Barrel Twist Rate and Bullet Stabilization

Do you need a 1:9 twist barrel to shoot a 55 gr. bullet accurately? Do you need a 1:7 for the 77 gr bullet? Can one rifle shoot both equally well? In this video we discuss spin stabilization and the cardinal rules of barrel twist rates.

[@bruceinoz8002]

There seems to be a rash of videos about bullet stability, lately. I IS possible to "overstabilize" a bullet. Like the rest of the general universe, we operate in FOUR "dimensions"; Z, Y, Z and T (Time). This is fairly important. We "spin" bullets to stabilize them in flight. Even round, lead balls. See the huge difference between smooth-bore and rifled muskets shooting round balls of similar diameter. This has been observed and studied for about three hundred years. A chap named Greenhill did a prodigious amount of work on this business and came up with the "Greenhill formula" just in time for the ballistic world to be turned on its head. The appearance of "cylindrical" bullets and then smokeless powder somewhat "overstretched" his initial envelope, but not too much. Once a bullet leaves the muzzle, it is subject to two major forces; Air resistance (drag), and GRAVITY. At supersonic speeds, MOST of this drag is in the form os high-presssure "shock cones, and not just from the pointy meplat. And, remember, serious rifle bullets are rotating at thousands of rpm,unlike supersonic aircraft. The entire idea of spinning a bullet is ti impart "gyroscopic stability"that will maintain alignment with the "line of departure. (Where the barrel is pointed at the moment of discharge). Great idea, BUT, gravity inexorably pulls the bullet,spinning or not, toward the ground.. Thus the "natural" flight-path is NOT a straight line and not even a simple "arc" but a PARABOLA, more or less. And so, If the bullet is spun too fast, it will try to stay on the line of departure rather than align with the parabola. Aerodynamics will "kick" it back on line, but not necessarily EXACTLY on the original trajectory. Optimally-stabilized bullets make many "micro-adjustments" as they fly. UNDER-STABILIZED bullets will wander all over the place. There is, essentially, a "window" of optimal stabilization.. Also, over stabilization" is not usually noticeable until well down-range, say five or 600 yards, when weird groups start occurring.. There are, or have been, several long dissertations about this on various sites. Even several barrel makers have chipped in with detailed articles. Also, higher altitudes and higher temperatures make for thinner air, which is less resistant. This is the reason the original 1:14 inch twist od the M-16 was changed f inches to 1:12 inches; in the high Arctic, stability was assessed as being sub-optimal.

[@sameraloreibi5750]

Very useful information, thank you

[@sdkweber]

You are welcome. And thank you for watching and taking the time to post!

[@wizardofahhhs759]

S&W uses a 1/9 twist on their AR-15s, the lighter 55gr bullets are super accurate whereas the heavier 60 something grain bullets not so much. I think gun manufacturers match the twist rate to the most common grains they know the average shooter will be using.

[@mrkenny1111112]

Great video. I have had extremely good success with a 1 in 7 223 wilde 26" barrel using an 80 gr Berger vld. Thanks again for very informative videos

[@sdkweber]

Thank you for watching and posting Ken. Glad to hear the Berger is working out well for you.

[@aussiesteveakastevecallagh2280]

Thanks Mate That that was an awesome presentation very clear and easily understood , I have always wondered why different Bullets require fast or slow twist rates , thanks very much for the video I now have more understanding of the subject , Steve .

[@sdkweber]

You are welcome Steve. Thank you for watching and posting.

[@fjc11212]

@@sdkweber You can over-stabilize a bullet. MY 1:8 twist FN AR-15 will prove it to you. 50 GR federal varmint loads shoot erratically at 100 yards. 75gr hornady can shoot quarter size groups at 100. Get a new job.

[@fjc11212]

Because of the fast twist rate, light bullets drift high right from fast twist barrels

[@sdkweber]

@@fjc11212 The problem you describe is indeed a dispersion problem but most likely not due to over-stabilization. Problems with over-spinning bullets normally appear as the bullet enters the transonic zone which for a .223 or 5.56 bullet would occur much further than 100 yards.

[@fjc11212]

@@sdkweber During the transonic stage bullets become "unstable" not overstabilized. Meaning, it will impact in an unpredictable location. Overstabilized bullets will be consistently right and high also scattered.

[@onpsxmember]

Could it be that the jacket spins around the core like a tyre on a dragster? Just a wild thought based on different forces on the cross section but it's so small, probably not the case. The destabilisation slowing down to trans sonic as you said seems a good bet. Btw, very nice video and audio, easy to follow.

[@sdkweber]

Thanks for watching and posting. interesting idea but probably not the case, especially with today's bonded bullets.

[@justice1327]

Technically it is not heavier or lighter...twist rate is matched up to the length of the bullet. Highly over-stabilized varmint bullets almost always explode in less than 100yds.

[@sdkweber]

You are correct DW None. I tried to communicate that in the video... length is really the important factor. Since a lot of folks do not know the length of the bullet they are shooting, especially with factory ammo, referring to heavier vs lighter in the same cartridge is a good way to look at it.

[@glenphillips7222]

Great information, thanks.

[@sdkweber]

You are welcome. Thanks for watching and posting Glen.

[@josephk.1842]

good job. thank you

[@sdkweber]

You are welcome Joseph. Thank you for watching!

[@katrinadarling3271]

I would suspect that no bullet is perfectly balanced around its rotational axis. That leads me to suspect that the faster it spins, the more effect that rotational imbalance has on the flight path. If a bullet is spun at a much higher rate than what is required for stabilization, wouldn’t that have a negative impact on accuracy? Also, can the rotating frequency of the bullet interact with gun barrel vibration? I have wondered if I placed an acceleromitor on the barrel and tapped it with a hammer, what the frequency of vibration would be? Is it even close to the bullet rotating speed? I have never heard any concern expressed, so I have to assume there is no issue.

[@sdkweber]

Exactly. That is why a bullet can be over-spun. We have a video on this topic coming out soon. I do not know if the rotational frequency has that effect and it would be hard to isolate and test as the barrel vibration is also impacted by the burning of the powder and the initial explosion of that event. Thanks for watching and posting.

[@impactodelsurenterprise2440]

How does subsonic ammo affect twist rate requirement? What's the recommend for say 308 sub?

[@sdkweber]

The velocity of the bullet has very little to do with stabilization and the required twist rate. It is effectively determined by the length and weight of the bullet. A .308 caliber bullet that is sub-sonic is normally very long and heavy. As a result, a higher twist rate is required to stabilize that bullet.

[@ferdinand2506]

how about l/d ratio???????

[@guardianminifarm8005]

Thank you

[@sdkweber]

You are welcome. Thank you for watching.

[@mohba01]

The disintegration part is BS for sure. As someone who comes from a lot of high power PCP experience i can assure you that there is no such thing as the trans-sonic tumble or yaw which ppl claim. Airguns also have the advantage of very little velocity spread and SD when regulated and this has been proven wrong many times over.

[@lancmac]

Ever think the friction created at 4000 fps is enough to melt lead tips creating even more friction till the spin on the drop of molten lead decides vaporize???

[@sdkweber]

Good question Lanc Mac! I don't think that will happen though. Yes, the temperature of the gases is very high, and the friction as the bullet travels down the bore will also produce heat, but the duration of that heat is so brief that is unlikely to melt lead. The heat on the polymer meplat of a bullet as it flies through the air can start to melt however. That is the reason why Hornady uses a heat tolerant polymer in their ELD bullets.

[@lancmac]

@@sdkweber If an aircraft at 80,000 altitude going mach 3 (3300+ fps) experiences leading edge temperatures exceeding of 2000d F. Lead melts around 640d F depending on any alloys.. So a bullet moving 3000 or 4000 fps is hot, really hot. and that does not take in to consideration surface heating by barrel friction or powder burn.

[@sdkweber]

@@lancmac The factor in heating that you are omitting is time. The U2 aircraft flies at 70,000 ft ASL(I do not know of any aircraft that flies at 80,000 ft ASL) and it does so for hours. A bullet leaving the muzzle at 4,000 fps has a total time of flight of 4 seconds. Its time in the bore is only a fraction of a second and that is not enough time for the heat to melt an exposed lead tip. For something to melt the correct temperature needs to exist as well as sufficient time. Try this experiment. Put a block of cheese in a preheated 350° F oven and leave it there for 4 seconds. Did it melt?

[@lancmac]

@@sdkweber SR 72 has a disclosed service ceiling of 81,000+ AT MACH 3.2+ above those numbers are classified but rumors have mach 3.5 and 85,000 have been tracked on radar for a few minutes.

[@fric8489]

Good video. Technically you can overstabilze a bullet (too high rpm) and cause a cheaper thin jacket bullet to fail and throw the jacket. Ask me how i know... $200 later for a new magneto speed replacement bayonet. No more cheap bullets for me.

[@sdkweber]

Thank you. So your bullet more or less vaporized itself on exiting the muzzle? I have heard of severely over-stabilized bullets vaporizing when they hit the transonic zone before but not on exiting the muzzle. That is too bad to hear it destroyed the Magneto speed bayonet.

[@jackjohn6063]

I thought 1 in 10 twist rate can stabilize bullets out to 220 grains. Your telling me it can’t stabilize that little squirrel round?

[@sdkweber]

It really matters on the length of the bullet. A traditional 220 gr. lead core bullet will be much shorter than a full copper 220 gr. bullet. The RPR has trouble stabilizing a 175+ grain bullet but does very well with the lighter 168 gr. bullet.

[@jackjohn6063]

@@sdkweber I see, that makes sense. Do manufactures put recommended twist rates on the box of there factory loads? And are they at all accurate?

[@sdkweber]

@@jackjohn6063 Some manufacture do put the recommended twist rate on their box of bullets (bullets for reloading). Loaded factory ammo rarely puts recommended twist rate on the box. For .308 Win. I have had good success with 168 gr and less. What are you shooting?

[@jackjohn6063]

@@sdkweber .308, but I’ve been shooting the 180’s as I plan on going moose hunting with them next year. There punching clean holes through paper at a hundred yards. I would at least see them key holing slightly if there not stabilized wouldn’t I?

[@jackjohn6063]

My rifle has a 1 in 10 twist I never thought that wouldn’t be enough for 180 gr. I’ve even heard that 1 in 10 can over stabilize bullets in the 150 to 160 range. Some guys are using 1 in 12 twist for the 150 gr rounds.

[@brianpencall4882]

ICBMs and man carrying rockets are stabilized with gimballed thrust

[@rondonovan4293]

😅use the equation to get correct rate

[@sdkweber]

Do you mean Greenhill's formula?