question

[44man]

Other sites show a love affair with short barrels and large calibers.
Some have .500's with 2-3/4" barrels.
How can we find out what the RPM's of the boolits are with different barrel lengths? Take one load, say for the .500 S&W and keep cutting the barrel from 10" down to 2-3/4", how does the rpm's change.
Can a short barrel spin up a boolit to stability? Velocity is decreased of course but does the length of the twist in the barrel also decrease the spin?

[felix]

No, twist is twist unless there is some kind of intentional twist modification (gain-twist). ... felix

[Doc Highwall]

What felix said. The only two things that affect RPM of a bullet are twist and velocity. If you double the velocity you double the RPM and if you slow the velocity to half you will half the RPM and this is why when people try to shoot a heavy bullet at slow or subsonic velocities the bullet becomes unstable. For a bullet to be stable it needs a minimum RPM and with too many RPM the bullet is not over stabilized but it contributes to the bullets drift in relation to the twist.

[danski26]

Smith and Weson 500's have gain twist rifling i thought?

[AZ-Stew]

Spin (RPMs) is equal to:

velocity divided by twist rate (1:12, use "12", 1:9, use "9", etc.) times 12 times 60

3,000 fps divided by 9 = 333.33333 times 12 = 4000 times 60 = 240,000 RPMs

3,000 fps with a 1:9 twist gives 240,000 RPM

Barrel length has no effect, other than to change velocity.

Regards,

Stew

[stubshaft]

Smith and Weson 500's have gain twist rifling i thought?


The .460"s had gain twist barrels. The twist rate listed is the nominal rate. Twist rate will not increase as the boolit leaves the barrel. Gain twist barrels are usually designed to minimize the distortion of a boolit by the rifling or in the case of S&W to reduce pressure.

[bobke]

so, given you are able to compute rpm, is there a chart/data/info somewhere that would exhibit the minimum rpm that a bullet of a given caliber, overall and/or bearing length and weight should spin (hence velocity) to give it proper stability at range. should be able to determine velocity as it falls off at distance, therefore predict it's stabilty and likely accuracy, to a degree, at same. sure would be usable info.

[Doc Highwall]

Velocity will fall off a lot faster then the RPM. Their are some ballistic programs that determine the twist for a bullet if you know the length and play with the velocity. I have played with one and knowing the length of the bullet and the twist of my gun it gave me a idea of how slow I might possibly be able to shoot it.

[sagacious]

If the rpm is too high, the bullet can indeed actually become 'over-stabilized.'

The bullet follows a curved trajectory from muzzle to target, and consequently, the axis of the bullet must change and follow the changing curve.

If the rpm is too high, the bullet becomes gyroscopically 'over-stabilized' and the axis of the bullet cannot change to follow the curving trajectory. The bullet's trajectory and the bullet's axis will diverge, and the result is the bullet travelling (falling) sideways at extended distances.

http://hochmoulds.com/

For practical purposes, as long as we spin a bullet fast enough to be stable, a little exra spin doesn't seem to hurt. However, if you want the gun to shoot well at long ranges, you don't want the bullet 'over-stabilized' very much. As the trajectory arches over and starts down you want the bullet axis to follow the trajectory line. A bullet that does this is said to 'trail'. If the bullet is spinning too fast, it will continue to keep its spin axis pointed along the bore line and end up flying through the air partly sideways. That's not good.

[S.R.Custom]

...Can a short barrel spin up a boolit to stability? ...

Generally speaking, yes.

You often hear complaints that the twist rates of handgun barrels --Ruger single actions in particular-- are too fast, especially in the larger calibers. Indeed, a short, stumpy bullet like the specimens found in the .45LC don't need a 1:16 twist. But over-spinning a bullet is less detrimental to accuracy than under-spinning it, so the MFRs anticipate a worst case velocity scenario and put enough twist in the barrels to sufficiently stabilize a slow moving bullet.

Which is great for a 4-5/8" barreled .45. But I would think the twist rate of a 2.75" barreled S&W 500 would be entirely academic; I don't see how you could realistically expect anything resembling accuracy from such a set up, regardless of how fast you spin the bullet. :roll:


so, given you are able to compute rpm, is there a chart/data/info somewhere that would exhibit the minimum rpm that a bullet of a given caliber, overall and/or bearing length and weight should spin (hence velocity) to give it proper stability at range?

Yep... (Aside from twist, the only parameters that make any difference on stabilization of the bullet are length and diameter.) Do a Google search on "Greenhill Formula." You should get a hundred sites that tell you how to figure optimal twist.

[Down South]

Smith and Weson 500's have gain twist rifling i thought?
That is also my understanding but of the .460. If you were to cut the barrel back to before where the gain stops then the RPM would be different.

[44man]

Made me wonder because I looked down the bore of one of those short things and there is so little twist it looks straight so it seems the boolit would need to be going full velocity at the get go right out of the cylinder.
Since the boolit is going to skid some, that leaves about an inch or so of twist to spin it up. I can't picture it reaching full RPM's.
Reminds me of one of those rifled choke tubes in a shotgun that never works very good.
Besides paper figures I wish there was a way to measure it from actual guns.
It would be very interesting. If it could be measured, each load for each barrel length would have to be exactly the same velocity.
Now all of you know I do not believe in paper figures and predictions for the twist needed for each boolit. The proof is always in the shooting. Look at the Marlin .44, the barrel can be 3' long and it will not stabilize a boolit but what happens if you shorten it to 2", is there enough twist left to spin the boolit to the same RPM's as the 3' barrel given equal velocities?
Shorten the barrel to 1/2", can it spin a boolit? I have a feeling that there is a minimum length.
I do not want to argue in any way, just trying to think mechanically and get some thoughts about it.

[danski26]

I believe Dr Mann was able to measure the RPM of his fired projectiles in some of his experiments. If i remember right it involves "screens" of a rice type papaer placed along the flight pathe every 12 inches. For 100 yards. Did he notch the boolit then to see the notch in the hole cut through the paper??? Can't remember now.........

[S.R.Custom]

...Shorten the barrel to 1/2", can it spin a boolit? I have a feeling that there is a minimum length...

Mechanically (and theoretically) speaking, if the bullet is hard enough, yes, it'll spin the bullet. The problem with those shotgun choke tubes is they try to put the spin on the slug after it has developed its momentum. Not happening.

But the big problem with really short barrels is getting a consistent burn of the powder charge. If your velocities vary a lot from shot to shot, so will your spin rates.

But to indulge your thirst for real-world data, it's been my experience that "acceptable" handgun accuracy (2" at 25 yds) can be achieved with full power loads in a .38/.357 with a 3" barrel. The .44s and .45s need 4 inches. I've not fooled with the 500, but my experience with the SuperMags indicates 5" would probably be a minimum for the 500.

[Doc Highwall]

I wonder how much of that is related to expansion ratio making high muzzle pressures with lots of unburnt powder exiting with the boolit.

[S.R.Custom]

My guess would be just about all of it.

[Doc Highwall]

Yep mine too. Especially with slow and ball powders with low sectional density bullets.

[44man]

Mechanically (and theoretically) speaking, if the bullet is hard enough, yes, it'll spin the bullet. The problem with those shotgun choke tubes is they try to put the spin on the slug after it has developed its momentum. Not happening.

But the big problem with really short barrels is getting a consistent burn of the powder charge. If your velocities vary a lot from shot to shot, so will your spin rates.

But to indulge your thirst for real-world data, it's been my experience that "acceptable" handgun accuracy (2" at 25 yds) can be achieved with full power loads in a .38/.357 with a 3" barrel. The .44s and .45s need 4 inches. I've not fooled with the 500, but my experience with the SuperMags indicates 5" would probably be a minimum for the 500.
Good thoughts. That was what I was thinking about. Just enough would work but if the rifling is almost straight like a real short barrel, I don't feel it is enough. It will start the spin but then the boolit is out in the air.

[sagacious]

It will start the spin but then the boolit is out in the air.
Once the spin has started, that's all that is necessary. As long as the bullet does not skid in the rifling, all that's needed is enough constant-rate rifling to start the spin. That need only be an inch, or less.

That may not seem right, but it's a simple question of physics. Once the spin has started, only an opposing force will slow it. The instant the bullet conforms to, say, 1-in16 rifling, then it'll stay at that rate until opposed by a counteracting force. Even an inch of rifling imparts enormous torque on the bullet.

Modern rifled shotgun chokes are a somewhat separate matter. The dead-soft and undersized lead slug cannot reasonably be expected to conform prefectly to the rifled choke, as it is not even designed to conform tightly to the bore.

The definitive test for the shotgun-- and probably for this matter entirely-- is perhaps the Hastings Paradox barrel. That design is smoothbore, with only the last few inches of barrel rifled. A slug specially designed and fitted to the barrel is used, and as is famously known, it works fine for it's intended application. This argues strongly that once you get that bullet spinning, that's enough.

I read a test where a-- as I recall-- 38Spec revolver barrel was progressively cut down to less than an inch in length. Accuracy continued uninjured throughout the test, and did not vary with barrel length. Perhaps someone recalls this test and can retrieve it from an online source.

Many handgun cartridges specify a rate of rifling twist that results in 'over-stabilization.' The 9mmLuger and it's 1-in-10" twist is well-known for this, although over-stabilization might at first seem contradictory. At long ranges, the 9mm bullet fired at standard velocity though 1-in-10 rifling may be expected to tumble. Radar tracking of the trajectory has shown this, but given the short effective range of most hanguns, ballistic performance at extreme long range is obviously of no consequence.

I mention this because the concept of 'over-stabilization' is critically instructive in this topic. It illustrates that what one might expect is not necessarily what actually happens. One might expect that an inch of rifling is inadequate to impart the required spin, but preconception can cloud comprehension here, and must be guarded against.

As others have mentioned, rate of spin is constrained by rate-of-twist and velocity-- if our measurement is to be in revolutions per minute (rpm). Increase velocity, and the bullet is pushed though the rifling faster, and thus more rpm's.

Rpm's can be expected to vary as velocity is reduced by cutting a revolver's barrel shorter. Compensation for shorter barrel length is one reason that many handguns have a 'faster-than-needed' rifling twist rate.

Hope this helps, good shooting. :drinks:

[44man]

OK, I have to agree.
Now for the twist rates on most revolvers, my BFR's are much faster then what other guns have, at least the revolvers. Accuracy has been not only better but easier to attain with a larger range of boolit lengths. It is so good I wish I could afford a faster twist on my SBH .44.
The rates are;
45-70 and .450, 1 in 14".
.475, .50 AE and .500 S&W, 1 in 15".
.460 and .454, 1 in 20".
Desert Eagle .44, 1 in 18".
I feel M.R. has REALLY gotten this right.
Compare to the piece of junk Marlin .44 with a 1 in 38" rate! When I was young I could do better with a slingshot! 8-) I could shoot pigeons off a roof at 100 yards with a slingshot, something a scoped Marlin leaves to luck. And just think, there is no spin with a slingshot. Never figured out how to rifle them! :bigsmyl2:

[sagacious]

Compare to the piece of junk Marlin .44 with a 1 in 38" rate!
You're right-- seems there's almost no room for comparison.

One is inclined to believe that if the 1-in-38" twist rate seen in some carbines were increased to 1-in-16 or 18" those guns would immediately display a greater propensity for reliable and good accuracy.

The subject of rifling twist rates could just be too esoteric to be periodically reviewed critically by the firearm manufacturing industry at large. The Marlin rifles, et alia, are not the only offenders-- there is a long history of rifling rates chosen at seeming random, and of cartridges suffering in popularity as a result.

Or, in the case of some contemporary manufacturers, it could be nothing more complicated than it's easier/faster/cheaper to run the barrels through the rifling machines using a slower twist rate. That might explain the occasional twist rate disparity between those manufacturers offering popular firearms to price conscious dealers and sellers and buyers, and higher-end maunfacturers offering a performance-oriented product to a more discriminating market.