Bullet Stability, or Lack Thereof

[RiverRider]

Because Rem700's thread got me to thinking about stability, I thought I'd run this past you guys. I've often wondered about this incident.

When I was much younger and even less wise than I am now, I decided to try some W748 in my pushfeed Model 70 in .223 Rem. I believe I was shooting 50-grain BTs at the time.

I had been using H335 and never saw any sign of excessive pressure all the way to the maximum load...it seems like the max was 26.5 grains...but that's neither here nor there. I made a foolish decision and assumed that since the max load of H335 shot the best in my rifle and was safe, that the max load of W748 from the same manual should be the most accurate AND safe in the same rifle.

So, I find myself out at the range one beautiful Idaho morning. I post my targets at 100 yards and walk back to the firing line with anticipation. Not soon enough, I find myself behind the rifle and making some noise.

I guess my scope wasn't all that great...I couldn't be sure what I was seeing through the glass as I fired successive shots. And my chronograph---I was certain something was wrong because I'm not supposed to get 3550 fps with a 50-grain BT out of a .223 with a 22-inch barrel. After four shots, I decided that my chronograph might be right, and I really need to get a close look at my target so I call a cease-fire (I had the 100-yard range to myself) and amble on down there.

Every one of those 50-grain BTs had gone through the target sideways. They all hit the target, but there was nothing I would call a group.

I always figured (duuuuuuhhhhh!) that the tumbling and excessive velocity (and attendant pressure) were related, but I've never figured out the connection. I think my best guess to this point is muzzle pressure too high, causing the bullet to tip, but I really don't know. So I am here asking for alternative theories for my edification.

Anyone KNOW what happened? Good theories would be welcome too.

 

[Oldtrader3]

I had a .22-250 once (a Remington) that put bullets through the target sideways but only up to 52 grains. The velocity of the 50 grain bullets that I tried in this rifle was close to 3900 fps. Some of the bullets never made it to the target at all. I always figured these light bullets the were over stabilized and came apart from heat and gyroscopic spin forces in the spinnning bullet. Many of these bullets would not only key hole but they would leave lead spatter on the target around the bullet holes or keyholes.

 

[RiverRider]

Well, I was hoping for more feedback, but I guess no one has anything to add. Not that I don't appreciate your response, Charlie.

I saw no evidence of bullet integrity compromise when I had this happen, and really would not expect to at 3550 fps. I find myself searching for a little more understanding, and since you have an engineering background maybe you can answer this: would internal friction in the core due to radial "swelling" cause the rotational velocity to diminish enough to make the bullet lose stability? That's about the only thing I can come up with considering the relatively low velocity I was getting (compared to the hotdog .22 centerfires).

 

[nvbroncrider]

I've heard stories of pushing light bullets fast and having them come apart in flight or keyhole and not be accurate but I've never seen it myself.

 

[JD338]

It sounds like the 50 gr BT was not stabilizing in the twist.
Keep in mind that in addition to the weight, the length and profile of the bullet will determine whether or not it will stabilize.
Sometimes you can over drive the bullet with a higher speed to get stabilization.

JD338

 

[Oldtrader3]

Somebody like Army Natick labs needs to study this issue. They probably will not because it invloves softpoint bullets. What is needed is some really highspeed multiple camera film of these bullets in flight at various ranges between the rifle and the target. There are huge centrifugal forces at work and I would not be surprized to see jackets slipping against core rotation and creating tremendous friction heat inside the bullet jacket, on the surface of the lead core. Plus thes forces may even create bullet yaw and instability?

 

[Guy Miner]

RiverRider - how heavy a bullet would that rifle stabilize? Do you know the rifling twist?

I've seen the .22 plastic tips fail to stabilize - I think because they're sometimes a tad longer than their lead tipped, or hollow point counterparts. These bullets produced the keyholing/sideways impacts and poor groups you mentioned. The best example I have of that is a .22-250 Dad and I were working with. His plastic tipped 55 gr bullets wouldn't stabilize. We switched the load to 52 & 53 gr hollow point bullets and suddenly the same rifle was producing really tight groups.

Another "twist" on the subject: :grin:
Yes, I've seen bullets come apart in flight. It was fairly common ten or fifteen years ago when more target shooters started messing with very fast twist barrels and pushing the longer bullets faster - all to get better long-range ballistics. Wasn't unusual to have some of the real fast .223/5.56 twist barrels (1:7 or 1:8) destroy various target bullets. A little blueish "puff" 10 - 100 yards from the rifle could sometimes be seen as the bullet came apart, from being spun too fast.

This actually spurred Berger to go to heavier bullet jackets for their Target bullet line - while interestingly leaving their Hunting VLD's with the thin jacket.

A long time ago I had a Nosler 168 gr competition bullet come unglued from my very sedate 1:12 twist .308 Win. The bullet jackets were damaged in that box though, it was visibly obvious, and I loaded and shot the bullet out of curiosity. Could hear a "flapping" sound as it went downrange towards the target. Never made it there...

Dad destroyed some .25 cal bullets long ago - shooting them from his .257 Weatherby. Simply pushed them too hard and the bullets came apart in flight. That was long ago and I don't know exactly what kind of bullets they were.

FWIW, Guy

 

[AzDak42]

With the limited data available, it's really hard to make any educated guesses, there are just too many variables.

- I've gone out and bought factory ammo loaded with the same bullet help me figure out if it's my load or the bullet itself.
- Was it a bad batch of bullet? Even the best companies can have a hungover employee doing QA at some point. One group of rounds with a bad concentricity problem and you might see this.
- What about your hand loads in another 223? You could have a rifle with a slightly oversized bore pared with some slightly undersized bullets, and you could get weird behavior.
- Also, a bad batch of powder that was hot + a heavy charge and you could see bullets losing integrity as they spin themselves apart.
- How would it behave with longer/heavier and shorter/lighter bullets?

The fun of this sport is getting to tweak all the variables in the quest for perfection. The pain of this sport is having all the variables to tweak to find perfection!

 

[RiverRider]

Thanks for all the participation. There's some interesting stuff here!

As I previously mentioned, the rifle was a pushfeed Model 70 and I would assume it had the standard 1:12 twist. It shot the 50-grain BT reasonably well at 3400 fps propelled by H335. It was not remarkably accurate with that load, but at least it made round holes in the target. It was only when I loaded W748 WAY too hot that I witnessed the keyholing.

I've heard many accounts of the old conventional 50-grain softpoints vaporizing when pushed to extreme velocities out of .220s and .22-250s. I've never done so myself, but I do plan to find out how a 50-grain SX holds up to 3900 fps out of my current .220 Swift. Bullet disintegration is pretty self explanatory but this thing I've been pondering seems mysterious.

 

[onesonek]

Yep, too many variables, proving each gun is a law unto itself. I would have to play with such myself for awhile, in order to figure out what may be the cause. Guns can just be plain fickle at times, but I have found it's best to load for the guns wishes, rather than my wants in general. In agreeing with all and any above, I can also see muzzle pressure to crown relationship as you suspected.

 

[FOTIS]

I had a Swede mauser that would keyhole everything under 160 gr.....

 

[onesonek]

Puzzling, ain't it Fotis. Then again, who said life was to make sense all the time. lol

 

[RiverRider]

That IS an interesting point, Fotis. I've seen several illustrations of over-stabilization, and they depict the over-stabilized bullet's attitude not remaining tangent to the arc of flight, but rather maintaining the same attitude all the way to the target. I wonder if the degree over-stabilization actually can cause tumbling. I know some people say this is so but I always believed they were mistaken.

 

[BeeTee]

As I previously mentioned, the rifle was a pushfeed Model 70 and I would assume it had the standard 1:12 twist. It shot the 50-grain BT reasonably well at 3400 fps propelled by H335. It was not remarkably accurate with that load, but at least it made round holes in the target. It was only when I loaded W748 WAY too hot that I witnessed the keyholing.

I've shot more than 1000 50gr Nosler BTs through my Rem 700VLS 223 with a 1:12 twist, mostly with W748. It's the most accurate bullet I've tried. Never had a keyhole, no matter hot hot they were loaded. In the loads I've assembled, 28 grains of W748 filled the case to the top of the case neck. Tapping it allowed the powder to settle to the junction of the shoulder and neck (100% load density).

With the lots of W748 I've used, I settled on 27.7gr, which produced about 3450-fps muzzle velocity. I've never seen evidence of excessive pressure with any 223 load I've assembled. No loose primer pockets, no extractor marks, no cratered primers, no change in bolt lift effort, etc... Some of my oldest Winchester cases have been loaded more than a dozen times, and they don't appear any different from when they were new. BT

 

[Oldtrader3]

I think that you have to remember that a varmit bullet is rotating up to 240,000/second. This put terrifically high gyroscopic forces on a bullet. I think with my .22-250 and the bullets of the 1960's, that they just had jackets too thin to hold together under the rotational stress. If the jacket breaks out of its ogive profile with a slight jacket tear or something, under those force vectors of the lead inside the jacket that it is going to lose stability and tumble with possible self destruction to follow.

 

[RiverRider]

That's something I've considered, Charlie. Quite honestly, it's hard to imagine how a cup-n-core bullet even stays together at all and it seems to me that if anything goes wrong, the bullet jacket would fail and the whole thing would disintegrate.

It makes sense to me that what I experienced was the result of some kind of structural failure that upset the bullet. But I suppose I could ponder this forever and come up with theory after theory, and it would be pointless.

 

[AzDak42]

That's something I've considered, Charlie. Quite honestly, it's hard to imagine how a cup-n-core bullet even stays together at all and it seems to me that if anything goes wrong, the bullet jacket would fail and the whole thing would disintegrate.

It makes sense to me that what I experienced was the result of some kind of structural failure that upset the bullet. But I suppose I could ponder this forever and come up with theory after theory, and it would be pointless.

But it's fun to speculate about.. so not entirely pointless.

 

[onesonek]

That's something I've considered, Charlie. Quite honestly, it's hard to imagine how a cup-n-core bullet even stays together at all and it seems to me that if anything goes wrong, the bullet jacket would fail and the whole thing would disintegrate.

It makes sense to me that what I experienced was the result of some kind of structural failure that upset the bullet. But I suppose I could ponder this forever and come up with theory after theory, and it would be pointless.

I agree

Also, I believe most know all or part of this, but thought it might be helpful to some that might not. As it's related to the discussion in part.
It's thought that the ideal twist for a bullet would be a gyroscopic stability factor of 1 (each caliber/weight/profile has their own GSF), but one needs to account down range rpm degradation, so a GSF of roughly 1.5 would be better if figuring for any range beyond spitting distance for most other than short very range guns. GSF is not really of concern except for those into extreme range or comp shooting, as most production guns cover that with their standard twist rate. Oh, and of course maybe those that notice their bullets flying sideways

I haven't looked into it deep enough to say at what GSF that specific bullets typically begin to be overspun enough that might create stability issues. That can be a fair amount of work on my ballistic program, if I don't have ready access to the profile measurements, but have done my own when having a barrel made with general bullets in mind for it's intended use. I do know when in doubt, it's better to be faster than slower however. Again, most standard twist rates of production guns cover the range of hunting bullets produced.

If faster twist is used, say a 7" or 8", specially with any low sectional density for caliber bullets used, then it takes more range for the yaw to stop, often refered to as sleep. Then too with overspining rpm far more than needed, creates not only excess yaw around the bullet's axis, but creates rotation around the bore's axis as well. Yardage of when the both axis sleep depends on how much excess rpm the bullet has. This is why one can see poor accuracy up close, but much better somewhere down range. Ex., my son's friend purchased an AR .223 a few months prior to Matt and I. In playing with the 55 gr. Varmagedon, he/it was shooting 1.5-2" @ 100yds. Suspecting the over stabilization on with 9" twist when asked, I told him to try further away, and low and behold, at 200 it was shooting 3/4" groups at 200 and stayed just under 2" at 300.

I always went with the general rule that tumbling is most often connected with, too slow of twist for profile. However, I believe Murphy's Law is possible with most anything, so I won't rule out tumbling happening outside accepted rules of stability. Also I can see a good rifle with normal twist rate for bullet that tumbles, it could be possible a slight uniformity issue with the bullet base in a production lot bullets. It wouldn't take more than .0001", and likely less to cause a tumble with all that pressure behind it In my thoughts. But that could be a stretch, and would be a rarity with todays bullet production controls I think also. But at the same time, it seems to me the more complex a design, leaves more things to go wrong too.

While not really tied to anything discussed here for the most part, with the exception of accuracy, when purchasing a new to me rifle of any kind, I always carry a magnifying glass of some kind (I have a rather inexpensive 10X Loupe, something like this to closely look at the crown. It don't take much of anything suspect on the crown, to really muck things up.

It just takes that part of the rifle out of question should some issue come up. Bad crowns usually destroy accuracy of an otherwise good barrel,,,period. Not often, but I have seen some horrendous crowns even visible by the naked eye get by quality control on new rifles. The 10X glass will show any defect well enough. If one feels more comfortable checking with a 20X or higher, won't get any arguement from me. And it's just wise I think, to check crowns real close on used guns.

 

[RiverRider]

Is decay of rotational velocity documented? I have a hard time accepting that it is anything more than negligible and insignificant because that motion is all relative to the bullet itself and displaces nothing. A mechanical gyroscope takes a long time to spin down from full speed, and it has to spin against the friction of bearings. The friction of air will cause forward velocity of a bullet to decay but there is very little friction from the air on the lateral surfaces of a bullet in flight.

 

[Antelope_Sniper]

The decay of rotational velocity is virtually zero.
It has something to do with the law of conservation of angular momentum....