Progressive Twist

Progressive Barrel

Used LM-105 from progressive Twist. Caught after 500 m Flight.

Hello Lutz,

Was a bit amazed about the message on my mobile, (I was 'down' for a while, my mobile is not my best friend :-))) . I checked and checked but really didn't receive any emails from you after the mail below. Can you resend them ? Perhaps you have sent them to another "Ronny" or something ?

Regards, Ronny, Thursday, October 26, 2006 9:00 PM

p.s. Small question : 'progressive' 6.5 mm Lothar walther barrels, are they worth the money ?

Dear Ronny,

Progressive Lothar Walther Barrels give You much less Spin around the Bore axis then conventional Barrels. Therefore the Rifle moves less. Therefore You shoot with less Dispersion.

I do not know, how much they charge for that, but I actually talked them into it, after we won the Lapua Sniper Cup with such a Barrel (for the LM-105). I only noticed this beneficial Behaviour after I actually shot the Rifle. I asked Walther to make it in the first Place to reduce the Torque on the Drive Bands and never thought about the Spint.

When I first shot the SAKO TRG-42 with progressive LM-105 barrel with 19 g 8,5 mm Bullet from .338" LM I noticed less Twist than my Heym SR 20 N in 6,5x65 with 7g 6,5 mm Lutz Möller Bullet. Now that was a big Surprise: Less Spin from an almost 3-times heavier Bullet! I discussed that Experience with Walther Management and pointed out, Sport shooters would enjoy progressive Twist with the Advantage to generate less Movement upon the Shot around the Bore Axis i.e. less Spin than before and therefore would be able to shrink Group sizes. That is the main Advantage of a Progressive twist barrel against an conventional fixed twist barrel. You will feel the Difference when You pulled the Trigger and You will see it on the Target with smaller Groups.

So I am leased to hear, Walther has these progressive Barrels now in the Market and think, especially with the 6,5 x47 Lapua Cartridge some new Records are in the Pipe. May You shoot one?

Sincerely, Lutz

Varying Groove Width


The notion for a “squeeze groove” barrel occurred to me about 10 years ago, but I think the notion wasn’t new then… The idea is that as the projectile runs down the tube, you put work into the driving surfaces on the bullet &; heat the driving surfaces on the bullet up to their melting point. This causes band material loss, which when combined with material lost from local roughness of the driving edge of the lands, shrinks the portion of the bullet that originally occupied the barrel grooves more narrow, as the bullet approaches the muzzle. This leads to leaks, which are likely to be detrimental to dispersion, if they occur in random locations around the outside diameter of the bullet, shot-to-shot. Large artillery pieces avoid this problem by having a plastic obturator, which is free to rotate on the projectile, thus sealing the gas behind the bullet.

The notion would be to size the groove width to limit the size of the leak path, based on the expected band wear. The wear of the band is a function of the work you put into the projectile to accelerate it rotationally, along with the melt temperature of the band material. By shrinking the groove narrower as you approach the muzzle, you keep the leak path small, which helps keep the gas behind the projectile, where it belongs. Since the band wear of most small cal bullets is small, the change in land width with “normal” small cal bullets would also likely be small. However, Lutz’s design is outside of most “normal” small cal designs, so I think it’s prudent to at least have a look at it &; then decide later if it’s worth doing.

With gain twist rifling, there’s additional band wear and material movement at the bourrelets. Since you don’t start at the final rifling angle, you must actually put in more torque-time product to the projectile during in bore travel, causing more band wear than seen in a constant twist barrel, assuming the driving area and band material stays the same. The rifling angle change, by itself, is beneficial to keeping in bore angles low. Please see:

The other benefit that occurs when you use a squeeze groove barrel is you see an increase in the surface area of the land tops. This helps to keep the contact stress between the projectile body and the rifling lands as small as possible. The attached picture is what happens to a medium caliber bullet fired in a constant twist barrel that’s hot (Do not use without permission). The things that helped solve the barrel life problem for this medium caliber system are the same things we’re going to use here in small caliber to help keep the contact stress between the projectile and barrel low, the projectile in bore angle low, and the resulting dispersion small. Making the grooves more narrow near the muzzle means the lands are wider, which keeps the contact stresses low…

I don’t have any first hand knowledge of the application of squeeze groove rifling configuration in any applications, the reasons for not using it are mostly manufacturing limitations. With small cal weapons, hi rate production is done w/ rotary forging in many instances, which is incompatible w/ squeeze groove/gain twist. In medium cal, gain twist can be accommodated w/ broaching or ECM, but only ECM would be easily compatible w/ squeeze groove. You would need a reason to do this, in medium caliber there isn’t a need. For large caliber, gain twist has been tried w/ various degrees of success, but they have the room/volume to solve the leak problem w/ a plastic snap on obturator…

Hope that helps some, please let me know how you’d like to proceed.

Best regards, Jeff, Montag, 10. Dezember 2007 22:51

More thoughts on Varying Groove Width

Hi Lutz!

With your design approach where the projectile engraves in discreet locations, a gain twist rifling design will cause a “fight” of sorts among the locations where the land contacts the driving surface of the projectile as the rifling angle changes (before wearing/melting). Therefore, I’d recommend a robust drive location (as you already have) at the aft end of the projectile.

I’ll take your word that most small caliber barrels are more likely to die accuracy (e.g. dispersion) wise from throat erosion than from other sources. This means that the projectile jump to the rifling from its initial position in the case influences the in bore angle attained by the projectile. If that’s true, then we can make the projectile more robust dispersion-wise by paying attention to it’s mass properties (Transverse moment of inertia must be low for good dispersion, e.g. hollowpoint or plastic tip) and there must be an adequate separation between the projectile center of mass and the aerodynamic normal force center of pressure. This makes the projectile center of mass move aftward relative to a solid bullet of identical shape, keeping the load on the forward bourrelet (due to spin or gun dynamics) low.

Regarding the squeeze groove/blow-by problem, if we are to extract maximum velocity from the projectile/advanced propellant, we will be expecting higher than normal down bore pressures, along with lower than normal gas molecular weights, meaning the gas has an increased likelihood of escaping…. In fact, I’d like to see gas ports EDM’d into the bore near the muzzle (after a bit) to ensure the gas pressure as the projectile exits is low prior to releasing control of projectile from the barrel bore (lands). On the contact area of the lands, I’d only considered barrels with lands that have roughly vertical sides. As the projectile runs down the barrel, the material that originally occupied the grooves will no longer continue to do so. This problem is worse w/ gain twist barrels as the band wear increases with gain twist. The band wears at the driving surface and a leak path opens up between non-driving surface of the land and the trailing edge of the band. If the grooves get narrower as we move from chamber to muzzle along the barrel, this problem is eliminated or greatly reduced.

The object is not to save the “dispersion life” of the barrel so much as it is removing the potential for variations in leak past the projectile which may hurt the dispersion. I know from medium and large caliber work that such leaks can cause steel projectile bodies to engrave, making for very large dispersion if it occurs randomly. Since we’re trying to achieve the ultimate in small dispersion, it seems to me a small design effort to undertake, particularly since the band wear computations are imbedded in the interior ballistics analysis and don’t require additional thought or analysis… We can make the numbers on band wear and decide later whether the predicted wear is sufficient to warrant making the rifling a squeeze groove configuration….

Best regards, Jeff, Dienstag, 11. Dezember 2007 23:10

Gain twist groove width

Hello Jeff,

I suspected the groove widening comment would get a reaction, but follow the reasoning: We are not talking about an automatic weapon with the high heat, and attendant barrel expansion. Gas leaks are anathema in any circumstance. However, how much of a copper jacket needs to be displaced in the process of engraving, through a gain twist, to create a seal where accuracy is the prime consideration?

The driving side of a land progressively displaces (and seals) at the forward end of the projectile. The idling side of the land progressively displaces (and seals) to the rear of the projectile. If land width is constant, the displacement continues to seal on both sides. It just uses less of the total engraving footprint to create a renewed sealing surface. The land width can be reduced, and still form a renewed seal. The only question is, at what point do you trade lower friction (less fouling) for gas leakage? Maximum jacket displacement width should roughly equal the width of a functional drive band. The "tipping point", defines optimum land narrowing rate.

In the context of a gain twist, Lutz's projectile relies upon only the rear band to maintain a complete seal. The others should evidence an increasing gas gap, going forward, on the idling side as they are rotationally overridden by the prime band. A recovered bullet, with a failed rear seal, would be a good way to confirm expectations (... feedback Lutz?).

LM: Noel, You are right!

You are probably right Jeff about this bullet being treated, in terms of sealing, as having a single band. The attendant benefits of "squeeze groove" rifling geometry would all apply to the prime band.

I think it is purposeless to use moly on bullets generally, and this one especially (It will never remain in adequate concentrations where needed.), but treating the entire bore with a moly/alcohol suspension followed by burnishing with dry moly does make sense to me. My preference for an effective, stand-alone non-strippable, anti-fouling bullet coating, is the maintenance free aspect.

I have an aesthetic prejudice for odd numbered lands also... strange when you put it that way... but if four could be made to work, there are demonstrated throat erosion pluses. It would be best to push the bullet to the wear limit rather than the barrel... the bullet is used only once.

Do you have some data on land side angle effects? I have been more focused on fouling drawbacks than engraving wear. I would have it raised if I could.

With the twist rate of 1:7 at the muzzle, I do not see how vent holes in the grooves are going to miss the ground. Can you clarify what you are thinking of?

Noel Donnerstag, 13. Dezember 2007 02:35

Jeff and Noel,

when the Gain twist lands increase the Angle to the Bore they increase to shear displaced Drive bands. To keep at least one Drive band Land cut tight, the Drive bands shall be be of different Width, so sacrifice the weak ones and let the strongest live uncompromised.

Regarding Gas ports, look at Mündungswolke, Rückstoßbremse and .50 Cal hdp-Brake (with Dimensions). Those ports need necessarily to be EDM'ed. They can be bored or milled, provided the Bore will subsequently be reamed to remove the Ridges. This functions well. The Blow by is no Problem, provided the Spheridian from the Bore to the open is not largened too much. Propellant Gases leave the Muzzle at the Muzzle in a Cone with about 80 - 90° Cross section Angle. Compare that to the Muzzle brakes Opening in the Front, and You know how much You "shade" the Gas cone, or how much Gas volume You capture to blow through the Ports. Gas mass is different, as with Expansion the Pressure drops, but Speed in creased (Johan Bernoulli). The Pictures in Mündungswolke obviously do not show Gas mass distribution, but with Bernouli's Knowledge in Mind, You should understand, what really happens. Concluding efficient Brakes need not to be very long (like 10 Calibers, only 5 do well).

If we want to lap small Arms forward, we need much more Performance, to move the non serious People to listen to the serious People. We need much more Powder behind the Bullet (simple). To keep the Barrels short, we need more Pressure (simple). To offer a meaningful Service live, the Barrel must withstand this "Overbore Cartridge" much better, than existing Barrels (Noels big Task). To allow the average trained User to shoot such Power house with Confidence, Ease and hence good Results, the Beast musts be tamed (possible). The Taming is an encapsulated gas dynamic Recoil brake, as shown in .50 Cal hdp-Brake and described in Sleeved Brake. Unless You sleeve Brake and the Barrel with enough Volume, the increased from the increased Power Cartridge will annoy the Shooter so much to ruin his, Ease, Confidence and subsequently Ability to hit the far Target. This You may observe already in Black operation sniper contests, where .50 BMG Shooters usually perform worse than 7,62 x51 NATO or .338 LM Users. When shooting comfortably You rarely flinch, but if You fear the Beast You may. So I believe, we can only jump a big enough Lap forward, if we bake the Cake with all necessary Ingredients, not saving or substituting one.

The LM-105 design uses Drive bands with a trapezoidal Cross section and should be engraved by equal shaped Lands (60° Flank angle) to yield a rather asymmetric Force distribution, when Bore and Bullet are Center, no Matter whether tilted or axially aligned. Only the trapez-induced asymmetry guaranties Self alignment between Bullet and Bore. This is one Reason, why my Bullet shoot so well. So while the Drive bands should not leak anyway, Body engraving into the Lands is unlikely to occur with my Design, as the Bullet rides with greatest Force with the Drive bands on the Lands, less so in the Groves, as there much less Tension exits. To reduce Gas leakage it still might be prudent to manufacture the Bore slightly conical and utilize the "Squeeze Groove" approach in Combination. Remember, the Drive bands melt in the Bore and Grove as well, at least near to the Bore, when speed is high an heating Friction work greatest. We may loose

While multiple Drive bands foster the Hope, the Labyrinth may bar hot pressured Propellant gases to blow by too easily, no Data or Experience exits or e are known to substantiate that Hope. A conical Bore with "Squeeze Gove" will anyway address to Annoyances. As we are about to greatly prop up the Charge, much further than today possible with exiting steel barrels we (mostly Noel) will have to face those Wear an Tear, that currently halts the possible Performance = Muzzle Speed un small Arms.

Regards Lutz, 13th December 2007