Greenhill again

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  • Last Post 14 August 2020
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Ross Smith posted this 03 August 2020

After playing with the greenhill formula and using the twist rate calculator on kwk.us, I'm still confused about velocity and accuracy. Using data from my 51 cal slug gun or my 308 bench rifle, I can get theoretical stability at 800 fps but have a low velocity warning. But cranking up the velocity so the warning goes away, then I need an absurdly slow twist rate. I can enter data with a shorter bullet and get theoretically good velocity and twist rate but my rifles won't shoot them worth a hoot. I'm sure that my bullet casting and swaging isn't right(yet), but this external ballistic stuff is getting more confusing, not easier.

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45 2.1 posted this 03 August 2020

Greenhill is a artillery calculator. Use your actual twist with the intended velocity you will use and see what it gives for bullet length first trying the 150 constant then try a constant of 125. Post the length for each. Maybe someone will help with stability factors.

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John Alexander posted this 03 August 2020

Ross,

The way I look at it is that you NEED a twist fast enough to stabilize to get decent accuracy.

But you never NEED a slower twist to get decent accuracy.   My 8 and 9 inch twist 22s shoot 35 and 40 grain VMax very well.  Factory rifles in 14"twists don't seem to shoot them better. However,  if you look at what twist will stabilize such short bullets at 3,500 fps you will probably find 1 in 16" or maybe slower.

It is true, that theoretically the slowest twist that will stabilize should give the very best accuracy if there is any imbalance in the bullet at all. Maybe that can be shown by the very top accuracy in JB benchrest guns. (Regretable, they don't know for sure either because all they never try is slower twists.)

But with guns, and especially cast bullets, us mortal use, the advantage of the slowest twist that will stabilize is probably always in the round off error, completely dominated by other reasons for inaccuracy.

Short cast bullets are inherently harder to shoot accurately than long.  It is no accident, or just habit, that 30 or 32 cal. bullets in CBA matches are almost always from 180 to 225 grains. This has little to nothing to do with twist.

John 

 

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Ross Smith posted this 03 August 2020

Thanks for the wisdom. I do suspect that my casting and swaging of the two halves of the bullet is probably my biggest buggaboo. But you never know what elephants are out there if you don't look.

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Ross Smith posted this 04 August 2020

After playing with a few numbers on stability formulas ranging from Greenhill to Miller and some variations( Thanks to Tony from Australia) If I use a bullet of 1.7-1.8" and 780+ grains, at 1000+- fps I will have stability factors from 2.5 to 3.5. .512 caliber.  Since I can't change the rifle, I need to concentrate on building a better bullets. Would a single piece bullet be betteror easier?

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45 2.1 posted this 04 August 2020

 Would a single piece bullet be better or easier?

You would be well off to use a one piece bullet. Basically treat it as you would shoot a Whitworth of match and Civil war fame....... use a one piece paper patched bullet. A two piece picket style bullet was never regarded well for accuracy.

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MFSTEAKLEY posted this 13 August 2020

I, too, have questioned the subject calculation.  Obviously, all the inputs are variables that change the answer.  The SG needs to be accurately determined and the velocity measured.  However, I suspect that the bullet length and diameter may be more complicated that one might reason.  A bullet isn't the same diameter for it's entire length and if you measure the OAL it may be an over estimation, if the tip is pointed, or partly so.  What do you guys think?

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Ross Smith posted this 13 August 2020

Mr. Steakley:  The greenhill formula I think now, is a good indicator of stability. But . What happens with a stability factor of 3 or 4? There is precious little advice out there on the effects of high and low factors on ACCURACY. Which is what we are after. It is nice to have a program that will tell us if a certain bullet will be stable or not but that is all it is good for in my opinion. It tells me very little about shape, style, etc before I spend 200 bucks on a mold from Steve Brooks, and I'm working on a muzzleloader that doesn't have leades and throats and tapers and free bore, just a straight tube.

While I'm on it, any reccommendations on bullet diameter for a .504 bore and .512 groove, 17twist, muzzleloader. Paper patch or grooved?

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45 2.1 posted this 13 August 2020

Mr. Steakley:  The greenhill formula I think now, is a good indicator of stability. But . What happens with a stability factor of 3 or 4? The stability factor is over twice the minimum to be stabile. It should be stabilized to a very long range. The bullet lengths given by Greenhill should be fine. There is precious little advice out there on the effects of high and low factors on ACCURACY. They are out there IF you know how to find them. The answers are in bits and pieces from several disciplines. Nothing like this is gathered in one place in coherent written word. There are a lot of 100 year old books that give information. Read about the Berdan Sharpshooters from the Civil War and the use of British Whitworth rifles by the Southern contingent in the same war. Which is what we are after. It is nice to have a program that will tell us if a certain bullet will be stable or not this depends on bullet parameters, bullet shape is a very big parameter. but that is all it is good for in my opinion. It tells me very little about shape, style, etc before I spend 200 bucks on a mold from Steve Brooks, and I'm working on a muzzleloader that doesn't have leades and throats and tapers and free bore, just a straight tube. You ever use the REAL system in a muzzle loading rifle? Look it up..............

While I'm on it, any reccommendations on bullet diameter for a .504 bore and .512 groove, 17twist, muzzleloader. Paper patch or grooved? You need to decide whether to use paper patch or a naked bullet first as the size between naked or PP will not be compatible. You need a tapered REAL type bullet so it self aligns as you seat it (given your parameters above). You will also have to decide whether you will shoot it wiped or dirty as that will effect bullet size and seating difficulties. There are some other factors involved, but those can wait until you make some decisions.........................

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Ross Smith posted this 14 August 2020

45-2.1    Again this is a 50pound muzzleloading rifle. The molds for the rifle and the swages to assemble the 2 piece bullet make a bullet anywhere from 1.4 to 1.8 inches long. 2 strip paper patched bullet. I would like to get a mold for a single piece bullet. If the bullet is to be paper patched, what diameter and length would you recommend? Same for a greased bullet.

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45 2.1 posted this 14 August 2020

45-2.1    Again this is a 50pound muzzleloading rifle. Old (as in about 1840 time frame since you have a two piece bullet with swage) or new?The molds for the rifle and the swages to assemble the 2 piece bullet make a bullet anywhere from 1.4 to 1.8 inches long. 2 strip paper patched bullet. It sounds like you have a false muzzle (removable and blocks the sights where you don't shoot it off with at least two alignment holes on the muzzle face), is this so? Are you presently using the two strip patch? I would like to get a mold for a single piece bullet. Greenhill gave you a 1.7" to 1.8" bullet length which is fine for your rifles twist (which is considered fast for a 50). If the bullet is to be paper patched (that depends on the accoutrements your rifle has), what diameter and length would you recommend? If you use the present patch system, you need to have or adjust the size so you can muzzle load it without forcing it down the bore, that is the bullet and patches have to be at bore size or slightly smaller dependent on whether you lube the patch or not and whether you wipe the bore for each shot or shoot it dirty. Same for a greased bullet.  It needs to be about bore size (so it loads easily without upsetting it while pushing it down the bore). A greased bullet could be reversed tapered as per the REAL (rifling engraved at loading) so it remains straight while seating. The base band would need to be 0.0005" under bore with the front band being 0.0015" (at most) over bore. Steve Brooks makes excellent molds for the BPCR crowd. The bullet design you need for accuracy does not match what they use. I would have at least 70 to 75% bearing length with a short blunt smaller meplat nose if you are interested in accuracy. Check out what a Wentworth muzzle loading rifle bullet looks like. They have round molds as well as hex for that rifles bore. You will be interested in the bullets profile.

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