Home Forums Hunting & Shooting Ask The Gunwriters Ask John Barsness Questions About "1-12 vs. 1-9 Twist in .223's"

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Gatehouse,

Yeah, it does sound like the 53 TSX is a little long for the 1-12, at least at the velocity you're loading it to. Which is yet another reason I prefer the twists in my rifles to be a little too fast rather than a little too slow!

Thanks.

Do you mean if the 53gr had higher velocity it should stabilize in the 1-12?

I might try the 45gr TSX's. See how thier accuracy is at 200 and 300 yards.

Yeah, it probably would. The problem is getting the extra velocity.

I'll just ram more powder into it. I hear the Remington action is pretty strong, I am sure it can stand some blown primers...;)

Yeah, I have blown a few primers in Remingtons myself. They never hurt anything, but you've got to clear the pile out of the magazine now and then....

No worries, I will use a Dustbuster with the slim nozzle attachment

John,

Thanks for a well written, timely article.

I shoot a fast twist upper on one of my hunting AR's. It is twisted 1:7 and is 20" long. It shoots 60 grain Vmax's into little, itty bitty knots with 25.5 grains Ramshot TAC.

I've a touch of time behind various .223's twisted various ways with similar bullets. My killing is mostly bigger stuff like fox, bobcats, and coyotes instead of prairie dogs.

My question to you, if you get a minute, is whether or not you have observed terminal damage varying by faster or slower twists with the same bullet.

My question results from a medium sample size the past few years of larger varmints and watching them take a 60 Vmax from this 1:7 at 2900 MV. Their "behavior" after the shot is quite different than a 1:12 shooting the same bullet. I'll just say the 1:7 animals die extremely fast. The bullet seldom exits, and 95% of animals are DRT. The terminal reaction of the 1:12 animals is not as impressive. Maybe coincidence, maybe 2 years isn't long enough to draw conclusions.

What say you sir?

Thanks...

I haven't shot a 1-7 very much so really couldn't say about it. But it would make some sense that bullets from a 1-7 would tend to expand more violently than from a 1-12.

Between a 1-9 and 1-12 there doesn't seem to be all that much difference, but the only 1-12 .223 we have left in the house is a varmint-weight Rem. 700, and the barrel is 26" long. It gets more velocity than my 1-9 24" 788 (a stainless E.R. Shaw rebarrel) and a LOT more than my 16" barreled Bushmaster carbine.

In the 700 my standard loads of 28.5 TAC and a 40 and 26.0 grains of TAC and a 50 get around 3850 and 3450, which is kind of cranking. The 788 gets 100-150 fps less, and the AR-15 gets 300-350 fps less, so that has to make a difference as well.

Thanks John...

One of the good things about the Vmax, I have found, vs. the BT is their ability to perform at lower velocities. Plus they're cheaper and more accurate, on average, for me. Slow a BT down and you'll get lots of exits, at least with 50's-60's. Slow a Vmax down and you'll still lots of goo. Anyways, back on topic.

Personally, I do think there is something going on terminally with my 1:7. It is not a bad thing...grin.

Thanks for the reply.

TAC is some great stuff.

I've got a 6x45 on the way. It will be 22" and be twisted 1:9. Ramshot strongly recommended I not use TAC in it to my dismay.

Thanks and have a great weekend.

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Bruce

This is certainly an interesting thread, but there are a few statements that have been made which need to be disputed.

Remchester informs us that: �This is because it takes a while for the bullet to "settle down" in flight, like a football will start off with a small wobble but then stabilize during mid flight (and then begin to wobble again toward the end of its flight). Below is a table that shows the approximate relationship between bore twist and bullet weight (based on using plastic tipped bullets in a 24 inch* barrel).�
Really? I�ve never seen that and I have been shooting at very long distances for some time now. It certainly runs counter to my understanding of external ballistics. Perhaps Remchester could point us to some sites that buttress his claim. At any rate, the way I understand it to work is as follows.

The reason for a spin on the bullet in flight is to user the statically stabilization of the gyroscopic effect of the spinning bullet for it to remain point on in flight. When the bullet leaves the confines of the barrel, where it has been spinning around its center of shape, it encounters something called air resistance, a force that acts on the bullet and attempts to cause it to destabilize and have it spin around its center of mass. Air resistance also causes the bullet to shed its forward momentum and one way to (partially) mitigate this effect if to give the bullet an aerodynamic profile. The greater the aerodynamic profile however, the more unbalanced the bullet becomes. You can see that the bullet is very light in front, where the ogive is, and has more weight in the rear half of the bullet. This imbalance is what the spin is designed to counteract. So, the longer and more aerodynamic a bullet is, the higher the pin rate required to statically stabilize said bullet.

Now that we have that out of the way, let�s continue. As the bullet leaves the barrel, it has achieved two maximums (maxima?). The first one is its forward velocity, aka MV and the second one is its spin rate. The spin rate is a function of the MV and the twist rate of the barrel. For example, at 3000FPS, a 1:12 twisted barrel will produce a spin rate of 3000 RPS (revolutions per second,) or 180,000RPM.

As time advances, the forward velocity of the bullet will decreases according to various formulas. The main factors in this decrease are BC (ballistic coefficient, how �slippery� the bullet is in the air) and SD (sectional density, how heavy the bullet is for its diameter. The higher the BC and the SD, the less rapidly the bullet loses its forward velocity. Indeed, the SD is used in the calculation of the BC, but I like to keep both out in the open. The air may be more humid or colder or the barometric pressure may be higher or the elevation lower, and those things do affect the amount of resistance the air puts up to the bullet, but is does not make too much difference. The single biggest environmental element is temperature, provided the elevation is the same. So I guess you could say elevation is also a big environmental element. So the bullet loses forward velocity as time goes forward. The bullet also loses rotational velocity, but this is shed at a MUCH lower rate than the forward velocity.

Now, as the forward velocity decreases and the rotational velocity stays about the same, the destabilizing forces diminish, but the stability remains the same. The bullet will remain stable, and will actually become more stable as time goes on, all the way until one of two things occurs. 1- The bullet becomes subsonic and the transonic passage knocks it around. 2- The bullet arrives at its destination. The vast majority of bullets will go subsonic and not be affected. I am here to tell you that I have shot enough bullets at long range that went subsonic hundreds of yards from the target to know that to be true. On the other hand, there are bullets known for their transonic instabilities; the 30 cal 168gr SMK comes to mind.

To say that a bullet is unstable at the beginning of flight and then becomes stable further down is fantasy. A bullet may well be buffeted by the muzzle blast, but it is very stable if its spin rate is sufficient for it to be statically stable at the muzzle.

Gene L. then asks: �bullets out of a 12 twist should rotate the same number of times regardless of caliber or forward velocity: i.e. one turn in every twelve inches, limited by the interior ballistics.�
As long as we are talking about inside the barrel, that is correct but once outside, that ceases to be true. Bullet spin is a function of time, not distance, once outside the barrel. Let�s continue with our example of 1:12 twist and 3000FPS. At the muzzle, the bullet is turning over once every 12 inches, at 3000RPS. Let�s say it�s a mild-loaded 52gr SMK. It takes 0.11 second for the bullet to get to 100 yards; in that time it will have spun around 330 times. Since there are 300 feet in 100 yards, if the bullet really turned over once every foot, it would have done so 300 times by the time it got to 100 yards. At 500 yards, the travel time is .78 second which would have caused the bullet to spin 2,340 times. 500 yards is 1500 feet and if the 1:12 rule would have been followed, the bullet would have spun over 1500 times.

IndyCA35 says: �For awhile there was a fad of using 90 grain bullets for 1000 yards because the 75 and 80 grain loads would not remain supersonic at that distance.� While I have not been able to keep the 75gr A-Max solidly supersonic at 1000 yards in a .223, I have absolutely no problem doing that with the 80gr SMK or JLK. In fact, they arrive at Mach 1.2+.

Then Remchester makes the following statement: �However the improved BC of plastic tip bullets actually reduces the need for a faster twist barrel because flight time it (sic) reduced. So plastic tip bullets should be causing a trend to slower twist barrels, at least when compared to appropriate twists rates for hollow point bullets of otherwise similar design.�

I am sorry to say that I have absolutely no clue what is being postulated here. I fail to see the relationship between faster twist barrels and reduced flight time. As I explained above the longer and more aerodynamic a bullet is, the higher the minimum spin rate required to stabilize it. This is exactly the reverse of what Remchester is stating. Certainly my (and those of multitudes of others) observations have borne out the need for faster twist for the longer bullets.

I can fill in some detail in this story. I'm a Service Rifle Competitor, and one of the major drivers in the fast twist, heavy 224 story is my chose sport. I've also been trying to track the history of the AR/M16 as a Match Rifle.

Back in the 1960's, Rodman Lab of Rock Island Arsenal put out some Match AR's for Highpower Competition as a test. They shot wonderfully at 200 and 300 yards, but suffered from wind at 600 because the heaviest available bullets were somewhere around 63 grain (and Semi Spitzered IIRC). They used 9 twist barrels.

About the same era, the gubmint was researching extending the reach of the 223 with their SALVO project. Bill Davis Jr, the father of the VLD was involved in that project and sent me a couple of sample bullets. They were made by a small company in California that went on to become Sierra Bullets.

To my knowlege, the Rodman Project never met the SALVO Project.

Someone already mentioned the rationale behind using a 7 twist in the M16A2. The optimal twist for the SS109/M855 62 gr round was actually 1/9 according to the USMC Major in charge of the project, but the Tracer round required the faster 7 twist in very cold weather.

9 twists would have remained the fastest commonly available twist to shoot the 68's and 69's (and the 7 twist M16A2's would have remained an anomaly) if it weren't for the Service Rifle Competitors pushing the development of the M16 as a Competitive rifle. It was first the privateers like Bill Wylde and Jimmy Knox of JLK in the 80's and 90's. But in 1992 or so, the watershed event occurred. The USAMU (Army team) was mandated by their commander to set aside their Match M14's and develop and shoot the M16 in competition. Somewhere along the line, Sierra made 80gr 22's mainstream when they introduced their 80 Match King. In 1994, the Army ran the table and cleaned house winning big with their M16's versus the previously dominant M14.

Everything went black from there and we saw the introduction of the 75's, 77's etc.

The 90gr bullets are an interesting side note. Someone mentioned a clean score being shot at 1000 yards. Rost495 and I had a ringside seat to the development of the 90 shooting AR. They can work...very well. But folk seem to have trouble getting them to work consistently. The JLK 90's have won the Porter Trophy (1000 yard Service Rifle at Camp Perry). They've also shot the first clean at the Interservices 1000 yard Match with Chris Hatcher of the Army on the trigger). On paper they're wonderful. Yes they require a 6.5 twist. 7 if you push them very hard. (FWIW, FTR_Shooter, I've been of the mind that the upsetting/destabilizing forces increasing at a greater rate than the stabilizing forces of increased spin due to increased velocity. Unfortunately of late, the empirics seem to be overruling me and I'm an empiricist!)
They're largely out of favor these days because of inconsistent results and even the Army has shelved them last I heard.

Here's a bit more empirics. Hornady says "for 9-twist" on the box of the 75 Amax's. Similar to Mr B, most folk I know find they need at least an 8. Similarly, Sierra says at least an 8 for their 80's, but I and more than a few others own 9 twists that have stabilized them out to 600 yards.

I'm surprised I haven't seen a single reference to Greenhill (or was I just skimming too fast?)

ChrisF, nice post, thank you. A lot of people think they can make up for a lazy twist with higher velocities. This can work up to a point, but as you increase the velocity, you also increase the minimum required spin rate for a statically stable bullet. This is why you can actually get a 22-250 or a .220 Swift with a 1:9 twist actually stabilize bullets that need a 1:8 twist in .223. If one does the math, one can see that spin will increase substantially with an additional 400-500 FPS, but it won't make much difference with an additional 100-150FPS.

Of course one must also realize that by dramatically increasing the velocity and therefore the spin rate, one can actually run into the problem of dynamic instability that can be engendered by less than perfect bullets. In this day an age, this happens less than it used to due to greatly enhanced manufacturing methodology.

As was stated earlier, the increased twist creates higher internal pressure and can probably cause more bullet damage if the bullet does not engage without slipping.

As for Greenhill, he's so last millenium. And that formula is really aimed (pardon the pun) at cylindrical bullets or yesteryear and lower MVs. With today's incredibly aerodynamic bullets the formula needs tweaking. When I look at my 80gr JLK or the 180gr JLKs, I am simply amazed at how streamlined these bullets are, and they need a good twist.

The 75 Amax is my go-to bullet in my 223WSSM with a 1/9 Pac Nor. The bullet behaves beautifully down to 3250 (at 5,000 foot elevation).

Sounds like that bullet is right "on the edge" as far as stability is concerned with a 9 twist. FWIW, Dutch.

That makes sense in the .223 WSSM, with its extra velocity--and at that elevation.

I haven't yet gotten it to shoot consistently in a .223 Remington with a 1-9 twist--and at 4000 feet. It has teased me now and then, but....

At 3250FPS, in a 1:9 twist the spin rate of the bullet is 4320RPS. I know that I stabilized it properly in my 1:8 twist barrel at 2800FPS, with a spin rate of 4200RPS. As we can see the spin rate is virtually the same, however because the WSSM has a higher velocity, the minimum spin rate required to stabilize the bullet is somewhat higher but this required increase is tempered by the higher elevation. However, I would think the WSSM should be able to stabilize it in a 1:9 at sea level.

It would be interesting to work up some sort of chart where you can see the minimum required spin rate to stabilize any given bullet in a range of velocities. I don't think such a chart exists. But my guess is that at sea level, a spin rate of about 4000RPS should statically stabilize the 75 A-Max at velocities under 3000FPS.

To calculate the spin rate in revolutions per second, I use the following factors:

1:7| 1.7 * MV = xRPS
1:8 | 1.5 * MV = xRPS
1:9 | 1.333 * MV = xRPS
1:10 | 1.2 * MV = xRPS
1:12 | 1 * MV = xRPS
1:14 | 0.86 * MV = xRPS

As elevation increases, the 4000RPS figure will decrease, but I do not yet have an algorithm for that. As temperature decreases the 4000 figure will increase, and again I am not certain of the rate. Conversely as the temperature increases the value will go down.

And again, above 3000FPS in muzzle velocity, the 4000 threshold will go up, but I am uncertain of the rate. Empirically, I do not think the increase in minimum spin is a 1 to 1 relationship with the increase in muzzle velocity, I believe it is less.

Robert Rinker agrees with you. His calculations show that the overturning moment of the bullet increases slower than the increase in stabilization of the bullet by increasing muzzle velocity. In other words, stabilization increases with muzzle velocity (all other things equal), but the increase is smallish. FWIW, Dutch.

I am I the first person to ask...A .375 H&H on prairie dogs? Do you find anything left?

With conventional big game bullets of 260-300 grains the bullet often just puts a hole through them, though the bllets do start to expand. With 220-grain Hornady flat-points the result is much like a .22-250.