Bullet 'attitude'

[geordiesniper]

This may have been answered but if for example a Berger 180 gr VLD was fired perfectly vertical at a speed of 2900 fps, how high would it go and would it hit the ground at the same point as being fired? But realistically if the wind moved the bullet the answer is no, but what if there was no wind?? Over to the ballisticians amongst you!

 

[swt5]

...if for example a Berger 180 gr VLD was fired perfectly vertical...would it hit the ground at the same point as being fired? Over to the ballisticians amongst you!

Hmmm, where does one go to get an accredited ballisticians diploma?
The force of gravity is a constant that is perfectly aligned with the center of the earth so if one could perfectly align the bore axis with that plane and fire a perfectly balanced/stabilized etc etc bullet into a perfect vacuum, you should be able to retrieve the bullet, oriented base down, sticking up in the muzzle from which it was fired. Even a small deviation from straight up results in gravity acting on the bullet to pull it away from the bore axis as soon as it leaves the muzzle. Add to that the inevitable deviations in manufacturing tolerances (chamber/bullet/barrel/crown etc) and unknown wind speeds at 200/400/600 yards straight up from where you're firing (even though at ground level it seems perfectly still) and you have all the ingredients to make that bullet land several hundred yards away. US Army studies (Hatcher's Notebook) conducted several decades ago were able to confirm the straight up/straight down conjecture when a bullet mark was observed in a piece of wood beside the rig firing the rifle straight up and the imprint of the bullet base could be clearly identified.

 

[Laurie]

Hatcher gives the answers to that question for the miltary ammunition of the day - 0.303", 7.92X57mm 'S' and various forms of .30-06. A 180 7mm Berger VLD at 2,900 fps has more performance and a higher BC than any of the above, but the results won't change too much.

The height attained in all cases was estimated at around 9,000 ft. The total flight time was always around 55 seconds, 19 seconds needed to reach its apogee, 36 seconds or so to fall back under gravity. Return velocity of a 175gn 30 cal FMJBT was estimated by Hatcher as around 300 fps producing around 30 ft/lb energy - enough to hurt but not to seriously injure or kill unless as noted before the victim was unlucky.

Even when rounds were fired absolutely vertically using a fixed mount and instrumentation to adjust the firing angle, no bullet landed anywhere near the firers in any of the various experiments done by various people at different times and in different places with different calibres. 100-200yd away was the norm, but it could be much further in which case strikes weren't seen or heard (done surrounded by water in all cases to hope to see a splash.) The cause is overwhelmingly wind plus a truly vertically fired bullets will either tumble (boat-tails) or fall base first (flat base) that causes minor deviation from a vertical flight. Hatcher found a problem when carrying out tests on Florida's Daytona Beach that the winds were different 9,000 ft up from those oin the ground and no strikes were seen until balloons were employed to identify higher altitude wind direction and speed.

If the muzzle angle moves from vertical to being angled, a point is reached where returning bullets may fall tip first or tumbler giving very different performance. Go beyond that critical angle and the bulet falls further away but also tip first and presumably at much higher speeds, so the likelihood of causing serious injury or death rises dramatically.

 

[Laurie]

US Army studies (Hatcher's Notebook) conducted several decades ago were able to confirm the straight up/straight down conjecture when a bullet mark was observed in a piece of wood beside the rig firing the rifle straight up and the imprint of the bullet base could be clearly identified.

swt5,

if you reread the bit of Hatcher that covers this, the gun cew adjusted the firing angle slightly to fire into the winds and hopefully cause bullets to land back on the firing platform. Even so, only four shots out of 500 in the adjusted firing sequence hit the 10 ft square platform and one the boat the team used.

 

[geordiesniper]

Hi Laurie, thanks for the input I can now sleep soundly tonight without pondering the question of up/down impacts. Also my original post I mentioned that my barrel was pointing up a few degrees. Just remembered that when I shoot at the Diggle range here in the UK, the firing point is lower than the targets, so the barrel is angled a few degrees or so.
Les

 

[glo]

I am not a ballistician. However, I can read and do some math.

One, bullet rotation stabilizes a bullet nose forward.
Two, bullet rotation is to stabilize a cylinder shaped objects.
Three, a bullet shot exactly straight up will have gravity overtake it and return to earth at terminal velocity (about 300fps).
Four, there is no appreciable friction from bullet spin: yes enough to cause spin drift but no, not enough to effect bullet attitude.
Five, because there is little or no friction on the spin of a bullet, it spins until it is forced to stop.
Six, a bullet shot exactly straight up will continue to spin at almost its exact spin rate as when it was fired.
And seven: hence number one; the bullet will stay nose up and drop like a rock back to earth, nose up while spinning at tens of thousands to hundreds of thousands of RPM.

P.S. Back in the 60' and 70' there was a .510 caliber wildcat cartridge made as a back-up gun for Browne Bear guides. They cut a .50 BMG bullet in half and seated it upside down. It worked just fine. So number one will work in the reverse. A bullet shoot base forwards will also be stabilized by spin.

 

[swt5]

...if you reread the bit of Hatcher that covers this, the gun cew adjusted the firing angle slightly to fire into the winds and hopefully cause bullets to land back on the firing platform. Even so, only four shots out of 500 in the adjusted firing sequence hit the 10 ft square platform and one the boat the team used.

Hey Laurie,
I agree a revisit to many passages in this fine book are long overdue but the point I'm trying to make, though obviously not very well is that the result, no matter how remote, tends to support any theoretical assumption. For example, if I were to say it was theoretically possible to roll a marble perfectly along the top OD of a length of copper pipe it may scientifically be a sound proposal in my mind but my gut would say such a feat would be near impossible to accomplish. Raising one end of the copper pipe and rolling 500 marbles across the top only to have one or two make it almost to the end, to me, at least supports the initial supposition that it is theoretically possible...if that makes sense? Like my marble example above, I'm not so sure it's possible to achieve a perfect straight up and down because once gravity is offered an advantage, no matter how minute, it begins to pull things off axis.

Four, there is no appreciable friction from bullet spin: yes enough to cause spin drift but no, not enough to effect bullet attitude.

I agree with most of what you said except this one. In the spirit of light hearted rebuttal, may I offer that the scored rifling marks on a fired bullet make it's surface no longer smooth and shiny. In addition, even air moving across a smooth shiny surface offers viscous drag and we are talking about air moving across the surface of something not only moving downrange at several thousand fps but spinning away from or into (depending on whether the left or right side of the bullet) that air at over 100,000 rpm's. The disparity between the number of air molecules subsequently rubbing against the left and right side of the bullets surface then becomes quite relevant...which brings us back to the resultant torque translating the nose of the bullet downward. I also believe this is why heavier bullets require more spin to stabilize. The drag induced torque may be enough influence on a given bullet weight but not enough to properly co-ordinate the flight of heavier bullets simply because more weight=harder to push so the rpm must be bumped up to increase viscous drag in order to overcome the extra weight and thus, make the nose dip to regain a co-ordinated flight path. My brain hurts now!

 

[geordiesniper]

SWT5. Your brain hurts!!!! All I wanted to know is what attitude a bullet stays/changes going downrange.
Did not expect to get such brain challenging responses to a simple question and now have the mother of all headaches! So thanks to all you ballisticians out there and keep those theories/concepts/thoughts/facts coming as they do make interesting reading to those of us who are mentally challenged!!
Thanks, Les

 

[Laurie]

P.S. Back in the 60' and 70' there was a .510 caliber wildcat cartridge made as a back-up gun for Browne Bear guides. They cut a .50 BMG bullet in half and seated it upside down. It worked just fine. So number one will work in the reverse. A bullet shoot base forwards will also be stabilized by spin. [glo]

There are many examples of people reversing bullets and shooting them base first usually to get non-expanding bullets to expand. The reports usually suggest acceptable or even high accuracy, although it seems conceptually really ugly. The BC must be terrible, but if it's a short-range application then I suppose it's OK.

The one I read of that seemed most 'wrong' was in a Gun Digest annual many years back with Holt Boidinson (I think) writing about small calibre centrefire leverguns for small game. This involved loading a hollow base wadcutter the wrong way round for shooting jackrabbits somewhere hot and sandy like Texas or Arizona. Apparently at short ranges they were amazingly accurate and real killers!

The ancestors of us Brits found out all about the problems of dealing with tough hill tribesmen and suchlike on what they called the Indian North West Frontier Province 100 years before your special forces guys (re)discovered that full metal jacket bullets don't always work well in the manstopping role on the Pathans' great great grandsons in the same region. Only in the British and Indian armies' case it was 210gn 0.312" dia RNFMJs not .22 cal FMJBTs. Reversing bullets was used as was weakening or removing the jacket at the bullet tip, then officially sanctioned and produced remedies in the form of HPs from the Indian arsenal at Dum Dum, a name remembered and (ab)used long after most people have forgotten about these battles and their weapons. (I suppose that's why we're back in that godforsaken part of the world. Our guys have been ordered in by people who never read a history book, or at least never read the right ones before going to war in this region!)

 

[Hondo]

At the equator a point on the surface of the earth moves approximately 1000 miles an hour or .2777 miles a second to the east. Now assuming total flight of 5 seconds of a vertically fired bullet not counting wind said bullet should fall 1.38 miles west of firing point. Please forgive me.

 

[Outrider27]

For those of us blessed/cursed with an excess of curiosity, I highly recommend the 2nd edition of Bryan Litz's book, "Applied Ballistics Fot Long Range Shooting", which has significant discussion of bullet attitude and the related subjects mentioned above, including corriolus (sp?) effect from the earth's rotation. Having read it through once over the last three weeks, I will be revisiting it often, trying to further comprehend what's happening between the muzzle and the target.

 

[geordiesniper]

At the equator a point on the surface of the earth moves approximately 1000 miles an hour or .2777 miles a second to the east. Now assuming total flight of 5 seconds of a vertically fired bullet not counting wind said bullet should fall 1.38 miles west of firing point. Please forgive me.

Hi Hondo, I thought of that one, but would I be correct in saying that as we (the earth, rifle, bullet once fired) are all moving relative to each other and spinning on the earths axis at the same rate. Therefore once the bullet is on its way upwards it is still travelling in a forward motion relative to the position of the rifle and the bullet would not suddenly 'fall behind' me and the rifles position? Plus I believe that Laurie stated that a bullet say Berger 180 gr at 2900fps approx would take approx a total of 50 seconds or so to reach a height of 9000ft and hit the earth at approx 300 fps and not 5 seconds. Comments awaited/appreciated.

 

[Laurie]

At the equator a point on the surface of the earth moves approximately 1000 miles an hour or .2777 miles a second to the east. Now assuming total flight of 5 seconds of a vertically fired bullet not counting wind said bullet should fall 1.38 miles west of firing point.

Yes .... but it doesn't, as all the firing tests have shown. The flaw in the above argument is that it assumes the planet rotates at 1,000 mph but its atmosphere doesn't, likewise anything else above its surface. Since the atmosphere also rotates - if it didn't we'd be living in perpetual winds of around 1,000 mph speed which I suspect we'd notice - the bullet also moves at that speed carried by the atmosphere.

Anyway, if this thesis applied, all shooting over anything other than the shortest distance would be impossible as the target would rotate rapidly away (or towards, or across the line of the range depending on its axis relative to the Earth's motion) from the bullet path. A 0.308" cal Berger 155.5 at 3,000 fps MV takes 1.5 seconds to reach a target 1,000yd away and the firing point / target move 733 yards in that period if the surface is moving at 1,000 mph.

 

[NHgunNut]

Right you are. However a bullet dropped from 3' will hit the gound at the same time as a bullet fired perfectly level at 3'