I've decided to finish up translating the Warner-Lowe letters over to CD and it's kinda surprising some of the things that are showing up.

  As the subject line suggests there has been a lot of discussion and cussing as to the veracity of Greenhills formula for shooting cast Lead bullets in the rifles we use in the different SS venues. Today there are many ways of figuring proper twist or bullet length for a given gun, but for many years all most of us had was Greenhill. The problem seemed to be that using the formula as written didn't always work. So, many of us started using different constants to get a reasonable result. Sometimes this worked, and sometimes not.

  I've often wondered why! well, I've just come across a section in one of Lowe's Day Books giving his explanation as he saw in back in 1890. In fact, even then Greenhill wasn't thought of to well for shoulder fired weapons, and Lowe, as well as some others, did the experiments to in order to get better results.

  To lead into what's below..... a Maj. J.P. Cundill & a Mr. Hudcock came up with a table which they thought would better serve the small bore community than Greenhill. A Wm. E. Carlin wrote to Lowe fleshing out this table, and then Lowe gives his opinion on things. The following is a direct translation from Lowe's Day Book so don't be to surprised at what you see. 

  The really amazing part is that the last part below is what has started showing up on the various Lists/Forums in the last coupla years. One thing new to me, at least, is why would pure Lead bullets require less twist than hardened ones?

PETE

Wm E. Carlin says table calculated for bullets hardened about 1x16 lead & tin pure lead bullets would require less twist so far as calculation goes.
Prof Greenhill made his calculations so as to give about the least twist necessary in building new breech loading rifle cannon so as to save expense of too many experiments in that direction  There are two things impossible to allow for in these calculations. 1 the confussibility of the air and the affect of decrease or increase of the muzzle velocity would have on the projectile of course the higher the muzzle velocity the higher the velocity of rotation but the higher the velocity of translation the greater the resistance of the air which tends to upset and render unsteady the bullet. In some cases I have no doubt in the majority of cases a higher velocity will often steady the flight of otherwise unsteady bullet but it is not necessarily so  For an increase of velocity might cause a steady bullet to become wild i.e. take an excessive drift.

Rimfire,

  I can certainly understand your finding it hard to understand Lowe's use of the English language. Just remember that this info came out of Lowe's Day Books and as such are mostly memory joggers on things as he saw them. So as you can note periods are not something he wasted his time on, and some sentences don't seem to make a lot of sense.

  With that in mind his comments on Greenhill are one of the more coherent paragraphs he put together.

  What I was trying to present was that even 110 yrs. ago shooters had the same problem with Greenhill that we have today, and like today people like Lowe had figured out ways of figuring twist for a given length of bullet, or bullet length for a given twist, just as we've done today with some of the more esoteric computer programs you can log into.

  As for my opinion of what Lowe was trying to get across...... I'll agree with you that spinning a bullet a little faster than actually required is better than using a twist that is borderline. But it's a little more complicated than that.

  This is one of the things Lowe's experiments were about when he designed his own formula for figuring proper twists. What he wanted to end up with was a twist rate for any given bullet length that would be stable at any distance you cared to shoot at out to his test limits of 2000 yds. Also to be able to take a given bullet and determine the best twist to get the same results.

  But..... it can also be said from Lowe's comments that a quick twist that is to far out of line will result in more drift than what would result from a more ideal rate.

  One of the big problems with any twist formula, as Lowe mentions, is there are variables that can occur on any day that will influence what optimum twist will be. Until Lowe figured this out it about drove him crazy when he'd go out shooting and one day he'd get what he calls "splendid" results and the next day get "poor prints" and not changing a thing. what he eventually found out was that temp. and humidity were the main cause of his problems. If you've read the threads on Density Altitude you'll see that as air density increases or decreases so will your impact points.

  So what Lowe, and others, had to figure out was how to be able to figure an optimum twist rate that would give good results under as many conditions as possible. Lowe seemed to think he'd figured it out, but someone with a LOT more money than I've got will have to do the grunt work to see if he was right.

PETE

I have found this site 2 b very interesting concerning ballistics

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rimfire

Quote:

My understanding is a little different. Stability increases as the density of the bullet increases, decreases as the density of the flight medium (air) increases, and is affected only slightly by velocity. Then for a given velocity, air density and bullet density, the stability is a function of bullet length. For most normal conditions, i.e. air on earth, lead alloy bullets and velocities from 1000-2000 fps or so, Greenhill works fine. It has been reported numerous times that better bore and bullet quality allow slower twist/longer bullets. So perhaps Greenhill is conservative, cause faster twist and the perhaps mythological over-stabilization is better than slower twist and keyholes. The 80 grain NEI 22 bullet never keyholed in a 9" Savage, but is humerous to shoot at 50 yards in a 222.
I've never understood that bullets become unstable at low velocity, say at 1000 yards, because of the twist-length relationship. I've read allusions to such, but no hard data. 
If, say, 457125's are going ?800fps? at 1000 after a 1200fps launch, and if one suspects that they become unstable, why not shoot some at 100 yards and 800 fps? Would they be stable?
joe b. 

The issue with bullets becoming unstable at longer ranges probably has much more to do with trassonic behaviour - passing from supersonic to subsonic with a less-than-ideal shape causes buffeting and will destabilize an otherwise stable bullet because of the shockwave transition.  One just has to look at supersonic and subsonic shockwave profiles to see what will happen.  My take is that in certain cases (i.e. barely enough stability for the supersonic  situation), the trassonic buffeting will cause enough instability to make the bullet loose its gyroscopic stability permanently, and start  up a yaw pattern which will increase up to the point of overturning.  Early supersonic airplanes also suffered from this phenomenon - but airplane design doesn't have to take into account the rifle bullet shape restrictions, so they could minimize the effect by aerodynamical optimization - which wouls not be that easy with a bullet, and which would also explain that some designs apparently suffer more from it than other...  Just my take on the subject, for whatever it is worth.

Pete,
Twist is indeed what keeps the bullet stable - but the transsonic region needs more of  it than the supersonic or subsonic region by themselves.  One of the reasons why Palma shooters try to keep their bullets supersonic all the way to the target.  And your spinning top doesn't fall over immediately - its starts with a slight wobble which will gradually increase (because it doesn't spin fast enough anymore) right up to stability loss.  now give the spinning top a good whack.  When it has enough rpm, it will become stable again rather quickly.  However, when the rpm is low, you could have just caused a 'fatal' instability a bit earlier than without the external cause (which is what we're talking about here - don't forget, before complete understanding of the problem, quite a few airplanes actually disintegrated while trying to pass the sonic barrier - the buffeting is that bad).  This could explain why some rifles shoot well out to a certain distance, and then accuracy comes apart - maybe they have a marginal but adequate supersonic twist, and the bullet permanently destabilizes at the distance where the transition of supersonic to subsonic takes place.  When the rpm is high enough, on the other hand, stability is re-acquired, bullet yaw is dampened out, and accurate bullet flight continues.   
There could be other causes too - but this seems to me one of the more important ones.   I won't guarantee I'm right, but it does seem logical...

Joe, 
Transsonic buillet behaviour doesn't have to be a problem - provided the twist is adequate (read fast enough).  But I am pretty certain that a marginal twist can go unstable during this transition phase.  The twist will be good enough to provide stability both during supersonic and subsonic bullet travel, but the transition is what gives the problem.  Indeed, the BP guys have to deal with it, and are getting such findings.  Marginally stable bullets can provide great accuracy, out to a certain distance - and then all goes to hell.  Which is why I am a fervent believer of faster twists - for me, overstabilization is by far the lesser evil.  Of course, there are quite a lot of variables there:  Bullet shape, temp, pressure,....  which all have an influence.

Amen Pete

The more that I learn the more I understand that all these new breakthroughs are the stuff that the oldtimers thought was common knowledge and didn't bother to write it down. The last couple of years the first place that I go is back to the old masters. 
Those that don't know their history are doomed to repeat it.

40 Rod