It has often been said of certain rounds that they will shoot bullets of different weight and style to the same or quite similar points of impact. Legend has it that the 270 (with 130s and 150s) and the 375 H&H (with 270s and 300s) will do this. Anyway, does anyone have any info on how twist might affect this outcome? For example, if you choose a rather fast twist in a bid to (over) guarantee stability, will you increase or decrease your chances of having a more uniform point of impact across bullet weights and styles?

quote:

Originally posted by 9.3x62:
For example, if you choose a rather fast twist in a bid to (over) guarantee stability, will you increase or decrease your chances of having a more uniform point of impact across bullet weights and styles?

9.3,

I have four rifles which are "overdriven" with twist rates faster than standard. I have not seen point of impact tighten up between bullet weights that I have noticed. I can't answer if it is a parameter on my .375 as it is 1 in 12" (standard) and does throw 270 and 300 pretty close.

What I have noticed FYI is worse accuracy with bad bullets. If a bullet is out of balance for some reason it will show up on the target. My first fast twist in .284 was tried first when completed with 139gr bullets.. 2" groups @ 100 yards! Luckily I didn't give up right away and tried 130gr Speer. 1/2" @ 100! And then is drives tacks with the 175's which was my goal for the 1 in 8" to begin with.

I think for a hunting gun the overdriven twist is a great thing! African hunters are finding this out more and more listening to them. Even my buddy who has killed more African, Asian, Aussie game than anyone I know, especially with the 9.3 x 62, told me his pre-64 Model 70 re-barreled to 9.3 has a 1 in 10" twist. I asked him why.... "Straight and head-on penetration on the big stuff; especially with solids..."

This is my reasoning that 1 in 10" for the .35 Whelen is also the best for 250gr and up bullets.

The more I use overdriven twists the more I feel for the hunter they are an improvement in terminal performance.

BigRx

BigRx:

I would be interested in a controlled experiment to test the hypothesis of terminal performance of over-twisted bullets, both expanding and non-expanding. I find it hard to imagine that the difference between a 10" and 12" twist would be consequential (assuming that both do in fact stabilize the projectile), and in fact over-twist seems like it would be a liability with expanding bullets.

I think we can take a hint from exterior ballistics, wherein the bullet is "penetrating" the air. Is a sufficient twist for exterior stabilization a function of air density? Anyway, just thinking aloud here...

9.3x62, I'm speculating but doubt twist rate is responsible for this phenomonon. More likely is that different bullet weights and their powder charges create diffent harmonics in the barrel and by luck or genius(your call) the bullets are leaving the muzzle at complementary points in the oscillation or whip of the barrel. Not the same mind you, but at points in the sine wave that allow like impact at some range. I have a .358 Win. in Ruger 77 that does this with a couple of loads, one light and fast(so to speak), the other a lumbering freight train. Of course I could be full of crap but that's my guess.

quote:

Originally posted by 9.3x62:
and in fact over-twist seems like it would be a liability with expanding bullets.

This could be a possibility...... I usually hunt with premium bullets and don't see necessarily more frontal area just a straighter bullet path. Those bullets like Bear Claw and others that peel into segments will show torsional load transmitted into the petals and terminate in a twisted albeit intact full weight retaining form if the bullet is bonded.

The thread below this one has another's comments on this subject in: "Penetration - What factors are most important?"

Also in "Big Bores", the thread "Water Barrel Test", a thread on the first page by "500grains" quoting an "Andy" who lists several tests showing test results with solids in various calibers and enhanced twists.... vs. standards.

BigRx

Yes, I can imagine that solid penetration may be improved. But expanding bullets, even premium ones, would need to expand almost exactly uniformly or the extra twist will, it would seem, more quickly cause the bullet to yawn while penetrating (falling over itself perhaps).

IIRC, the 9.3x62 made its rep with a 14" twist - so any effect can't be that problematic.

Another question would be: What do we lose when we go with an over-twist barrel? Is it a free lunch or not? I suspect not...

quote:

Originally posted by 9.3x62:
Yes, I can imagine that solid penetration may be improved. But expanding bullets, even premium ones, would need to expand almost exactly uniformly or the extra twist will, it would seem, more quickly cause the bullet to yawn while penetrating (falling over itself perhaps).

IIRC, the 9.3x62 made its rep with a 14" twist - so any effect can't be that problematic.

Another question would be: What do we lose when we go with an over-twist barrel? Is it a free lunch or not? I suspect not...

9.3,

I see just the opposite than stated in paragraph #1 Preventing the "going sideways" with more twist.
This is my objection of Remington being stuck on the old .35 Remington twist (16") even in their Whelen barrels. Winchester had 1 in 12" even a century ago in the .35 Winchester for 250gr @ 2200fps.

Paragraph two; Lacking enough first hand experience I had a conversation with my friend mentioned before on 9.3 x 62 twists. Even mentioning the original 14" twist.
He told me he has used all three. I asked what he meant and he said he had two 12" twist guns. His opinion is good, better, best in the order he has used them.... 14, 12, 10. He sticks with 286gr bullets and hasn't used the 320gr Woodleighs even in his latest 9.3 x 62 with 10" twist.

Paragraph #3:

quote:

Originally posted by 9.3x62:
Another question would be: What do we lose when we go with an over-twist barrel? Is it a free lunch or not? I suspect not...

Sorry, I lost the third paragraph quoted above.

Yes you are correct here. The no free lunch I find is the quicker twists usually (most of the time for me) develop more pressure quicker than their slower counterparts. Even though I have more lead usually (deeper throat) I may not get to the velocity I could with the slower twist. I think that "Alan" test was experiencing this as well. Even with this handicap I think the faster twists helps the hunter.

BigRx

Let me explain my idea. Suppose (for demonstration purposes) that upon impact half of the bullet nose expands perpendicular to the axis of rotation. With a fast twist, this "arm" will trip the bullet into tumbling (sideways probably) that much sooner than the slow twist.

In general small bores with big case capacities are more likely to group different loads together and the bigger the case capacity the more likely they are to do it. A 270, 7mm Rem or 300 Win are more inclined to put different loads closer together than say a 30/06. They are also more likely to put a shot from a barrel with fouling conditions into the group.

375s with good barrels tend to shoot different weight bullets to different elevations. 458s do the same but to a much greater extent.

I think slower twists all round make the rifle easier to live with from an accuracy point of view and that includes putting different loads closer together.

I have never done enough to draw a firm conclusion but I think that twists that are far too fast for the bullet increase wind drift.

Mike

quote:

Originally posted by 9.3x62:
Let me explain my idea. Suppose (for demonstration purposes) that upon impact half of the bullet nose expands perpendicular to the axis of rotation. With a fast twist, this "arm" will trip the bullet into tumbling (sideways probably) that much sooner than the slow twist.

9.3,

I won't say this won't happen... I will say what I have found. Along with hunting, I have shot many experiments in other penetration mediums. You don't always get a recovered bullet on game; especially with the larger calibers. You do in wet phone books however.... I know it is not the same but still there is something to learn I think...

I think a tire on our car may react to your scenerio above. A large wheel weight flys off and the wheel shakes at 40 MPH and almost puts you in the ditch at 80 it is so violent!

Our fast twist bullet has more gyroscopic stability we have induced into it. Its longitudinal travel also tends to keep it point on. That's theory...
What I have seen is bullets losing petals, maybe even a whole side and still staying point on...

But I have seen this happen when the twist becomes too slow!
I wanted a grouse load for my Hornet. I thought a heavier 55gr Spitzer at a slower speed to allow it to just mushroom a little might be the ticket. At 1500fps into wet paper I would find the bullets sideways and the tiptorn to one side in the start of expansion! It would "flip" them sideways like clockwork! I ended up loading the same bullet and velocity in a .223 with a 10" twist. Here they mushroomed to about .35 caliber and always stayed point on. (Still hard on grouse)
A similar test with a .35 caliber wildcat in a Contender pistol (14" twist) had me testing 250gr bullets for minimum expansion velocity. For convience I used a .35 Whelen (Remington 16" twist) and loaded bullets to simulated down-range velocities of 1500fps. Again, I saw the tip peel to one side and the bullet immediately flip sideways! (according to paper trail) I ended up with a 200gr bullet load for the Contender and it is the only bullet weight that seems to remain gyroscopically stable in the 16" twist for me. (or slower mv in 14")

I don't see the fast twist flipping. I have seen them turn somewhat in an arc when influenced by something hard (bone?) But I have seen standard twists do this as well.

I plan a test with a steel plate 1/16" thick 2" into wet paper. I will leave the edge half way across and have it located to hit it with half the bullet's diameter. That should be an "extreme" test to one sided expansion!

I speculate the bullet will shed part or most of one side and remain head-on in an overdriven twist..... We'll see...

Best, BigRx

It seems like dramatic uneven expansion is going to destablize the bullet regardless of it's spin; in this case all the rotational force is then released, resulting in a more dramatic and unpredictable bullet behavior.

Imagine a spinning top - it you flick it while it's rotating fast, it destabilizes and reacts violently, spinning wildly out of control. Spinning the stop again, but this time wait until it is rotating slower - it too destabilizes upon flicking, but does not react nearly as violently.

This is all just conjecture on my part... I suspect the reality is that, within the recommended expansion velocity window (in fps) of the average bullet, the difference between a 10" and 12" twist in, say a 9.3x62, will be almost entirely academic.

quote:

Originally posted by 9.3x62:

Imagine a spinning top - it you flick it while it's rotating fast, it destabilizes and reacts violently, spinning wildly out of control. Spinning the stop again, but this time wait until it is rotating slower - it too destabilizes upon flicking, but does not react nearly as violently.

This is all just conjecture on my part... I suspect the reality is that, within the recommended expansion velocity window (in fps) of the average bullet, the difference between a 10" and 12" twist in, say a 9.3x62, will be almost entirely academic.

9.3,

Your top has no rapid logitudinal travel direction along with its spinning on its axis. Also you know when said top spins slow enough, it will fall over all on its own. (even with longitudinal direction)

A good measure I rely on is back to the wet paper. If a bullet is marginally stable, I notice the very last of its penetration will be canted or even sideways. A bullet loses RPM as it penetrates as well. It could even be a deep penetrating and a fine game load. But I know not to try a heavier bullet in that gun twist configuration. Optimally I strive for just enough twist to terminate point on. We are not talking "spinning top" differences just that some calibers are good and some aren't as to twist.

Based on this wet paper stability I shoot (or prefer) the following in lieu to standards in non-hyper velocity:

.25 caliber 120gr 1 in 9"
7mm caliber 175gr 1 in 8"
.338 caliber over 250grs 1 in 9"
.358, 9.3mm over 250grs 1 in 11"
.375 caliber 300gr and up 1 in 11" -- 1 in 12"OK
.416 caliber 400gr up 1 in 12"
.458 caliber 500gr..... Here probably 1 in 14" is fine, the standard. But a good lesson is here to be learned. The 500 grain .458 bullet started out at half the speed (1200fps) in 1873 with a 1 in 22" twist!!! It has gone up all the way to 1 in 14" twist with almost twice the velocity! Why? Big, mean, DG that need a lot of straight-line and deep penetration!

By the way, I don't have 11" twists.... I wish I did!
10" or 12" in your favorite with 286gr or lighter is probably academic as you say... 320gr go for 10" (11" if available) When one starts talking heavy bullets and 14" and 16" twists....... Well, let's just say I know I'll find them terminated sideways (maybe even deep) and I would rather not have that happen.

BigRx

There is no doubt in my mind about the fact that faster than normal twist rates benefit terminal ballistics with premium bullets at average hunting distances. The spinning top analogy is good to a certain extent as it is an experiment that was done, albeit somewhat crudely.

Setting up a disc to spin on the table of a vertical mill and spinning it at various speeds, we found that dropping a marble on the edge would destabilise the disc and induce a wobble from which it does not recover when spinning at low speed. As the speed is increased, the wobble resistance increased and at higher speeds, after the initial impact and wobble, it would stabilise again. I think that it would require a more powerful impact to completely destabilise a spinning top at higher speed than at lower speed and then it would destabilise more violently.

The accident of a rifle shooting different bullets, particularly different weights of bullets at different velocities, to the same point of impact is just exactly that: An accident.

The grouping of bullets within the "cone of fire" of a particular rifle is largely determined by where, within the oscillations of the muzzle, the bullet typically exits the barrel. Nothing about the rate of twist can in any way influence different bullets to exit the muzzle at the same point in its oscillations. If two different loads happen to hit the muzzle just right, then they very well may group around the same center at a given distance. Of course, if the two different bullets have different ballistic coefficients and different velocities, then the group centers will necessarily diverge with distance.

I'm not sure if I can find his site, or even if it is still up, but there used to be a ballistician type poster on here who did some experiments with the .458 WM and varying twists. End result was that a couple of steps up in twist from normal improved penetration of solids dramatically.

Stonecreek:

It is not an accident and can be done in many calibers. The key factor to bullet placement on target is not barrel vibration, although, of course they play a part, but time in the barrel and how much recoil any given bullet is generating during that time. A simple experiment with pistols shows this to be true. Take a .44 mag, sight it in with .44 specials at say 50 yards, and then shoot a reasonably hot .44 mag load and see where it hits. So by adjusting loads to adjust barrel time, it is usually possible to make different wts of bullets shoot reasonably close together at reasonably close ranges. As you said, get farther out there and the BC becomes more obvious.