bullet length, weight, and stability.

[bruce m]

the recent thread on 32/40 bullets brought home an interesting fact.
i always thought that bullet length was the only factor influencing stability for a given twist at a given velocity.
playing with the jbm calculator revealed that a heavier bullet for the same length has greater stability.
the term stability here refers to gyroscopic stability, often represented by Sg.
for greater weight in the same length, a bullet must of necessity have a longer shank/shorter neck, or a blunter nose profile that holds more alloy than a sleeker one, or both.
both these situations make the bullet more nose heavy, a thing which in itself adds to stability, extreme examples of which are minie bullets, hollow base wadcutters and certain rifles slugs.
also adding to stability is more weight further from the axis, making a more effective gyroscope.
i remember dan theodore trying to explain this and can only now see it.
which brings us to the old sharps long range bullet.
examples exist and have been measured and weighed to be 1.5" long, and 550 gns, with a cupped base.
particularly with the weight loss from the cupped base, as well as the alloys of 11:1 to 14:1, the only way to achieve the weight is in the nose and shank.
could this explain why they were routinely successfully shot in 18" and possible 20" twists in the day.
modern nosed bullets have trouble making 550 gns, even at 1.5" long.
they are lighter, and therefore less stable for the same length.
this has brought ther max ideal length for an 18 twist ot 1.450", with a 16" twist being almost mandatory for a 1.5" long bullet if windshear is lurking around.
i have shot a 1.5" long 540 gn pp bullet successfully at 1000 yds, 18 twist, but would not trust it in dirty conditions.
so here comes the real question.
having discussed Sg, we would be remiss to not look at the other kind of stability - dynamic stability.
this cannot be calculated, and is not fully understood by ballisticians to this day.
an example of a bullet with poor dynamic stability, but good Sg in 10" or 11" twists is the 168 gn sierra matchking.
it is unreliable at 1000 yds, often tumbling before it gets there.
another is the 577/450 martini henry bullet which is ok to 900 yds, but not 1000.
the sierra at least has been proven to have poor drag characteristics, with uneven airflow in the transonic zone creating the problem.
our bullets do most of their lying transonically where airflow problems are at their most extreme.
we want the highest b.c to minimize wind deflection, and seek nose shapes that offer this.
these nose shapes reduce weight which reduces stability for the same length.
the martinin bullet was quite blunt and therefore had higher nose pressure, causing a greater tendancy to overturn.
somewhere in this soup will be found an optimim trade off.
it will be interesting to revisit this in 10 years time.
keep safe,
bruce.

[gunlaker]

...
also adding to stability is more weight further from the axis, making a more effective gyroscope.
i remember dan theodore trying to explain this and can only now see it.
...
bruce.

This is an interesting point. The moment of inertia about the bullet's axis increases as mass is moved further away. It also goes by the square of the distance so is not linear.

It's interesting to note that the max length of bullet that most of us use is also about the max length that can be made in Saeco mold blocks which are what most mold makers use. Longer blocks would enable other designs to be tested.

Chris.

[BFD]

Chris, there are always tube moulds that can be made in any length for paper patching.

Note that the gyro aspect is greater for paper patch bullets than for groove bullets, yet GGBs are more stable than PPBs all else being equal.

[gunlaker]

Interesting Brent. I'd never heard of a tube mold.

thanks,

Chris.

[MikeT]

BFD,

Are you saying that the GG bullets will stay "nose forward" longer than a PP bullet?
Are the grease grooves acting similar to the fetching on an arrow?

Keep on hav'n fun!
MikeT

[gunlaker]

Mike, there is a paper written by Richard Gunn that discusses the stabilizing effect of grease grooves. The paper is in the ASSRA archives, although I haven't read it. Certainly any drag that is behind the center of mass is a stabilizing force.

Chris.

[BFD]

Interesting Brent. I'd never heard of a tube mold.

thanks,

Chris.

I'm probably using the wrong name for it. But it isn't a clam shell design like normal moulds. I have one that looks like a bubba copy of an Ideal mould that was made a century and a half ago.

[BFD]

BFD,

Are you saying that the GG bullets will stay "nose forward" longer than a PP bullet?
Are the grease grooves acting similar to the fetching on an arrow?

Keep on hav'n fun!
MikeT

yup, exactly

[bpcr shooter]

Are you saying that the GG bullets will stay "nose forward" longer than a PP bullet?

yup, exactly

I do see how the GG bullet being nose heavy compered to a PP bullet but if it were true that they will stay nose forward longer, wouldn't we still be using them in our modern rifles?? The modern long range bullet is long, sleek and has a long taper, Nothing like the bullets of the past if anything a paper patch bullet more resembles the long range bullets of today. (Ex Money bullet PP)

maybe there is something I am missing, but to me a PP bullet would be better for long range.

[beltfed]

Interesting "modern" History of PP bullets -work by Harrison and Harris /NRA
on 30 cal PP bullets in 30-06, 308 , etc.
1. First tries were with turning 311334 bullets to 0.301". (Impt: left shallow remnants of the GGs)
These shot well.
2.Then they had molds made, same dimensions, but w/o the shallow grooves.
The smooth bullets did not shoot as well.
3. Had another mold made to duplicate the #1 lathe turned molds with shallow grooves:
Again , they shot well.
Speculation, as mentioned above about body drag with the grooves.
Also helped them to line up front edge of PP in a groove.
I have both the Lyman 301618/160gr and the 301620/200 gr, I need to get back to
trying again. Have thoughts of BP loads in my 32-40 Rem Hep and my 30USA Ruger No 1
And "the other powder" loadings for Schuetzen and CBA Vin Mil matches
beltfed/arnie

[MikeT]

Arnie,
So the next generation LR PP bullets will have shallow grooves in the rear third of the bullet!
Grooves a couple of thousandths deep will lighten up the bullet, especially if one puts a recess in the base, about 0.050".
We could call that a 'bitcoin' bullet.

DT are you out there? You would be the most logical person to attempt this endeavor: good machinist; good shooter; good bullet designer.
What more can I say?

Keep on hav'n fun!
MikeT

[beltfed]

MIke,
After having reviewed the PP articles from the '60s in TAR and recently again in my
rare copy of the NRA CBH, I got interested in the idea of light grooves in BPCR PP bullets.
I have already asked DT if he could cut me a 40 cal PP mold with grooves.
Now our lately project work has been with the Dual Diameter EPP bullets for gg chambered rifles.
And they are shooting WELL for four of us now.
beltfed/arnie

[bruce m]

what we know is that the single most effective way to reduce drag is to lengthen the ogive for the same meplat diameter.
with jacketed bullets this is simple.
the current rush on long range jacketed bullets exhibits longer ogives as the primary gain.
our bullets are limited in this area as the longer the nose, the more of it is likely to bump up into the rifling, thus shortening it to a max useable length.
in 40 cal this has proven to be 1.5 calibres, and in 45 cal a little less, even with 12:1 alloy.
for the same bullet length, lengthening the ogive reduces the weight, and thus the stability.
reducing the weight also reduces the b.c., which is not what we want in a long range bullet.
and we want a nose shape that has stable airflow characteristics when trans and sub sonic.
the original money nose had a meplat 55% of calibre, unlike some more recent versions.
this would offer greater stability for length than the new ones.
there exist some extremely long very blunt nosed greaser moulds in 45 cal that i have had serious doubts about, but now wonder.
possibly i owe users of these bullets an apology.
keep safe,
bruce.

[Kurt]

I got this PP mould for a hunting bullet. It has very narrow and shallow grooves and it is a fine shooting bullet. Yes it's a homely looker but it hits the 1023 yrd target at Baker 8 out of ten in the normal winds on that Big hill And I also have one of those mini grooved .446 for the .44-90 bn and the .44-100 rem. that I patched to .451" and that only hit 3 out of 10 on the same target the same day. must be the homely design that makes it work better then the sleek looker

IMG_0102 (2).JPG

[Distant Thunder]

bruce,

This is some very good info to think about and discuss. It's possible that chasing the highest possible BC is not the way to the ideal bullet design for black powder velocities. What you're saying is pretty much what I have found to be true in my shooting and watching other shooters using various bullet designs in both paper patched and grease groove. I've use the things I've learned in the design of my own bullets and while they may not be perfect they have been shooting very well for me over the past several years and every kind of wind I've had to deal with. I have had good and very consistent results following these principles.

The very first long-range match I won at Lodi in 2009 was with a grease groove bullet that I modified and I switched to paper patched bullets after that match and have stayed with them since. All the long-range wins I've had at Lodi starting in 2010 have been with one paper patched bullet design and there are many 2nd and 3rd place finishes in there too. So that design works pretty well. I'm sure it could be improved. Would shallow grooves on the shank be the answer? I certainly would not change the nose design or the length and I'm not a fan of cupped bases. Grooves?

Our bullets are not supersonic for very long and applying supersonic designs to our transonic bullets is probably not the best approach. No high power shooter would even consider using a load that allowed his ultra sleek, high BC bullet to fall below the speed of sound and for very good reasons. It DOESN'T work!

So what is the best design for our long-range shooting? Almost anything will work with steady cross winds, what works best even in those nasty, twitchy tails winds and those quartering head winds?


DT