45 Colt Twist Stability in Lever actions

[RC20]

As way of explaining what I am pondering, I have the 45 LC in Conversion cylinders on a couple BP revolvers. I have wound up shooting those quite a bit.

So I am thinking, I always was intrigued with lever guns. How could not not just seeing a John Wayne Movie or the Rifleman? Dang, it was a BAR before there was a BAR!

So I have settled on two different mfgs model wise. Marlin who has made a come back under Ruger and Henry that continues to build good guns (granted there seems to be a poor example showing up in any of the levers, a bit tricky vs a bolt action.

Its not a serious endeavor , more a play with thing though I can shoot the 45LC out of the two models I am looking at. I do plan on a scope and both accommodate that (my eyes do not do well in iron sights). The eye pal works pretty well to allow pistol shooting.

The twist to the plot is that one has a traditional 1-16 (or close) and the other uses a 1-38 twist.

No one I have watched video wise does reloads and the stability extreme is obvious to me, that its quite a wild variation in twist. I have seen some bizarre groups and some mumbo jumbo involved.

My bullets that I would use are 200 and 255 gr lead, Speer and Hornady respectively. Doe not mean I don't load up some jackets stuff but only for the rifle.

Velocity is whatever works though I tend to run 850 fps in the Conversion cylinders and mostly shoot the same.

Ideally I shoot 100 yards though I have 75 and 50 yard options, means moving the target.

I ran stability calcs on the 1-38 twist and wound up with a Green label (good) but it came up at 9.0 something. Same with 1-16 and got down into that 1.5 area.

Does the calculator (JBM) reflect flat nosed bullets in that large a caliber or is that a different kettle of fish?

[Unclenick]

The calculator is very broad and assumes things, like homogenous material density. This one lets you enter more bullet specifics and gives you a plot of velocity v. twist for a stability factor of 1.5, which is a value good for accuracy. The bottom line is that many pistols have faster twist than is strictly necessary, and a 38" twist is still good at the problematic sound barrier in a standard atmosphere.

[Doug Lee]

Hello All,
Unclenick,

How does a particular twist relate to the problematic sound barrier?
Thanks,
Doug Lee
*

[Unclenick]

As you may know, the speed of sound reflects the limit of speed that air molecules move, so it is a speed limit for air. Its value primarily depends on temperature because temperature is proportional to the square of how fast the molecules move in Brownian motion. This reflects how fast the air molecules can pass momentum to each other when they collide.

The "sound barrier" is an abrupt increase in drag on any object moving through the air because the air in front of it that is being compressed by the object's forward motion can no longer escape compression by running forward away from it. As a result, a high compression zone appears in front of the object that is only free to escape laterally, which takes more energy from the object for the same reason blowing air through a constriction takes more effort than blowing it through a wide space. That lateral expansion forms the edges of the shockwave associated with faster-than-sound travel.

For a bullet, the abrupt increase in drag requires a quick change in its yaw of repose (angle into the wind) in order for the bullet to precess nose-downward at a rate that keeps it tracing (remaining nose-forward into the headwind). If it didn't make that change in yaw, it would stop tracing and would have headwind drag that was off-axis, causing it to tumble. So, what happens as it goes through the sound barrier is that the yaw of repose is initially off, causing drag to try to make the bullet tumble. To prevent that from happening until precession has time to correct the yaw of repose, the bullet needs extra gyroscopic stiffness, which comes from having faster spin than it needs on either side of the barrier.

By the way, air travels at different speeds over different parts of the bullet. The air going around the curvature or along the taper of an ogive is going faster than the air along the sides of its bearing surface. So you can actually start to see some increase in drag coefficient at about 80-85%, depending on bullet shape, and transitional effects can continue up to 120-125% of the speed of sound (aka Mach 0.80-0.85 to Mach 1.20-1.25), so that whole range, including the speed of sound itself, where the relative magnitude of the effects peaks, is called the transonic velocity range.

Because drag increases as the square of velocity times the drag coefficient (which, in turn, is dependent on shape and Mach number), the rate of spin needed for stability is generally higher with higher velocity (except for that transonic yaw shift, where it is disproportional). Fortunately for shooters, a fixed rate of twist produces faster spin as muzzle velocity increases, and this actually overcompensates a little bit, which is why some bullets that are unstable at moderate velocities will become stable if you shoot them faster. A second influence is that the rate of spin in flight is decayed by air surface speed friction due to rotation, and not by headwind air speed friction. As a result, spin falls off much more gradually than forward speed does, and this tends to increase downrange stability.

An exception to the above is when a bullet shape has a dynamic instability (a tendency for the shape to try to overcorrect for perturbations). The 168-grain Sierra MatchKing, for example, when launched from a 10" twist at typical 30-06 or .308 W velocities, will go maybe 700 yards in a crosswind before falling into the transonic velocity range. At that point, it starts tumbling and keyholing. This seems to be due to a combination of spin rate and its 13° boat tail, as the 9° boat tail on the 175 grain SMK doesn't do it, and it doesn't happen to the 168 reliably when fired from a 13" twist Palma barrel.

There is a lot of funny stuff in ballistics.

[Doug Lee]

Unclenick,
Thanks for that!
Doug Lee
*

[RC20]

I continue to be confused.

The calculator gives some odd results, I think I am seeing a salability of 6 on up.

This is for a 45 caliber pistol bullet, aka 45 Colt. Flat nosed in both cases, Hornady 255 (.454) and a Speer 200 (.452).

Speer has what I call a Truncated nose, the Hornady quasi round nose but flat tip.

I am trying to sort out if which barrel twist aka 1-20 or a 1-38 is suited to these bullets.

If you get over 1.5 is that good?

[Unclenick]

At a gyroscopic stability factor of 1.0, a bullet nose trying to fix its position into the headwind never stops its coning motion (the nose orbiting the yaw of repose) but never gets worse, either. Below that value, the orbits get progressively larger until there is so much overturning drag that the bullet tumbles. With a stability factor above that value, the coning spirals inward smaller and smaller toward settling at the yaw of repose. The practical effect is that if the bullet is subject to perturbations, meaning a sudden change in air density or wind on its way down range, when the stability factor is between 1 and 1.3, these perturbations can still throw the wobble bigger, and groups are just not great. A common recommendation for rifle "hunting accuracy" is no lower than 1.3 and no higher than 3.0. A value of 1.5 is considered optimal for rifle accuracy by many, but 1.4-1.7 is a widely accepted range for high precision rifle shooting. Bigger values exaggerate the lateral bullet jump at the muzzle at bullet exit caused by bullet tilt and/or mass asymmetry around the bore axis, which, in turn, depends on RPM, and high-power rifle bullets usually spin at hundreds of thousands of RPM, so it doesn't take much bullet center of mass offset to cause groups to open up. However, modern precision match bullets can often be overspun quite a bit and still shoot very well as long as you don't accelerate them so fast you get core stripping or centrifugal distortion.

Pistol bullets are often fired with higher stability factors. The ranges are shorter, the precision level is lower, and the lower velocities make for lower RPM from a given twist. The added gyroscopic stiffness from spinning more than they strictly need to prevent tumbling lowers initial yaw and avoids the need to give them distance over which to settle that yaw.

[mehavey]

1,000 Words....

[RC20]

Nice group. Not just the size, the pattern is good.

I get what Unclenick is working around, but I need specifics as I am looking at 1-16 twist up to 1-38. Its not the theory or stability calculator, its the application and does it apply for what I am asking?

In the case of your group, 1410 FPS and 330 gr bullets, nice.

What I am asking is will a Marlin 1894 with its 1-38 twist do that with cast lead bullets in the 200 and 255 weights? And likely to be 1000 FPS as they are 850 in my pistols. Again plan on the same rounds and loads in both.

My brother has a 45-70 in Marline that shoots that well. But that is 300 gr or bigger bullets and of course no 45-70 hand guns.

I will scope the rifle (whichever I got with). Henry is a more traditional 1-20.

[mehavey]

By the stability twist calculations for 200gr/0.58" and 255gr/0.68" bullets both at 1,000fps in a 1:38 twist ...
Very comfortably.

But I'll take some MagTech 250gr L-Flats (~900fps/Marlin) out tommorow and see (again).

[RC20]

Be glad to see real world data in an area that is relevant to me.

Right now I can't get the rifle I was looking at, so its a strong interest but no purchase until stock returns.

I would think Marlin would shift more to 45LC as 357/44 types are available, make the guns that sell faster!

[Unclenick]

Originally Posted by RC20

What I am asking is will a Marlin 1894 with its 1-38 twist do that with cast lead bullets in the 200 and 255 weights?

Oh yes. Anytime you shoot a physically shorter bullet made of the same material and general profile shape (stubby rather than long and sleek in this case), your stability increases. It's longer bullets that need faster twist. But 200 and 255-grain bullets will be shorter than the 330, and so should be fine.

[mehavey]

1000 Words:



Nothing to write home about -- but nothing tumbling either at low/Cowboy speeds.
(Note: MagTech advertises these as 761fps out of a 4" Test barrel

I'll load some "quality" rounds w/ some soft/30:1 Accurate 45-250D's * and 4.5gr Red Dot and see what happens.

* Classic 454190 Clone

[Doug Lee]

Mehavey,
Good shootin' !!!
I've got an 1894 in .44 mag.
Doug Lee
*

[RC20]

(Note: MagTech advertises these as 761fps out of a 4" Test barrel

I think that is pretty good as well. Certainly starts to give me some idea of what I am looking at. Also what a longer barrel does with and for a lower speed load.

All the reasonable cost Marlins in 45 Colt are gone. I can get a Henry but that is $900 range.

So thoughts shift to 41 magnum as I shoot that a lot as well. Same issue, $200 more expensive.

[Unclenick]

Keep in mind a 4" SAAMI test barrel has a 0.008" wide vent between the throat and chamber to mimic a barrel/cylinder gap, and the gas that bleeds off there will not be lost in a rifle barrel, giving the latter a bit more pressure and total gas than a revolver has.

[mehavey]

Fascinating.
`Did not know that.
So it's actually/truly mimicking a 3" revolver barrel (if tested in revolver mode).

On that note, should we assume traditional rimmed handgun rounds are tested with revolver-ported test barrels as above ?

[Jim Watson]

No, it is a 4” barrel after a chamber faking a cylinder, then the gap. Pictured on SAAMI www.

Standard revolver test barrel is 7.7” straight for most calibers.

[mehavey]

Got it.
True representation...
(if) done as SAAMI standard)

[Unclenick]

Yes. Several revolver cartridges offer the manufacturer the choice between a 4" revolver mimicking test barrel and a longer single-shot pistol mimicking barrel. SAAMI distinguishes the two as "vented" and "solid" in their test barrel drawing dimensions. Most commercial ammo velocities clearly came off the 4" revolver mimicking barrel, while you'll see Hodgdon's loading data usually uses the longer single-shot barrel, which can fool people comparing the loads to commercial box performance data. In the special case of the 44 Remington Magnum, the pistol standard has a 5.763" total length vented barrel (4" after the vent gap), an 8.275" single-shot barrel, and if you hop over to the rifle standard, there is a 20" carbine-length test barrel for it, too.

[mehavey]

2,000 Words . . . .



I don't think there will be a twist problem... even with "DEI" Cowboy loads.

[RC20]

Agreed! Nice group.

Any idea why the wide different twists on most levers vs the Marlins way out there 1-38? I think I saw 1-28 on one, most were 1-16 or 1-20

[mehavey]

I've found the 38 twist on th 45 Colt is "sufficient" for any practical bullet load.

On the other hand, that same 38 twist on my '94/44-Mag will not shoot anything
much past 280gr in that 44 family. (But it does shoot that weight cast very well)

[Unclenick]

Shorter ones are adopted from handgun barrel standards. 1-16 has been 45 Auto since Browning designed it in 1904, and is now the SAAMI standard handgun twist for all 45 cartridges except 454 Casull, which has a 24" twist like the old 45 Schofield did. Browning originally used a 200-grain bullet in 45 Auto (the Army changed it to 230 grains), which closely copied the ballistics of the 200-grain 45 Schofield load, which had worked in the Philippines, except that the Schofield used the old 0.454" 45 bullet diameter. I don't know why he upped the spin rate of the bore, other than the short-range reduction in yaw, which would increase penetration by reducing initial yaw to prevent tumbling in the target medium.

20" is standard for 44's and for 45-70 rifles, so it is suitable for 300-grain and up bullets in 44's and for up to 500 grains at higher rifle velocities, but is not required for the shorter 250-grain bullets.

44 bullets will be slightly longer for weight than 45's, but if you stick to really blunt shapes (almost wadcutters), 44 Mags have been used with 325's and 330's and worked. Length has proportionally more impact on stability than weight does.

[RC20]

That puts the Marlin 1-38 for the 45 Colt on the far extreme.

Still scratching my head.