RPM Threshold

[Bass Ackward]

Bass

The harmonics of the barrel have absolutely nothing to do with the defects in cast bullets as we cast them or the defects (unwanted obturation) caused by accelleration. Larry Gibson

Larry,

I bought Veral Smiths little Blue book that has cost me several thousands of dollars to date. My first purchase was a 700 Remington LSS in 30-06. First thing I did was to free float it. Next thing I did was try to shoot high velocity cast in it and I stuck the bolt. 300 rounds later I started HV work again.

I used the 56 grains of RL 19 that I always quote as this is where groups peaked. Group sizes were about 2 " with defective fliers running out to 6". So I bedded it with 2# of pressure. Groups came in a little to let's call it 1 3/4" and fliers stayed about 6". I upped the pressure to 4#. This time groups stayed the same and fliers came in @ 4". Hmmmm I says. So I upped the pressure to 8#. Groups came right in to about an inch and fliers now are 1 1/2". That's a bullet with the grand canyon running up the side.

My "guess" is that a bad bullet or load causes a rotational change that alters timing and that change alters barrel vibration causing the bullet to exit at wild points. Uncontrolled, that vibration is going to get worse the higher the velocity goes. Bedding pressure minimizes that effect. The only time I get truly wild fliers now is when my lube thickens and sizes down my bullets enough that they DO let go of the rifling. Then every shot is a flier, good bullets and all. But based upon my results above, I disagree that bad or unbalanced bullets are the cause of most fliers. Otherwise bedding would show no change.

MOST of all cast bullet damage is done from internal ballistics while the bullet is still in the barrel, and not from external ballistics at least until longer range. Even at high RPM.

[Idaho Sharpshooter]

Actually, everything we do to a cast boolit after it falls from the mould onto a terry cloth pad is bad for it. One reason the breech seater boys shoot so well, is they do nothing to the bullet until they seat it into the barrel with a .0003"-.0005" interference fit that has the effect of sealing the the bullet off from ignition gasses and causing deformation as the bullet obdurates to fill a throat and then pressure swages back down to fit the rifling. Their bullets are pre-obdurated, for lack of a more precise scientific term. Their bullets are also softer, which would allow a smaller void to be swaged closed inside the barrel. J-bullets are swaged twice or three times in the forming process...which pretty well swages any imperfections, occlusions, whatever term you prefer; out. Plus that copper skin is about 100Bhn, and they are always swaging an increase is diameter which helps.

Rich

[felix]

Rich, what you are saying is fine, but pay attention to these definitions:

1. ob·tu·rate: To close or obstruct.

2. ob·du·rate: Not giving in to persuasion; intractable.
Synonym: inflexible.


... felix

[AnthonyB]

Guys, this is a fascinating thread. Please don't let the different opinions halt the discussion.
Tony

[Rrusse11]

Guys, this is a fascinating thread. Please don't let the different opinions halt the discussion.
Tony

I'll second that!
My 2 cents; handmade cast boolits can rarely be as consistent as swaged/jacketed, lower rpms and slower powders mean less centrifugal deviation for cast. My slow twist 444 and 454 Colt Marlins have a speed/RPM threshold with heavy boolits that is clear. Never tried to test the upper end.
Cheers,
R*2

[Larry Gibson]

Bass

Whens the last time you EVER saw a bench rest rifle with fore end pressure on the barrel, even cast bullet bench rest rifles? If what you say is correct then all of them ould have pressure on the barrel instead of none of them. Those boys shoot to win. If it works they use it, if it doesn't they don't. The only time I've seen or heard of pressure on the barrel helping accuracy is with bad barrels or really light weight ones. There are muzzle attachments that tune the harmonics to the load or you can tune the load to the harmonics. However, good barrels shoot well without either when consistant loads are used. Had a fellow HP shooter who had a target rifle (6.5-308) that shot 1/2 MOA with MKs. He thought if he put a Browning Boss on it he could get 1/4 MOA so he had one put on. Well he tuned and tuned and tuned and couldn't get the Boss equiped barrel to shoot under 1 moa. Took the Boss of and it shot 1/2 MOA again. So does this one incident mean the Boss doesn't work? No, it just means that good barrels shoot well and in this case the Boss did not lesson the harmonics but increased them.

If you can keep a bullet with a "grand canyon running up the side" from being a flyer just by applying pressure to the barrel then you must part your hair in the right spot. I'd think you be too busy doing that magic (and making a fortune) to all the bench rest rifles and match rifles out there to be discussing this with me. But let us continue our conversation anyway.

"MOST of all cast bullet damage is done from internal ballistics while the bullet is still in the barrel, and not from external ballistics at least until longer range. Even at high RPM."

Bass, now when have I EVER said RPM damages the bullet? I haven't, and at normal velocity I don't think it will unless there is a gross defect in the bullet from some other reason. Of course the damage to a bullet is done in the barrel, that's called obturation. Obturation can be good or it can be bad but most obturation is bad because it creates inbalances in the bullet. The inbalance means the center of gravity does not coincide with the center of form. While in the barrel the bullet rotates (regardless of the twist) around the center of form. When the bullet exite the barrel the bullet rotates around the center of gravity. If the two do not coincide the bullet is inbalanced. It is during the external ballistics (that's outside the barrel) that RPM accentuates the inbalances creating yaw, pitch and wobble. The centrifical force created by the higher RPM causes the yaw, pitch and wobble to be worse the higher the RPM is. This translates into larger dispersion or larger groups. There is a very good description of this (pictures too) on pages 16-19 in Hornady's Fourth edition, Vol 1 reloading manual. Pressure on the barrel will have no effect on how the centrifical force causes this dispersion during the bullet during it's flight.

I shot your bullets too in a 30-06 rifle with 10" twist that is MOA capable with several loads both jacketed and cast. Using your load of 56 gr RL19 (actually worked up from 54 through 58 gr) and groups ran from 2.4 to 14". Most were in the 3-4" range. I actually got better accuracy with H4831SC of 2-3" groups in the 22-2350 fps range. But guess what? I also shot your bullet over 27 gr of 4895 (that's my accuracy load with the Lee C312-155-R). Your 154 gr LBT bullets ran 1905 fps/137,160 RPM and went into .98" (call it 1"). So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying.

So, yes I can get "reasonable" accuracy of 2-4" (if we call that "reasonable" with a MOA capable rifle) at a higher velocity of 2000-2400 fps with your bullet and your load. However, best accuracy of MOA (consistant accuracy also) comes in or under the RPM threshold. The higher the velocity the higher the RPM and the larger the groups. Not to hard to understand.

Larry Gibson

[joeb33050]

Joe, take out the ones with estimated velocities. Use only the ones with chronographed velocities.

There's a column on the sheet that's either C = chronographed, or E = estimated. I wish somebody would put this up, here.
joe b.

[Pat I.]

It seems as if this topic could go on forever with no agreement but I think if we look at what jacketed BR shooters are using for bullets it takes the theory out of the equation. The bullets they use are made on J4 jackets which have .0002 run out on the jackets compared to maybe .0005 with factory jackets. The reason they use them is to keep the CG and the center of form as close together as possible to maximize balance. It has nothing to do with internal ballistics and is all about the bullet once it leaves the barrel.

I suppose it might be possible to attain usable accuracy with a cast bullet at speed but I'd be willing to bet that in most if not all instances best accuracy will be found by following the basic principal of an RPM threshold and I also believe it could be predicted for twist rate and caliber with a little study. It has nothing to do with the bullet vaporizing or anything else, it's all about velocity and spin rate accentuating the imbalances built into cast bullets by design.

My 2 cents.

Pat

[45 2.1]

Rich-
The government does fine work, usually to prove a point and isn't shy about throwing money at things. The point is they didn't do that work with a bunch of cast boolits though. I've seen enough things that those ideas don't explain to know that it's only a theory. Until a theory explains everything and is repeatedly provable, it's only a theory and not a law. Too many articles busting it have been published besides the things i've done and seen for me to accept it covering everything. Too much smaller caliber work going on for a long time that is repeatable to believe RPM theory.

This RPM theory thing seems to be evolving before our eyes as the parameters change with every post.

[felix]

Everything is RPM limited, depending on its mass construction and its diameter. ... felix

[Idaho Sharpshooter]

45-2.1,

until I built my 22-378 Wbee Ackley Improved I would have agreed with you. But, since I was able to blow up every bullet I was offered with enough speed; and that case could hold enough powder to do so, I have to stick with what the arsenal and my personal testing has shown me.

Going to the .223 as a primary infantryman's weapon back in the 1960's may make more sense today in light of the RPM issues. Small caliber, and heavy for caliber bullets = fast(er) twist. Faster twist and 3,000+fps MV = lots of RPM. Minute %-age of spin rate decay = increased radial/axial tissue displacement on impact.

Rich

[Larry Gibson]

I copied Buckshot's response on another thread here because it's a classic example of the effects of the RPM Threshold and gives a good view as how it is misunderstood.

"A shooting buddy has a Winchester M54 in 22 Hornet. He can shoot cast almost as fast and accurately as his jacketed loads. Well, to about 2500 fps anyway . Heck, he even shoots pulled 40 gr 22RF bullets. These over light charges for fun and the occasional crow in his back yard.

I've shot cast 56gr boolits form my Savage M112 single shot .223 accurately to 2200 fps. It has a 12" twist. Actually the accuracy is still useable at 2400 fps but it isn't going to win any matches..................Buckshot"

A lot of casters, experienced and inexperienced, say something like "wow, he is shooting cast bullets accurately a lot fast than I am in my '06 and 6.5 Swede. Wonder why I can't do that." They can tweek the alloy, lube, sizing or case neck thickness all they want but the still won't get there ("there" being accuracy with cast as good as with jacketed at "speed"). The reason why, if we bother to look closely, is that a M52 in .22H has a 16" twist. That 2500 fps load is only cranking 112,528 RPM, close to the bottem of the RPM threshold. The Savage with the 12" twist at 2200 fps is cranking 132,000 RPM, up into the RPM threshold. They go over that and accuracy is lost. The .22H, in my experience since '75 with the Hornet and cast bullets, is that he is maxed out case capacity wise with a slow powder. Switching to a faster powder won't help but maybe an alloy change will alow another 100 fps or so. With the 12" .223 a change to a slower powder, an alloy change or different sizing might also get him another 150 fps before accuracy falls off but that's questionable as he already is well up into the RPM threshold.

Those are good example of how we see someone getting higher velocity with accuracy than we do and we think we should be able to do that also. You can IF you have the same twist. However, if your rifles barrel twist is faster you won't. It's all about RPMs when we want to go fast with accuracy using cast bullets.

Larry Gibson

[felix]

More ammo for this discussion: The BR games during the 80's in the Houston Warehouse concluded without doubt the 22's are more accurate than the 24's by at least 1/3. Problem is nobody can realize that accuracy within realistic ambients. Folks who could shoot that good never chanced it, and I would not either considering the costs of attending the matches. There is a big difference between 22's and 24's, 50 grains versus 70 grains, where each is twisted at the preferred 14. ... felix

[cbrick]

Fascinating thread and quite a can of worms. Here's some more food for thought.

Has anyone given thought to it being normally easier to get better accuracy at higher velocities with smaller diameter bullets (say the Hornet and 223 discussed in this thead aside from the well discussed twist rates? The "theory" being that any possible defect in a smaller bullet, be it from casting, sizing, loading or firing (internal ballistics), is closer to the center of axis during free flight (external ballistics) than it could be in a larger diameter bullet. This coupled with the bullet being lighter in weight it also has less mass spinning off center regarless of RPM's. Sectional denisty was the "key" referenced in the previously mentioned Handloader article.

Rick

[leftiye]

Any given size and shape defect is also proportionately bigger in a smaller boolit. This means that the smaller boolit must be more perfect- have smaller defects- to be as precise, relatively as a larger boolit. Granted it is closer to the axis, and recieves less centrifugal force. Another factor favoring smaller bores is the lower sectional densities of their boolits (barring extreme designs). They will tolerate both faster accelerations (higher pressures) with a given alloy, and more RPMS (as you state).

[felix]

Must weigh each 22 boolit to get high speed accuracy, dividing them into 1 tenth grain offset piles. The lower weight piles must be remelted. Take only three piles. If those three piles do not amount to 90 percent or thereabouts of all the boolits from that pot run, strongly consider another pot run. Segregate multiple pot runs using the same lead or not. A pot run is defined by each cold start. ... felix

[scrapcan]

I will add something not mentioned and may not be of any value to this discussion, but here goes anyway.

In Robert Rinker's Understanding Firearm Ballistics there is a discussion about the Greenhill formula and the constant that is used. It is on pages 141-142. They make an adjustment for density of bullet material and also for velocities greater than 1800 fps (stated in the text as a change form 150 to 180 to account for a 20% slower twist and a 20% longer bullet).

anyway I don't know if this adds anything to this discussion or not. I had wanted to post this in another thread that discussed this topic, but did not.

[joeb33050]

There's a lot of "why it does or doesn't exist" here, but little that I can see about demonstrating that it, the "RPM Threshold", does exist.
CBA records certainly include high RPM cases, although the average is lower.
I'll get some more CBA records put together.
Could the threshold merely be a combination of accepted twist rates and accurate velocities?
If the support is history, "The good shooters don't (often) exceed XXXXXX RPM.", could that be because of 12 twist 30 caliber barrels and the ~2000/2200 fps accurate velocity ceiling?

There's some reason that the SS breech seating guys shoot as well as or better than the fixed bolt guys in CBA competition. I don't know what that reason is, but have always imagined that bullet jumping forward AND having to start spinning, all in an instant.
Something makes it necessary to use harder bullets at higher velocities; could it be the damage at the initial jump on softer bullets?

Maybe the secret of CB accuracy lies in the gain twist barrel, from zero twist at the chamber end to minimum required twist at the muzzle.

Where can I read about the alluded to government testing?

Thanks;
joe b.

[Bass Ackward]

Bass

Whens the last time you EVER saw a bench rest rifle with fore end pressure on the barrel, even cast bullet bench rest rifles? If what you say is correct then all of them ould have pressure on the barrel instead of none of them. Larry Gibson

Larry,

Time for another deep breath. Bear with me. Every time you have to stop and think about what I wrote, think about why you NEVER see a free floated military barrel.

You said yourself, jacketed accuracy can be found at low, medium or high velocity. Jacketed throws enough friction that as the speed increases, the vibration speed increases and the barrel whip is minimized. No bedding is needed because you simply alter your powder charge or your powder speed to get the vibration pattern needed for consistent shot to shot exits.

The wine glass with your finger routine. Slow and easy and the pitch is low and soft. Push harder and the pitch of the tone gets higher and louder. Ever play the geetar? A real lose string can be seen vibrating and drawn tight never seems to move.

With lead, you must stay with the slowest powder producing your best velocity or accept a lower velocity (RPM). So jacketed accuracy is a different bedding game. Stop here. You have to believe this or your wasting your time reading on.

Lead bedding logic SHOULD be different. It doesn't "have to be" if you can get best accuracy within the harmonics range you want to operate in with the barrel you have. (HV 30 caliber seems to be 2200 to 2400 fps) Bedding won't help then if you are accurate and it could make things worse. But lead, you can only throw so much friction before your lube breaks down and you lead. SO if your best (most accurate) load effort CAN"T reach the proper harmonics, then you MUST change the oscillating frequency which may be more pressure or more pressure farther out. Bedding does this. The farther out vibration is controlled, the less pressure is needed. But if you don't tune your lead, you live with the result.

(It's the same with jacketed and thus the 115 grin Speer reference in the 8.66 twist. While people believe it's the twist rate overstabilizing, it's the increased barrel FREQUENCY from speed (the finger on the wine glass again) making bullet harmonics and bullet exit at a single point (accuracy) even MORE difficult. We prooved in several rifles that light bullets COULD BE shot MORE accurately than heavy for which the 8.66 twist was designed. All we did was change bedding.)

Now, when a lead bullet reaches the point that lube breaks down and it .... grabs, that barrel and the vibration pattern is changed dramatically. (You pushed your finger harder on the wine glass and got a louder sound) So you get a flier. If you bed so that that change is trivialized, then you get less of a flier. Or so that is my logic. Same thing for minimizing humidy effect on lube and temperature, and metal fouling too. This is why militaries grab their barrel and hold it so as fouling alters accuracy, the effect is trivialized. A benchrester simply cleans his gun. Bedding does externally what a powder or charge change does. It's as simple as that. Maybe not as well, but well enough for lead.

Works as long as you aren't destroying bullet because of fouling, then all bets are off. You have to understand once you reached that point. Based upon my 35 Whelen tests last month with 4-6 BHN harder bullets and how that affect accuracy, I am going to get a .308 sizer and harden my bullets a tad and see if I can't shoot HV this winter.

When you slow the twist rate, you are in effect cutting pressure that alters the vibration pattern to allow higher velocity (RPM)s before the lead becomes unstable changing vibration again. Why not bed?

One more point: Larry "So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying."

No all that prooves is that your gun shoots that bullet more accurately at that velocity. Free floated aren't ya?

[Bass Ackward]

To those who feel this is an argument, I am sorry. It certainly is NOT on my part.

And while I am speaking to Larry, I will get to SHOW him later on when he comes out. I am really speaking AT everyone else. Minor subjects here never address a cast bullet gun. Only the bullet and load.

But I believe a cast bullet rifle is a different horse from what makes a good jacketed rifle and I have stated bits and pieces of this over the years, and Larry is the first to call me on it, forcing me to elaborate.

There are cases where we get a rifle that just does well with cast cause the conditions are just right. When they are not, we live with the RPM chart.

Sorry if I ofend anyone with my lecturing which is really just trying to express myself in as little space as possible.