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Posted: 11/3/2009 3:09:13 AM EST
Let's say you have a situation where you are pushing the envelope on the bullet your barrel can stabilize –– say, a 69 OTM in a 1/9. To take another example close to my heart, 55 gr 22-250 in a 1/14 barrel. In both cases, you are probably at the ragged edge of what the barrel can stabilize. Works with some guns, not so well in others.

Here's the question. Assuming you get the bullet to stabilize initially –– nice tight consistent groups at 100 yds, will the bullet generally remain stable thereafter until it goes subsonic?

Many bullets wobble and then self correct as they leave the bore as the center of rotation changes from center of form to center of mass. Assuming you get this initial stability, then usually you're okay at longer ranges.

I know for any particular gun the answer is "go shoot it at 400 yds and see", but I'm interested in people's general experience. Once you get initial stability, even in a marginal load, do you tend to be okay downrange or with a marginal situation, even where you get good initial stability, will you start tumbling fairly quickly down range? I'd like to be able to do my load workups at 100 yds. Ultimately, I'll verify the final loads at longer ranges, but I don't want to do that every single time if I can have some reasonable initial confidence that if I'm stable at 100, I will be stable at 300 or 400.
Link Posted: 11/3/2009 4:43:41 AM EST
If the only change is slowing down, the bullet should stay stable until at least the transsonic transition. How stable it will stay after that is a crap shoot-how stable was it? How ragged was that ragged edge? A transsonic transition is fraught with turbulence, even at the scale of a .224" diameter bullet, so odds are the bullet will destabilize and yaw as it transitions. How much and how far are completely unknown.

Since this transition is so rough on bullets, it's best to choose (or load) a bullet that matches the rifling much better than "dancing on the ragged edge."
Link Posted: 11/3/2009 10:55:44 AM EST
Link Posted: 11/3/2009 11:38:41 AM EST
Originally Posted By Forest:
Originally Posted By jgttrey:
Let's say you have a situation where you are pushing the envelope on the bullet your barrel can stabilize –– say, a 69 OTM in a 1/9.

A 69gr OTM in a 1:9 is NOT pushing the envelope.

Shoot 75 & 77 gr OTMs, now that is pushing the envelope.


Agreed. A 1X9 barrel should have no difficulty with 69 grain bullets. In fact, the 1x9 twist rate owes it existence and popularity largely to the appearance of the 69 gr. OTM bullet in competition. Generally speaking, most any 1X9 twist barrel should stabilize anything up to and including 73 grain bullets, with particularly good barrels stabilizing up to 75 grain.

But, as for the root of the original question, there is much greater resistance (friction) to the forward motion than there is to the rotational motion of the projectile. And then there is gravity. For all reasonably practical cases, the bullet will either transition to subsonic or hit the ground long before gyroscopic forces (or lack thereof) would become a problem.
Link Posted: 11/3/2009 11:42:11 AM EST
Originally Posted By Forest:
Originally Posted By jgttrey:
Let's say you have a situation where you are pushing the envelope on the bullet your barrel can stabilize –– say, a 69 OTM in a 1/9.

A 69gr OTM in a 1:9 is NOT pushing the envelope.

Shoot 75 & 77 gr OTMs, now that is pushing the envelope.


Fair. Most of what I've read says 69 gr is the max reliable weight, but some stabilize higher. Fine, but my larger point was that for any given bullet weight, you have to have sufficient spin by virtue of velocity and twist to get the initial stability. Once you get it, will it generally stay stable until it goes subsonic, when all bets may be off? Put another way, I'd like to know if I'm stable and consistent and accurate at 100 yards is that also an indicator that I'm going to stay that way thereafter until subsonic.

From the other reply, it sounds like having the right twist is mostly about getting that initial stability in the first 10 feet and, if you do, you're probably o.k. That would have been my guess, but is why I asked.

This is relevant to making ammo decisions about my ARs, which are both 1/9 and 1/7, but more important for my 22-250. It's 1/14. I know from testing it will stabilize 55 gr pills at 100, even 60gr Partitions. Not all do, but mine does if they are hot. Lower than 55 gr is no bueno in my book for deer, even though they are small hill country whitetail. Knowing that 55 gr is really pushing it for a 1/14 22-250, I was curious about the downrange performance as I work up loads. I don't want something that looks good at 100 but destabilizes at 200. Yes, I shoot targets at longer ranges and can check this myself and will as I work up more loads, but before I do I was curious about the feedback. With all the ammo experts around here, and the relevance to ammo generally, I posted here instead of the reloading forum.
Link Posted: 11/4/2009 4:15:32 AM EST
In general, if it is stable at 100 yards it is going to stay that way.

1. As someone already pointed out, although bullets lose a lot of forward velocity as they get down range, they do not lose a lot of rotational velocity.
2. A bullet needs to be stabilized because the aerodynamic center (the point where the total aerodynamic force can be thought to (modeled to) act through is forward of its center of gravity. As the bullet slows down (forward velocity), the magnitude of the aerodynamic forces which may tend to cause yaw decreases with the square of the change in velocity (in other words, the magitude of the force drops much faster than the forward velocity does). Therefore, less rotational stability is required to overcome the effect of destabilizing forces (a gust of crosswind, etc).
3. The impact of point #2 more than makes up for the small impact of point #1.

It has been a lot of years since my aerospace engineering days. Anyone feel free to correct me if my logic is flawed.
Link Posted: 11/4/2009 10:28:55 AM EST
69 and 75 gr OTM Sierra Match kings do not tumble out of my M-15 A4 1-9 SPR at 400 yards. The accuracy of the 69 is much better than the 75 at this range. Stability maybe?? Who knows.

My next SPR is going to have a 1-7 as I would like to take advantage of the heavier bullets that are much more available these days.
Link Posted: 11/4/2009 4:25:42 PM EST
Originally Posted By bowslngr:
In general, if it is stable at 100 yards it is going to stay that way.

1. As someone already pointed out, although bullets lose a lot of forward velocity as they get down range, they do not lose a lot of rotational velocity.
2. A bullet needs to be stabilized because the aerodynamic center (the point where the total aerodynamic force can be thought to (modeled to) act through is forward of its center of gravity. As the bullet slows down (forward velocity), the magnitude of the aerodynamic forces which may tend to cause yaw decreases with the square of the change in velocity (in other words, the magitude of the force drops much faster than the forward velocity does). Therefore, less rotational stability is required to overcome the effect of destabilizing forces (a gust of crosswind, etc).
3. The impact of point #2 more than makes up for the small impact of point #1.

It has been a lot of years since my aerospace engineering days. Anyone feel free to correct me if my logic is flawed.


I would add that with small bullets like those used in 5.56mm rounds, the mechanical center of rotation due to rifling can be significantly different from the axial center of mass; small bullets made in mass quantities WILL yaw more due to a mismatch between the axis and center of mass far more than small bullets made in high precision processes (like high dollar hunting bullets). Since these bullets are so small physically and weigh so little, any small variation in either quantity will have a strong effect on the bullet's stability and accuracy.
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