Bullet stability

[Patrick L]

First, I apologize. This thread will deal with store bought j bullets, but its still valid.

It's amazing how much you don't know when you think you know it all. For years I have just casually glassed over countless threads and articles about twist vs. bullet stability. Yeah, I got it, certain bullets need certain twists or they won't shoot well. I always perceived that as sort of a varmint/bench rest shooters issue, guys who were literally shooting fractions of an inch with highly specialized rigs. Since everything I shoot is fairly conventional, it never really was an issue. I figured an unstable bullet matching would cause a half inch rifle to shoot two or three inch groups, or something like that.

Anyhow, some of you may recall I put together an iron sighted .223 bolt gun to shoot NRA Highpower with last summer. That project went very well and I'm thoroughly enjoying it, shooting cast boolits of course. I did shoot a few generic 55gr FMJ loads when I was mounting the sights, etc. And it seemed to shoot them well. Anyhow, since I'm doing so well with cast, I figure what the hell, I remembered I had some of those super accurate Sierra 69 gr Matchkings left over from the AR days. I loaded 10 of them up with my old match load and whammo let them fly. Looking thru the spotting scope revealed not one hole in the paper. Recalling that I typically use what would be a out a 500 yd sight setting (jacketed)to be on at 100 with cast, I assume I sent the group over the top of the target. So I load up 15 more and this time I take our actual Highpower target, which is a 5 inch bull on a 2x2 foot piece of paper (so I can see poi) and I shoot 5 at 50 yds. All 5 in the black, scattered throughout but since I'm not particularly trying, just checking poi I think nothing of it. So out to 100 yds I go. This time I take one shot, and look thru scope. Hit just outside black at 2 o'clock. Great, I'm on paper. Shot #2, I can't see. Maybe it's in the black, sometimes those little .22 holes are hard to see. Shot #3, I still can't see. What the hell? I ask a friend to stand behind me and see if he can see where on berm I'm hitting. At shot 4, he said he thought over towards the left. Sure enough, #4 is at 10 o'clock outside the five inch bull. Shot #5 can't be seen. We walk down, and sure enough, one shot one either side of the five inch bull, and three shots completely miss a 2 foot by 2 foot paper. Wow!!!

It the dawns on my, my gun had a 12 inch twist. I recall seeing on the box of bullets that they were for faster twist guns only. Just for laughs I take 10 quick shots with some cheap 55 gr FMJ plinking loads. I get a nice 3 inch group, nicely centered and about 6 inches high. Just what I expected.

The final chapter, I went to LGS on way home, bought a box of Sierra 53 gr Matchkings. Bingo! 50 yd group the size of my thumbnail, 100 yds about 2 inches for 10 shots, which is about as well as I can do with irons.

I never would have believed that "unstable" meant that you would miss a four foot square target!! Like I said, I would have expected a six inch group, or maybe evidence of the bullets hitting sideways or keyholing, but not missing entirely.

You're never too old to learn I guess.

[44man]

Twist is king for the bullet. There is just no way out. Same with revolvers. Too many ignore it.

[Larry Gibson]

My match AR and Savage Comp rifles both have 9" twist and shoot the 69 MKs excellently. I've Mostly used them to shoot reduced matches at 100, 200 and 300 yards. They also shoot 52 - 55 gr bullets quite well also (though not the thinner jacketed explosive varmint bullets) especially the match variety. I have found that with my 12" twist M700V which is just as accurate at closer 100 and 200 yards begins to be more accurate at 300 yards than the 9" twists. My rack grade 12" twist AR is also as accurate at 300 yards with the 52 - 55 gr bullets as is my Match AR with the 9" twist. I don't even bother with the 52 - 55 gr bullet in faster 7 & 8" twist ARs past 200 yards as the heaver 69 - 85 gr bullets always shoot better past 200 yards out of those. The adverse effect of spinning bullets faster than necessary for full stability is a well known fact among match and bench rest shooters. The quality of the bullets balance is also essential, especially for maintained accuracy at the longer ranges.

I have for some years discussed the merits of matching the twist for minimal full stability for best accuracy with cast bullets, especially at higher velocities. The merits of doing so are well documented in several threads.

Larry Gibson

[runfiverun]

Larry.
he was spinning them too slow.

[Patrick L]

Larry, maybe I'm reading too much into your post, but in the last paragraph are you telling me I should have known better?

[AZ Pete]

A 1:7 twist will suit you well for any well constructed bullet in a .223 to 600 meters and further.

[tsubaki]

A while back I was comparing some subsonic loads in a .223 with a 1:12" twist barrel using a 55gr flat based pointed soft point and a 55gr spitzer boattail.
Both had the same charge rates. The flat based projectile produced a muzzle velocity of 1088fps and the boattail had a 1165fps velocity.
The interesting part was every round of the boattail keyholed. The flat based ones flew fine.
Just for the meanness of it I later loaded some more boattails at the normal charge rate of 2840fps and there was no more keyhole.

Picture of the boattail keyhole.
http://i175.photobucket.com/albums/w159/tsubaki3/647f5b8ebc2973eaf1fa1f9788747a73_zps445ec5f8.jpg

[tsubaki]

Oh, this result prompted me to get a BATOR mold and try again.
So we will see later.

[Larry Gibson]

Larry.
he was spinning them too slow.

Yup, he was. I was just agreeing with the OP and further discussing you can have problems the other way as he said; "Yeah, I got it, certain bullets need certain twists or they won't shoot well."

Larry Gibson

[Larry Gibson]

Larry, maybe I'm reading too much into your post, but in the last paragraph are you telling me I should have known better?

Absolutely not. I thought I was agreeing with and reinforcing the merits of your post as I agree whole heartedly with them. My apologies if I was misunderstood.

Larry Gibson

[Larry Gibson]

A while back I was comparing some subsonic loads in a .223 with a 1:12" twist barrel using a 55gr flat based pointed soft point and a 55gr spitzer boattail.
Both had the same charge rates. The flat based projectile produced a muzzle velocity of 1088fps and the boattail had a 1165fps velocity.
The interesting part was every round of the boattail keyholed. The flat based ones flew fine.
Just for the meanness of it I later loaded some more boattails at the normal charge rate of 2840fps and there was no more keyhole.

Picture of the boattail keyhole.
http://i175.photobucket.com/albums/w159/tsubaki3/647f5b8ebc2973eaf1fa1f9788747a73_zps445ec5f8.jpg

Benny

If you still have some of both bullets measure the lengths of each and I'll run the stability factor for them if you want? Would be interesting to see what it was at those low velocities.

Larry Gibson

[Patrick L]

Oh, OK, and I'm sorry if I misunderstood.

[M-Tecs]

With quality jacketed bullets overspinning is much less of a problem until they start to come apart. My favorite prairie dog load is a 40 Grain Nosler BT or Hornady V-max over 27.5 of H335. I shoots equally well out 8, 9, 12 and 13 twist. By well I mean sub 1/2 MOA out to 300 yards for 5 shot groups.

I have some Speer 52 HP's that come apart in my 9 twist barrels. If I push my 45/70 300 grain Remingtion HP's to hard the exposed lead spins of and they shoot very poorly.

[303Guy]

Bullet shape has a lot to do with it. Sure, the length is important but the stability factor changes with shape and a boat tail needs more spin.

I have a 16 twist hornet that stabilizes 60 gr Flat base spire points just fine. A 55 gr boat tail FMJ needed to be driven real hard to get it to sort of stabilize. I'm not sure a 16 twist 223 would stabilize any of those bullets because of the muzzle blast.


The flat based projectile produced a muzzle velocity of 1088fps and the boattail had a 1165fps velocity.
The interesting part was every round of the boattail keyholed. The flat based ones flew fine.

That 1165 fps would likely cause the tipping. Boat tails are also very unstable in the transonic zone. It would be interesting to see how they would perform at 1050 fps. Being boat tail they may not be stable at subsonic either but might not tip quite as much or quite as soon.


I have some Speer 52 HP's that come apart in my 9 twist barrels. If I push my 45/70 300 grain Remington HP's to hard the exposed lead spins of and they shoot very poorly.M-Tecs, that sounds reasonable but how did you determine that is what was happening?

[M-Tecs]

M-Tecs, that sounds reasonable but how did you determine that is what was happening?

On the Speer 52's most would hit the target but on some you could see the trace cork screw and some you see the puff 20 or 30 yards out.

Rifling form makes a difference. My 8 twist Schneider Polygon barrel http://www.schneiderriflebarrels.com/ did fine with them but not so much with my 9 twist cut rifled barrels.

On the 45/70 300 grain Remington HP's the target was speckled by lead dust and very tiny holes around the actual bullet hole.

[Ballistics in Scotland]

The OP's experience is exactly what you would expect, and Tsubakis, while less of a foregone conclusion, isn't surprising. A boat-tailed bullet needs more of a twist, and although velocity doesn't contribute quite as much as you might think, low velocity also may result in failure. He had the two together, producing keyholing when the bullet weight was such as you might otherwise get away with.

Note that not all 1165ft./sec/ velocities are alike. When the velocity falls to that figure far downrange, with an ordinary load, the rotational velocity hasn't fallen nearly as much. That is fact, although it is purely my interpretation that rotation is slowed merely by friction against the air, while linear motion is slowed by elbowing the stuff out of the way. At 1000 yards that can amount to four or five times its weight in air, and for .a .22 rather more.

Jeff Cooper describes bullets landing in sand and still hissing unpleasantly through rotation. TF Fremantle in 1901 tells how a bullet bounced, its linear velocity almost entirely lost, from the water-tankwhich was intended to catch it unmarked for examination. It was still spinning enough to stabilize it, like a top, when by some freak of chance it landed upright on its nose.

So an 1165ft./sec. reduced load is spinning far less fast than a normal one which has decelerated to that figure.

The idea of a bullet being overstabilized is less certain to be significant. A faster than necessary rifling twist is undoubtedly undesirable with cast bullets, and also with black powder, since the higher angle to the travel of the fouling gives more chance for it to lodge, especially if the rifling is deep as it is on rifles designed to resist the erosive effect of smokeless. With cast bullets and smokeless, it is doubly undesirable.

There is also a deflection of an unbalanced bullet on emergence from the muzzle. If the bullet's mass is .001in. off centre, it is describing a spiral .002in. in diameter. When it leaves the muzzle it obeys Newton's laws by reverting to a straight path, since it is no longer constrained into a curve by the rifling. That path will lead off in whatever direction the end of the bore spiral was pointing. .00 might not sound much, but it will lead to a group of about 27in. at 1000 yards when the twist is 10in., in addition to all the other things that stop bullets going into one hole. That deflection is inversely proportional to the rifling twist.

But the aerodynamic effects of excessive spin on the bullet once in flight are uncertain and often exaggerated. I planned my ex-P14 Enfield .300H&H around 190gr. bullets, MatchKings or SP, with a 10in. twist. But the best accuracy I got, about 1⅛ at 2000 yards over trials long enough to be consistent, was with Speer 125gr. spitzer bullets at around 3500 ft./sec. Sometimes a fast twist and that velocity can produce core melting, but if it doesn't, it doesn't.

[tsubaki]

Benny

If you still have some of both bullets measure the lengths of each and I'll run the stability factor for them if you want? Would be interesting to see what it was at those low velocities.

Larry Gibson

That's easy Larry, thanks.
Nosler #39526 .224" OAL 0.815". MV 1165fps. Their data shows OAL 0.810", BC 0.267, SD 0.157.
Remington PSP .224" OAL 0.690". MV 1088fps. Probably now extinct, haven't been able to find these for a couple of years. A shame, this was my favorite. It also performed exceptionally when driven at 3450fps.

[Larry Gibson]

That's easy Larry, thanks.
Nosler #39526 .224" OAL 0.815". MV 1165fps. Their data shows OAL 0.810", BC 0.267, SD 0.157.
Remington PSP .224" OAL 0.690". MV 1088fps. Probably now extinct, haven't been able to find these for a couple of years. A shame, this was my favorite. It also performed exceptionally when driven at 3450fps.

Using the Miller Bullet Stability program an "Sg" of 1.0 is the lowest for marginal stability. That is a measurement of the rotational stability of the bullet or the "rigidity of the spinning mass". The other force which affects bullet stability is the "over turning aerodynamic torque". For full gyroscopic stability of the bullet the first must be greater than the second. An Sg of 1.4 gives comfortable full gyroscopic stability to the bullet. However, since rotational velocity decreases a lot slower than forward velocity the bullet's Sg actually increases as the range increases.

The Nosler BT with the .815 length at 1165 fps has an Sg of only .75. There was not a sufficient "rigidity of spinning mass" to keep the pressure from the "aerodynamic torque" from pushing the nose away from the direction of flight and the bullet tumbled. That Nosler bullet with the 12" twist requires 2750 fps to attain an Sg of 1.0.

The Remington PSP with the length of .690 at 1088 fps had an Sg of 1.18 and was stable. That bullet out of a 12" twist at 670 fps has and Sg of 1.0.

Let us keep in mind that the adverse affect of RPM on accuracy has nothing to do with bullet stability. As an example most M2 bullets in the 30-06 are capable of 2 - 4 moa accuracy at 2700 fps out of a 30-06 rifle with a 10" twist. Yet Sierra, Hornady, Speer or Nosler 150 gr bullets are capable of moa or better accuracy from the same rifle at the same velocity. Why the difference as the bullets are all quite stable? The difference in accuracy is the result of the commercial bullets having much more uniform jacket form, jacket thickness and weight and lead core weight. In essence the commercial bullets are just better made and much better balanced bullets to begin with. The RPM adversely affects the greater imbalanced bullets in flight to a greater degree with less accuracy being the result, especially as the range increases.

The fact is the greater the RPM (or the faster the twist at a given velocity) the greater the adverse affect on accuracy any imbalance in the bullet will have in flight. The farther the flight (as in at longer ranges) the greater the disparity will be as the Sg increases. But for all that to occur the bullet first must have sufficient rotation for stability. Patrick L's and Benny's examples did not have sufficient rotation for adequate stability and thus any affect the RPM may have had on any imbalances in those bullets became a moot point because the RPM had no chance to adversely affect the unstable and/or tumbling bullets.

Larry Gibson

[Larry Gibson]

Patrick L

FYI; the Sierra 69 gr MK at 2750 fps out of a 12" twist has and SG of .95 and are not stabilized. Yours may or may not have been tumbling yet at 100 yards but were probably very close to it.

The 2750 fps is the velocity of my match load with that bullet out of my 9" twist AR. The same load runs 2930 fps out of my Savage Comp match rifle. My load is 25.5 gr Varget in LC cases with a WSR or Rem 7 1/2 primer. Shot our monthly local 300 yard prone match (using 300/600 yard reduced targets) this morning with that load in the Savage rifle. I was the only one shooting Match Rifle. The others were shooting F Class rifles....I tied for 1st.

Larry Gibson

[tsubaki]

Thanks Larry!
I ran the BATOR through that program and at 1200fps it came out at a SG of 2.62. :bigsmyl2:
I may like this thing. At 2840fps it comes up at 3.49.
Though I seriously doubt the latter velocity is doable based on most everybody elses experience.

[Ballistics in Scotland]

I described the straight-line deflection which must necessarily occur if the bullet's centre of mass is off its physical centre. This is precisely calculable, and the resulting error is twice as great at 1000 yards as 500. You can produce it experimentally by drilling little holes in the bearing surface of the bullet, and even produce several separate groups by orienting such bullets in different directions in the chamber. If the imbalance is slight, this may be the only source of inaccuracy.

But there is another source of inaccuracy in an imbalanced bullet, and this is the one which may be absent or trivial, or in extreme cases give rise to tumbling or breakup in flight, when the rifling twist was adequate or more than adequate. The bullet, once out in fresh air, tends to spin around its centre of mass. If that centre isn't on its centerline, a protruding bit of outside spins around the bullet, acting very much like a tiny rudder. This produces an air spiral or corkscrewing motion, which is far harder to predict or quantify than the effect of the bore spiral. It may remain constant in diameter, in which case a bullet can produce a bad group at short range, but a reasonable one at long. A bullet may settle down as it proceeds downrange, or its corkscrewing may deteriorate.

There is an interesting experiment by which you can determine what is going wrong with a non-keyholing but inaccurate bullet, although it takes a fair range to do it. Set up a sheet of tissue paper half or 60% down-range, so that you can aim under it at the real target, but the bullets' trajectory takes them through the paper. If you find all the holes similarly oriented (e.g. 3in. high and to the right at 150, 6in. at 300), that is likely to be the effect of the bore spiral. If the bullets appear to twine round one another in flight (e.g. one bullet being on the left at 150 and on the right at 300), an air spiral is coming into play.

[Digital Dan]

What you relate as an air spiral or corkscrewing is part an parcel of an epicyclic precession mode, or two modes occurring simultaneously. All bullets exit the muzzle in this dual mode of precession, but the real question goes to magnitude of each mode. One mode is a fast cycle and is caused by ever present bullet imbalance, the other a slow mode caused by various deflecting forces at the muzzle which can include wind, muzzle flip, gas jet upset, imperfections in the bullet base and gravity. The first nulls with fair rapidity, the slow mode never does so. Were a bullet's nose used to scribe a pattern on paper the movement would resemble the heavy dark line in the image on the right:

146330

[44man]

Over spin creates the cork screw too. Take the S&W 29 with a faster twist then a Ruger. Shooting a 240 gr bullet will show the cork screw, it is very fast. Quite amazing to watch through a good spotting scope. It never affected groups but as range changed the POI would be one side or the other depending on the cork screw position.
Bullets from a Ruger showed a straight path but I would expect a lighter bullet, shot faster would bring it in.
To cure the S&W only took going to a 250 gr bullet.
My pre 64 model 70 in 220 Swift showed the affect at close range and I never got the rifle to shoot under 1" at 100 but shot a 5 shot group of 1/4" at 350 yards, my sight in distance back then. I used the 60 gr Hornady. It would head shoot chucks beyond 600 yards.
This is the reason BR shooters use a slower twist for closer ranges. Long range shooters use a faster twist.
I never liked the 22-250 since it needed lighter bullets that did not work as good at long ranges.
The Swift was so good, many times I would spot a chuck at 200 and depending on the lay of the land, I would pace another 2 to 300 yards AWAY from the chuck. I used the Balvar 6 to 24 scope.
Crows at 400 yards just had wings flutter out of trees. But I could miss a chuck at 100.
No twist formula works, not a single one will be right.

[Larry Gibson]

......and when the centrifugal force is such that it over comes the epicyclic precession the bullets flight begins a slow increasing helical arc around the line of flight. Or it can go off on a severe tangent if the centrifugal force is great enough. The effects described by Ballistics In Scotland and Digital Dan occur with most all fired bullets to some degree. However, if the RPM is great enough and the bullet begins the helical arc or goes off on a tangent it will not follow the line of flight as the range increases. This threshold event has nothing to do with the stability of the bullet as it will still be flying nose forward and not tumbling. This event is easily demonstrated by shooting groups at 50, 100 and 200 yards.

If the bullets are under the threshold event there will be normal linear expansion of the groups as the range increases. The normal "group" size is formed by the imbalances in the bullet and the yaw and pitch when they are small enough for a given RPM under this threshold event. The better the quality of the bullet the smaller the group size. Normal linear expansion is such that if we shot 3 separate groups at 50 yards of 3/4", 1" and 1 1/4" we should have expect a group (same number of shots) size of 1 1/2 - 2 1/2" at 100 yards and 3 - 5" at 200 yards.

If the centrifugal force is strong enough to go over this threshold event and the bullets (some or all depending on how far above the threshold the bullets are) will begin the helical arc or go off on a tangent. When that occurs the expansion of the groups as the range increases will non linear. Basically the groups will get significantly larger than they should as the range increases.

I have demonstrated this with cast and jacketed bullets numerous times and most of the results have been posted in other threads. 44man is quite correct; "Over spin creates the cork screw too." If the "over spin" is great enough the threshold event occurs which induces the helical arc or tangent to line of flight to the bullet also. Again let me reiterate; the adverse effect of the bullet crossing threshold event has nothing to do with the stability of the bullet. It is an entirely different phenomenon.

Larry Gibson

[44man]

Very good explanations. If a bullet goes to sleep at range, accuracy can be great.
One I never figured out was the 30-30, 10" TC Tender. I always spotted for my friend after I sold him the gun.
At 200 meters, the boolit would swing right and go for the next ram in line. Then I would watch the boolit swing back and hit the one he aimed at. He had to hold different windage at 100.
I asked if he cast in a guidance system!