, the fit to leade, bore and groove diameter and by using slower burning powders that offer consistent ignition and burning. Basically the more balanced we can keep the bullet during casting, loading and acceleration the higher we can push the RPM threshold upwards.

    BTW, several of the anti RPM threshold pundits lay claim to marvelous accuracy at high velocity (well above the RPM threshold) using the very factory and milsurp rifles you say it can't be done with???? Sure would like to see how they do it...........

    Larry Gibson

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    Larry

    No problem.....inducing inflection and intent is just about friggin impossible through a keyboard......most of us with strong opinions one way or the other have simply "read/typed stuff wrong" before......besides it is not like it is the first time we have gently argued over simple terminology of the same meaning. You say RPM threshold....I say "the point at which the challenge begins:.......no biggie Sometimes I enjoy getting a wonky misbehaving rifle to fall in line and other times it sure is nice to grab an easy gun......depends on J.O.B/time/mood for me.

    Mike

  7. #87
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    Larry

    Did you play with that alloy Edd sent you yet?

  8. #88
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    Quote Originally Posted by Larry Gibson View Post
    goodsteel

    ..... I think that the reason the RPM theory exists, is because all the standard rifles out there, with a very few exceptions, damage a soft lead boolit before it gets out of the barrel. ..........At the pressures and speeds we are shooting, lead begins to act like silly putty, and any small misalignment can really have a detrimental effect on it as it is squirted out of the barrel. ie, what get's to the target is not the nice concentric boolit that dropped on the bench.

    You are for the most part quite correct. The problem is one of casting a balanced (concentric certainly is a large part of that) bullet, keeping balanced through the sizing/seating GC, keeping the bullet balanced during loading and keeping it balances during shooting i.e. shooting or the internal phase of ballistics. If the bullet is not balanced on muzzle exit the physics of the external ballistic phase take over. At some point of RPM the centrifugal force makes the bullet depart from the line of flight on a tangent or it makes the bullet (the most often occurrence to happen first) begin a helical arc around the line of flight which slowly increases non linearly in group dispersion as the range increases. That is the reason why most find the accuracy of their cast bullet loads decrease at some point.

    That point is not a limit as some pundits keep wanting to believe. I have pointed out many times that the RPM threshold can be pushed upwards by controlling the amount of unbalancing that occurs to the bullet during acceleration (shooting). We can control that by the design of the cast bullet, the alloy used, the fit to leade, bore and groove diameter and by using slower burning powders that offer consistent ignition and burning. Basically the more balanced we can keep the bullet during casting, loading and acceleration the higher we can push the RPM threshold upwards.

    BTW, several of the anti RPM threshold pundits lay claim to marvelous accuracy at high velocity (well above the RPM threshold) using the very factory and milsurp rifles you say it can't be done with???? Sure would like to see how they do it...........

    Larry Gibson
    That pretty much sums up my understanding of it. The thing is, if RPM threshold is just a measure of how jacked up the rifle/loads/boolits are, it seems like it would be able to be pushed quite far up the scale if everything is correct, or tricked into being correct. So the "threshold" is more a measure of the shooter rather than the components right? Or his level of advancement in the sport?
    I mean, for myself, there was a time when I didn't think cast lead was any good for anything over 1100fps. Now, I know how to load some better ammo and my personal "threshhold" is up there around the 2400fps mark. Course, you give me a really janky rifle, and I'll only get it to 1700fps before all the problems kick my groups out to copenhagen can size. So, the threshold is kind of a personal, case by case, measure of the shooter, the rifle he is shooting at the time, and how well he knows how to tweak it, right?
    Tim Malcolm
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  9. #89
    Boolit Master leftiye's Avatar
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    Then again, maybe it's not the rotation, but the boolit hitting the steeper pitches of rifling upon firing. We've been all over this stuff for years. I'd bet it occurs at the leade, Larry thinks it happens during flight. Most of it is looking at the same things and seeing different animals. Deformation as acted upon by rpms (or just deformation?).
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  10. #90
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    Quote Originally Posted by leftiye View Post
    Then again, maybe it's not the rotation, but the boolit hitting the steeper pitches of rifling upon firing. We've been all over this stuff for years. I'd bet it occurs at the leade, Larry thinks it happens during flight. Most of it is looking at the same things and seeing different animals. Deformation as acted upon by rpms (or just deformation?).
    This is a possibility, certainly. I don't recall reading here of any work done with gain twists, which aren't exactly common. I wonder what effects such a rifling form would have on castings.
    I don't paint bullets. I like Black Rifle Coffee. Sacred cows are always fair game. California is to the United States what Syria is to Russia and North Korea is to China/South Korea/Japan--a Hermit Kingdom detached from the real world and led by delusional maniacs, an economic and social basket case sustained by "foreign" aid so as to not lose military bases.

  11. #91
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    Quote Originally Posted by Larry Gibson View Post
    Dan

    That sound s like a plan to me. I plan on being in northern Wyoming next June for PD shooting. When hace you got in mind?

    Larry Gibson
    Just a big ol' shootin', lie tellin, testing different loads and cartridges, bonfire having, get together. Haul out a bunch of steel targets, and some paper ones. Maybe make some ballistic gelatin. Put some of these theories to the test, while having fun and enjoying how freaking terrific cast boolit shooting in Wyoming can be. The range limit where we shoot is dictated by how far you want to ride on the fourwheeler to check targets...
    I'll be a nice to you as you'll let me be, or as mean as you make me be.

    Polite society started dying the day it was no longer necessary for rude men to physically defend themselves from the consquences of their actions or words.

  12. #92
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    Larry,

    First, I want to mention that the "RPM threshold" is not the same as some would call a RPM limit. The swapping of terms means 2 different things in my mind. Secondly, you and I have discussed the theory and have agreed that your work on this idea was with a three part alloy which causes the theory to require some modification once an individual adds another metal to the mix, i.e. copper. The hardness of an alloy is not the same as the toughness of an alloy which also seems to be often swapped as meaning the same thing. What we Michigan hillbillies have theorized, is that the addition of copper to the mix changes the bullets ability to resist permanent deformation during the firing part of the cycle. We have exceeded the RPM threshold often, but, for the record, with a slightly different alloy that contains copper. We also theorize that copper has a point at which it gives diminishing returns as an additive, but have yet to prove where exactly the trade-off in toughness vs. malleability becomes significantly worthwhile. We in general have worked more toward the end results on game vs, target accuracy, which have 2 vastly different goals. Group size is great, but of little use if the terminal performance is lacking and vice versa.

    Added to this we have found (by accident) that muzzle pressure also significantly affects the accuracy of a cartridges performance. So, IMHO, RPMs, pressure, alloy strength, powder burn rate, and muzzle pressure are ALL part of the whole scheme of things. Added to that one has lube efficiency for their own area and climatic conditions, which is in itself a point of contention.

    Edd

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  13. #93
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    Quote Originally Posted by leftiye View Post
    Then again, maybe it's not the rotation, but the boolit hitting the steeper pitches of rifling upon firing. We've been all over this stuff for years. I'd bet it occurs at the leade, Larry thinks it happens during flight. Most of it is looking at the same things and seeing different animals. Deformation as acted upon by rpms (or just deformation?).
    The fact that the more gently and "slowly" we can introduce a boolit to the rifling yields increased accurate velocity tells us you may be correct, at least in part. As alloy "toughness" (as Edd described) also increases the accurate velocity, we can assume from the same effect as gentle introduction, and also from ability to hold center in the bore and not deform under stress as it approaches the muzzle at very high speed. The known effects of a paper jacket supports both of these concepts since the jacket strengthens the surface of the boolit that engages the edge of the lands and absorbs engraving force by buffering the permanent engrave marks on the boolit. Tough, resilient boolits shoot better at HV, that is obvious from the performance levels of copper-jacketed bullets, which can even tolerate a terribly crooked start into the rifling and achieve fairly decent accuracy at extremely high velocity. Surface toughness helps them self-align rather than permanently deform.

    Four ways RPM gets you at some point: The boolit either starts out bad, is damaged during loading, gets screwed up in the process of being fired through the gun (most common, probably), or exits the crown with some yaw due to any of the previous factors. The result? If the boolit is deformed in a non-concentric way, it will corkscrew downrange in a non-linear fashion as Larry suggests. If the boolit yaws excessively when exiting the crown, it cannot fully recover direction when it recovers its balance and departs the muzzle on a tangent. Decreasing muzzle pressure and ensuring base squareness, together with mastering the biggest bugaboo of all (HARMONIC MOTION), help ensure a predictable flight path as the boolit exits the crown.

    Lots of things to consider with any velocity/accuracy/RPM predictions, but boolit imbalance does rear it's ugly head very predictably. Balance is tough to master at high velocity, especially with fast twists, all aspects of the system interact on each other and compound the difficulty of the task of getting a true boolit out of the muzzle. Even when those are mastered and the boolit is launched without accuracy-damaging deformation, it is difficult to tell because harmonics are playing hoc with the works. But tune that part out and you have it licked. Your targets will tell you what part of the game needs to be played and when if you pay attention.

    Limiting rotational velocity is not a cure-all to the HV accuracy issue, but it helps. It makes it easier to get pretty decent accuracy with very little educated fine-tuning of the system, but will not get the notorious "1/2" groups" that some achieve even at exceptionally high RPM.

    Gear
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  14. #94
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    Quote Originally Posted by DanWalker View Post
    Just a big ol' shootin', lie tellin, testing different loads and cartridges, bonfire having, get together. Haul out a bunch of steel targets, and some paper ones. Maybe make some ballistic gelatin. Put some of these theories to the test, while having fun and enjoying how freaking terrific cast boolit shooting in Wyoming can be. The range limit where we shoot is dictated by how far you want to ride on the fourwheeler to check targets...
    jeez Dan,
    bout all we'd be testing in casper this week is wind drift.
    you must not be home.
    it's all an educated guess,,,, till the trigger is pulled.

    this opinion brought to you by mister low-tech solution..

  15. #95
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    Quote Originally Posted by runfiverun View Post
    jeez Dan,
    bout all we'd be testing in casper this week is wind drift.
    you must not be home.
    HA! Been home since the 20th! Watched a few tumbleweeds go supersonic as they crossed my place this morning. Nothing like a September snowstorm, followed by a few days of 40mph winds, to chase the sissy's outta my beloved state. Here's hoping it drops to -40 around Christmas and don't warm up until May!
    I'll be a nice to you as you'll let me be, or as mean as you make me be.

    Polite society started dying the day it was no longer necessary for rude men to physically defend themselves from the consquences of their actions or words.

  16. #96
    Boolit Master freebullet's Avatar
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    Dan congrats on the buck. I hope to be testing that boolit soon also. To further throw a monkey wrench in everything that's been said here already, there are guys shootin powder coated cast rifle boolits- jbull fast.


    Ps. You can keep -40 up there please.

  17. #97
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    I WISH California had -40* temps from Christmas until May. Such conditions would be much less attractive to the welfare queens and their baby-daddy consorts that infest this place.
    I don't paint bullets. I like Black Rifle Coffee. Sacred cows are always fair game. California is to the United States what Syria is to Russia and North Korea is to China/South Korea/Japan--a Hermit Kingdom detached from the real world and led by delusional maniacs, an economic and social basket case sustained by "foreign" aid so as to not lose military bases.

  18. #98
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    As long as the tourists leave, the elk bugel, and the deer get frisky I'll take any "bad" weather any day over Europeans in short shorts and bad attitudes.

    Quote Originally Posted by DanWalker View Post
    HA! Been home since the 20th! Watched a few tumbleweeds go supersonic as they crossed my place this morning. Nothing like a September snowstorm, followed by a few days of 40mph winds, to chase the sissy's outta my beloved state. Here's hoping it drops to -40 around Christmas and don't warm up until May!

  19. #99
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    Quote Originally Posted by 9.3X62AL View Post
    I WISH California had -40* temps from Christmas until May. Such conditions would be much less attractive to the welfare queens and their baby-daddy consorts that infest this place.
    Heck pard, come on out! You can park your camper on my place till you get your spot picked and your cabin built! Worst you'll have to worry about here is stray cows in the winter and rattlesnakes in the summer.
    I'll be a nice to you as you'll let me be, or as mean as you make me be.

    Polite society started dying the day it was no longer necessary for rude men to physically defend themselves from the consquences of their actions or words.

  20. #100
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    Me thinks most all are beginning to understand the RPM threshold and what is really occurring. Even Leftiye is on the right track as what occurs during internal ballistics (acceleration) to upset the cast bullets balance (no matter how carefully we cast and loaded it to maintain balance and concentricity) is exactly what the centrifugal force acts more forcefully on as the RPM (velocity with a given twist) is increased. Gear and Goodsteel are all over it because it's what we do to keep the bullet balanced as we increase the RPM/velocity in a given twist barrel that determines where the RPM threshold will be. Using a slower twist, if one has the opportunity, certainly is a benefit as it keeps the RPM lower and the adverse effect of the centrifugal force lower at higher velocity.

    357maximum

    Yes, I briefly flirted with Edd's alloy before I began the move from Washington to Arizona a year and a half ago (where does the time go.....). I met with almost immediate success boosting the 311466 from the 27.5" Palma barrel at 2600 fps/40,000 psi to almost 2700 fps with 43,000 psi (taking that off the top of my head just for conversation here-would have to research the records for the actual data). Accuracy remained the same with 1.2 - 2 moa 10 shot groups at 200 yards.

    The initial problem I ran into was simply I was out of case capacity in the .308W. Faster powders for increased velocity also increased the psi. Slower powders simply did not give even the 2600 fps so while the psi was lower so was the velocity. Solution; a larger case capacity. Hence the interest in this 30x whatever cartridge in the 14 or 16" twist barrel. I believe it can contain enough RL19, AA4350 or H4831SC at close to 100% case capacity to maintain 40 - 43,000 psi and 2700 - 2800 fps out of the 26" barrel with 150 - 160 gr cast bullets.

    Note the 26" barrel minimum I am getting for this cartridge; as mentioned muzzle exit pressure is very important. The longer the barrel the higher the velocity can be with less muzzle exit pressure. Several have posted or PM'd me asking about short barrels with slow twists getting higher velocities. The requests have revolved around 18 - 20" barrels but even a 16" barrel was considered. I try to explain that those HV, even with a slower twist, will necessitate faster burning powders, higher time/pressure curves and thus higher muzzle exit pressures. With a proper ternary alloy cast bullet the shorter barrels become a very difficult obstacle to overcome. Just part of all the things we must consider if we are to keep the bullet as balanced as possible to push it to higher velocity before it bumps into it's RPM threshold.

    Larry Gibson

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