Variable Twist Rate - Engineers Speak Up

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Has a variable twist rate ever been tried in a rifle? In other words start out with a slow twist and slowly increase the rate so that the last few inches of the barrel do the final speed up.

Seems to me that would be less stressful on the bullet and maybe allow a faster spin than otherwise possible due to stresses weakening the jacket due to a constant twist rate.

Comments?
Engineers?
Anyone, Anyone... Buehler?
 
I saw a progressive barrel at the Sioux Falls gun show a few years back on a sporterized Mauser. Evidently, the owner had it made so he could shoot cast bullets? At least that's what the guy told me. I don't know what the twist rate started at or ended at. I think he said the barrel was made by Lothar-Walther.

Ryan
 
Been tried a fair bit, as far as I've read. Cost of manufacturing and little perceived benefit seems to be the reasons it's not prevelant.
 
rbernie said:
Been tried a fair bit, as far as I've read. Cost of manufacturing and little perceived benefit seems to be the reasons it's not prevelant.
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+1. Lots of people have experimented with it over the years, and the near-universal conclusion has been that it's a waste of time and money. So-called "progressive twist" rifling is much more expensive to manufacture, and offers no real benefit in practice.

The bullet's rate of spin as it travels to the target is determined by the twist rate at the muzzle. It doesn't matter whether the rifling starts slow and builds to that rate, or is constant throughout the bore -- whatever the rate is at the muzzle determines the spin rate of the bullet. Therefore, the only possible benefit of progressive twist is that it could theoretically prevent bullets that are moving very quickly from stripping and failing to engage the rifling at all.

That's almost never an issue, however, because in a well-built rifle with suitable ammunition, the bullet engages the rifling before it is moving very fast. Think about it -- although the bullet may exist the muzzle at 3,000 fps or more, it engages the rifling at a much slower speed. In precision rifles with custom ammo, the bullet is already touching the rifling before the cartridge even fires. Therefore, the bullet will accelerate into the rifling and stripping is not an issue.
 
It's called a gain twist, and it's use goes back well over a hundred years. You can still get some barrelmakers to make one for you, but if there was any advantage to them, we would all be using them, wouldn't we.;)

Don
 
Gain twist in Caracno

The original Carcanos had gain twist rifling.
The Series 1891 Model 38 short rifle (21") and Model 41
long rifle (27") made before and during WWII are
supposed to be conventionally rifled.
 
krochus said:
It is called Gain Twist Rifling, Italian carcano rifles used it as well as the new 460xvr revolver by smith and wesson.
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Gain twist and progressive twist are two different names for the same thing.

The fact that S&W uses it in the 460XVR kind of proves my point. As I said above, there is generally no need for it because in a well-designed rifle with appropriate ammo, the bullet engages the rifling before it is moving fast enough that it could strip out and fail to spin. In a high velocity revolver, however, that's not the case. Upon ignition of the charge, the bullet has to pass through the cylinder throat and across the barrel/cylinder gap before it enters the forcing cone. It will engage the rifling at the end of the forcing cone. By that time, it may have already accelerated to a high enough velocity that it will strip if the initial twist rate is too high. Thus, if a high velocity revolver, there may be some advantage to starting out at a lower rate to prevent stripping the bullet, and accelerating the twist toward the muzzle to ensure a terminal spin rate that will provide adequate stability in flight.
 
Has a variable twist rate ever been tried in a rifle?
Click to expand...

Variable or gain twist is almost as old as rifling. It was quite popular back the 19th century, but no one can prove it really works better than straight twist.

Years ago I built a rifling lathe and rifled several muzzle loader barrels -- one of them with gain twist. It really isn't hard to do.
 
You asked for engineers. Well, my engineering degree didn't cover anything directly dealing with firearms. Gunsmithing seems to be a self-taught or OJT mostly. Not much demand in this neck of the woods for universities to put out those degrees or teach it at all.
However, if there was an Engineering of Firearms class, I bet a lot of students at A&M would have taken it. I would have.

Anyway, the above responses make sense. You are looking for an end result out of the muzzle. If you can get the same result more economically, so be it. Anyone seen an acceleration/velocity curve of .223 though a 20" barrel?

I wonder if you could see more difference in artillery shells?
 
Anyway, the above responses make sense
Click to expand...
Indeed they do.

My thought was that the total mechanical stress placed on the bullet would be less using a gain twist thus reducing damage to the jacket caused by the increased stress of going from no spin to rapid spin very, very quickly.

Less damage to the jacket would result in bullets being able to achieve higher spin rates without failing due to centripetal force.

Gyroscopic action being what it is I imagined that the greater the spin rate the less lateral drift due to wind forces.

I guess not though since the concept seems to be old and the technique well explored. If there was any great benefit - as someone pointed out - gain twist would be much more common than it is.

Seems I am way more than a day late and at least a few dollars short.
 
My thought was that the total mechanical stress placed on the bullet would be less using a gain twist thus reducing damage to the jacket caused by the increased stress of going from no spin to rapid spin very, very quickly.

Less damage to the jacket would result in bullets being able to achieve higher spin rates without failing due to centripetal force.
Click to expand...
Actually, gain-twist rifling has its own jacket-marring issues, since the angle at which the rifling engages the jacket changes as the bullet moves through the bore (meaning more area of the bullet jacket gets marred). So that's probably not an advantage over conventional rifling, unless (as with the new S&W revolver) the alternative is serious jacket stripping.

I do wonder--would a gain-twist barrel tend to be more prone to copper fouling, or not?
 
I do wonder--would a gain-twist barrel tend to be more prone to copper fouling, or not?
Click to expand...

I suspect the tooling marks would dominate the gain twist factor -- a rough gain twist barrel would foul more than a nice, lapped constant twist, and vice-versa.
 
Another downside, as seen with some carcano rifles that were converted to carbines while having gain twist rifling: Lets say you have a barrel made that's 24" long and later decide that a 20" barrel would be better, You now have a rifle that will no longer stabilize bullets properly.
 
Look in back issues of Muzzleloader, or in the Foxfire books (about mountain crafts.)

My lathe was made of 2X6s -- a shallow box, 12'X1'. That's the lathe ways.

The carrier was about 4'X1'2 or so -- 2X6 sides, 2X4 ends. This simply drops over the ways and slides up and down.

The rifling guides were made from oak, turned on a lathe. About 4" in diameter and just over 4' long. It had the ends turned down to ride in holes drilled in the ends of the carrier.

The rifling was determined by winding a thread around the rifling guide (there's a formula for determining gain twist), snapping it, and spraying the thread with paint. When the paint dries, the thread is peeled off, leaving a mark that shows the rifling. A router bit in drill press, and a makeshift jig was used to make a deep groove along this mark.

When the lathe is assembled, a board with a hole in it fits in grooves in the ways. The rifling guide passes through this board. A hardwood key, screwed to the board makes the rifling guide turn as the carrier moves forward and back on the ways.

I made my rifling rods of brass. The cutter was L-shaped, and cut from an old file. One leg of the L was pinned in a slot in the rod, and a screw through the end of the rod allowed me to adjust height of cut.

You assemble the lathe and clamp the barrel to the end of the ways. Run the rod through the bore and pin it in place in a hole drilled in the end of the rifling rod. Adjust the cutter so it barely scratches the inner surface of the barrel. Lubricate copiously. Do not pull the cutter all the way out of the bore at the end of the stroke -- it can only emerge on the forward stroke.

After several strokes, it will stop scratching. Push it out of the bore, unclamp and rotate the barrel (a quarter or eighth of a turn) and continue. When you've scratched out all the grooves, adjust the cutter and start on the first groove again.

It takes a day or two to rifle a barrel.
 
In the Rifleman

a number of years ago there was an article (maybe by C.E. Petty) which used physics formulas to show that a bullet cannot 'strip' or skim over the rifling, pretty much no matter what, so long as there's any rifling left at all.
 
Vern--

Great explanation of barrel making. It's always great to hear from a craftsman.

As for the physics of gain twist, let me toss a guess out there. When the bullet engages the rifling, it undergoes a sudden angular acceleration. Anything that acts on the bullet acts on the barrel. Maybe gain twist is a way to reduce the rate of change of angular acceleration and therefore prevent sudden torsional loading on the barrel--loading that can cause minute oscillations about the length of the barrel? Hell, maybe these oscillations can interfere with the rotation of the bullet and cause instabilities that only serious, diehard benchrest geeks would even notice or care about.

In the case of the S&W 460 XVR, I wonder if the round would be powerful enough to untwist the barrel liner if it was wasn't progressive rifled.
 
In the case of the S&W 460 XVR, I wonder if the round would be powerful enough to untwist the barrel liner if it was wasn't progressive rifled.
Click to expand...

In all the experimentation with gain twists, no one has yet to show a significant advantage. I think the torque issue is probably not operational -- the torque must be expressed anyway, and the dwell time is too short to measure any difference.
 
Hi Vern,

You probably right.

My thinking was not so much the maximum value of the torque (which, as you say, will be the same with either rifling design), but the intensity of its application. Progressive rifling may allow the torque to build more slowly and prevent an "impact hammer" effect--like the difference in the load on a propeller shaft when you dump the clutch compared to gently running the motor up to redline. This probably makes little to no difference in thick, one-piece rifle barrels, but maybe the thinner two-piece barrel design of XVR is more susceptible to sudden twisting forces. Of course, I am assuming that there is a technical (as opposed to marketting-driven) reason for S&W to add this expensive feature. But this is all total speculation on my part.
 
Shear_stress said:
Hi Vern,

You probably right.

My thinking was not so much the maximum value of the torque (which, as you say, will be the same with either rifling design), but the intensity of its application. Progressive rifling may allow the torque to build more slowly and prevent an "impact hammer" effect--like the difference in the load on a propeller shaft when you dump the clutch compared to gently running the motor up to redline. This probably makes little to no difference in thick, one-piece rifle barrels, but maybe the thinner two-piece barrel design of XVR is more susceptible to sudden twisting forces. Of course, I am assuming that there is a technical (as opposed to marketting-driven) reason for S&W to add this expensive feature. But this is all total speculation on my part.
Click to expand...

The question is, how slowly is "slowly?" The dwell time is so short that for all intents and purposes it might just as well be an ordinary constant twist.

And making gain twist barrels is no more expensive than making constant twist barrels -- the set up and processing costs are the same.
 
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