Rifling conundrum

[Streeker59]

For the techies and rifle builders out there, can you answer me this ?

 

When a round is fired in a rifle, two forces begin to act upon the rifle. These being Recoil from the pressure of the burning powder forcing the bullet out of the case and into the rifles chamber and Torque from the bullet engaging the rifling and starting to turn. Newtons third law being the explanation of each of the two forces.

As such, if one has a slower pitch to the rifling, there is less torque to contend with. The problem of course is that a slower pitch will limit the length and therefore weight of the bullet that one can use.

 

What if you have a barrel that has a slow pitch at the breach end that increases at the muzzle.

Example... A .308 with a 26" barrel. The pitch starts at 1 in 14" at the breach and increase in pitch until it reaches the muzzle at say a 1 in 10" turn.

 

My question is this... Would there be an advantage in having a barrel made with an increasing pitch to limit or control the amount of torque experienced by the rifle? Does such a design exist already ? Should i be getting down to the patent office in case i'm a genius ?

 

Also, when one looks at the rifling in a barrel at the breach end, there are small / short ramps where the rifling begins then the bullet has to bite hard into said rifling to begin its journey down the barrel.

 

What if the rifling was cut or buttoned so it was very shallow at the breach then gradually got deeper to its ultimate depth a centimeter or so from the chamber. Would this not smooth out the pressure curve and lead to a lighter felt recoil?

 

Am i talking ###### ???

[brown dog]

Sorry....google 'gain twist rifling'

[Streeker59]

Thanks brown Dog.. Guess nothing's new under the sun or Google..eh ?

[Andrew]

Gain Twist, indeed. It's been around for a while. As to the twist, torque, recoil thing: Who knows? I have shot Remington 700 silhouette rifles in 308 caliber with 1:7 twists and 1-14 twists and with equal loads there was no detectable difference in recoil. ~Andrew

[Streeker59]

Ah well, back to the drawing board...

[Guest martin]

Nice try Streaker mate,it did sound like you had cracked something there,better luck next time...............martin

[Guest varmartin]

How about ...Adrenalin..?....

[MarinePMI]

What? No mention of gyroscopic drift?

 

Seriously, I've only heard of the torque affecting a handgun. More accurately, it kept ejecting spent casings right into the shooters head/face. The solution was a left hand twist barrel to cause the pistol to torque the opposite direction. Dang if I can't remember the name of it, it was a custom/limited production pistol still being made today...

[ds1]

Just to add on the long lead to the rifling - freebore, Weatherby rifles do or used to feature it.

 

David.

[Ronin]

There is no getting away from the fact that torque has some effect on rifles, espacially those that are being shot from front reast and rear bag, the foreends on these rifles are 2.5 to 3 inches or more wide, some of this is because they "ride the bags" better this way, another reason is to stop the "twist or torque effect"

 

Developments in stock technology has seen lowrider design from laminate, composite and carbon fibre makers all to reduce twist and retain a reliable and accurate return to battery for the next shot.

 

I have had several phone conversations with the guys at Bartlein, Frank being particually helpful.

 

They have made several T twists (gain twist) barrels for mtach palma shooters in theb states using a 13 or 14 twsit start and finishing in a 12 or 11, this small gain has shown increased velocity and less twist effect although how the twist effect (torque) can be measured is beyond me - maybe the shooters have percieved this, maybe not, but the advantage of this technology have had success shown by medal wins.

 

Gain twists were originaly made in the days when smooth bores becam rifled barrels, the idea is that old,

 

Modern times has seen gain twist technology return in 1990's by Lothar Walther specifically developed for shooting lathe turned solids in 338 lap mags using LM105 bullets.

 

These started out at 14 and ended in 7 twist.

 

The barrels were expensive and the success measurable by wins in competition in specialised shoots in finland in 1995 - these are a very specialised barrel...

 

Bartlein now offer transition twist with any calibre, they have as iv'e written earlier that some 308 shooters are using very moderate increases in twsit rate and having success.

 

The larger calibres (8.5mms) that have been made have had mixed success, some shoot well others dont, this may be a reflection on bullet design rather than Bartlein barrels as some lathe turned solids are better than others...

 

 

I would have no problem ordering one for a 308 myself in say 1-13 increasing to 1-12 for 155g scenars, but I wonder if the advantage is percieved rather than real.

 

I have a t twist barrel on order at the moment for the 338 that is 1-14 ending in 1-675..

 

It may be an expensive folly, but I like to experiment and I have now obtained a reliable supply of good quality solids.

 

 

 

The Bartleins are an extra $150 for T twist rifling, if you order a barrel its a reasonable expense and with the extras like fluting and T twist it does make an expensive piece of steel,

 

 

Pays your money and makes your choice I guess.

 

 

FWIW, Bartleins make exceptional shooting barrels....

[brown dog]

Gain twists were originaly made in the days when smooth bores becam rifled barrels, the idea is that old,

 

Modern times has seen gain twist technology return in 1990's by Lothar Walther specifically developed for shooting lathe turned solids in 338 lap mags using LM105 bullets.

 

These started out at 14 and ended in 7 twist.

 

Ronin,

 

Interesting you mention a modern justification as solids. Dredging my memory, gain twist was first explored as a way of upping mv with lead bullets (ie before jackets) without causing the proj to strip itself on the rifling; ie it was seen as a possible way of minimising stress on the proj whilst getting it to the desired rpm....not sure how good it was..because it didn't stop the world going on to develop bullet jackets!

 

I may be being thick, but I fail to see how torque can be reduced by gain twist; if 2 projs exit a 24" 1:12 barrel at 2800fps they will both have the same rotational velocity regardless of the initial rifling rates...and they'll both have reached it in the same barrel time.....and Newton's Laws apply!

 

The difference between the 2 will lie in the rate of acceleration to that rpm......the bullet in the constant twist barrel started turning at 1:12 the second it engaged the rifling and accelerated rotationally as a product of its increasing forward vel ....whilst the gain twist bullet undergoes it's greatest rotational acceleration when it already has significant fwd vel

 

...intuitively, there could be an argument that the bullet screaming down a smoothbore and then, in effect, hitting rifling would experience greater stress than a bullet starting in a fixed thread at vel 0 and working its way forward as its vel increases.

 

You'd have to do some serious maths to work out which one actually receives the lowest rotational accelerations......but I would guesstimate that it's swings and roundabouts and there's very little difference in it at all.

[Ronin]

Hence my quote" it may be percieved that the gain twist have less torque effect"

 

I also dont see (or understand) why that two rifle barrels exhibiting the same end spin on the same bullet would produce different levels of torque effect if one were to start slower.

 

The gain twist in solids is usualy used to prevent stripping of the driving bands that most lathe turned solids have.

 

The solids are made a thou undersize and the bands are cut by the rifling imparting spin.

 

Interestingly jacketed bullets that have been used in these barrels have shown to produce better accuracy (which I guess is relative to the amount of proper testing done so far)

 

 

Without giving anything away, is this technology used in larger ordanace?

[brown dog]

Without giving anything away, is this technology used in larger ordanace?

 

Not in my experience; NATO 155 and 105 stuff is all subject to STANAGs and everything I've experience of, or been taught about, has fixed rate rifling .....and interestingly when reading across to machined rifle solids with driving bands; it all engraves copper driving bands on the steel shells in just the same way; and a 155 at top charge is doing about the same mv as a 5.56 (....at a slightly heavier bullet weight );

 

The only place you'll see anything different is in tank barrels, and that's all about choice of primary and 2ndary natures. We always used to mandate HESH as secondary and that needs spinning; everyone else had HEAT as 2ndary and that degrades if spun. Balance that with APFSDS as primary (which doesn't want spinning either) that left the whole world on no-brainer smoothbores with UK inventing complicated APFSDS slipping driving bands to allow it to be fired down our rifled barrels. That's changed a wee bit now though.

 

I think the bottom line is that if you want nutty mvs and no torque, you have to start firing fin stabilised stuff.

[Cornishman]

Isn't it annoying when you know you've read about all of this but the magazines have all been chucked. The articles were all 'Precision Shooting' ('PS') which regularly gives technical articles on this sort of stuff.

 

Well, the mags aren't to hand but the questions are so interesting I'll have to trawl through my memory and see what I can recall.

 

There were several questions. PS published two articles about throat angle and the writer argued that this should fit the profile of the bullet you're going to use. In other words your chosen bullet needs to meets the rifling at the same angle all at once. I suspect any mortal shooters wouldn't notice the difference if this angle was a wee bit out but there does seem to be a lot of science behind the way they cut reamers, they're not chance. I'm guessing that a gentle transition into the rifling would lead to gas blow-by, bullet cutting and loss of accuracy, barrel fouling etc. Not good.

 

Again, from memory, Parker Ackley did tests to see if a fast twist barrel absorbed more energy in getting a bullet to spin faster. He concluded that it did but that the difference was almost unmeasurably small. Having said that I can certainly feel the difference in torque when shooting a heavy bullet in a fast twist 308 compared with a light bullet/slow twist. The heavy bullet is harder to shoot.

 

This question of torque has always given me that cottonwool feeling in my brain and apparently I'm not the only one. There is a story (true? Who knows) that when the US army was looking for a new pistol before WW1 they whittled the competition down to just two: The Colt 1911, which was finally chosen, and the Savage, The Savage was unusual in that it had a delayed blowback action rather than a locked breach. The delay was caused by the torque reaction of the bullet entering the rifling. The story is that the Savage engineers got the rifling twisting the wrong way! Like me it seems they couldn't really get their heads around which way the torque reaction would twist the barrel and whether the reaction was just through the acceleration phase, from friction in the barrel and whether there was a momentum reaction when the bullet left the barrel. I'm really hoping there's no-one out there who really understands this stuff because you might try to explain it and then my head would hurt all over again.

 

Wikipedia gives a slightly different version of this story (see link below). It's possible theirs is more accurate but it's not as good a story.

 

http://en.wikipedia.org/wiki/Blowback_(arms)

 

 

 

Cornishman.

[ds1]

Taking the idea of gain twist to an extreme why would you need rifling in the first half or 3/4 of the barrel anyway? Twist needs only be long enough to stabilize a bullet. 2 1/2" revolvers achieve that and a test by Bill Calfee showed 3/4" of rifling will stabilize a 22LR. The rest of the tube in effect is there to gain velocity, nothing more unless you use iron sights.

 

Imagine a shotgun but with rifling instead of the choke - bullets may gain more velocity in an un-riffled tube but would need to be coated or ribbed, throat wear would probably be much reduced with a smooth consistent surface to pass over and no wear at the rifling as the hot powder particles would then be gas. Velocity could be increased and pressure lowered. The issue would be making the smooth bore tight enough to achieve a good transition into the lands but bullet obuation could be less than from a traditional case to chamber to throat jump.

 

I believe a barrel maker could do it easily - I don`t believe they are daft enough to cut their own throats so to speak.

 

Something to think about - some rifles, throat burners can shoot way after predicted life spans and some shoot out very quickly, often how a rifle shoots depends on how the throat is eroded - if one or two lands see most damage the barrel is a tomato stake, if erosion is even it usually shoots much longer. This was one of the ideas behind rounds like 6mm Competition Match where a vigorous "cleaning" of the throat area after every 100 rounds or so with an abrasive kept it shooting well.......so why not a really long, perfect free bore?

 

David.

[brown dog]

Imagine a shotgun but with rifling instead of the choke - bullets may gain more velocity in an un-riffled tube but would need to be coated or ribbed, throat wear would probably be much reduced with a smooth consistent surface to pass over and no wear at the rifling as the hot powder particles would then be gas. Velocity could be increased and pressure lowered. The issue would be making the smooth bore tight enough to achieve a good transition into the lands but bullet obuation could be less than from a traditional case to chamber to throat jump.

 

David,

 

Sort of what I was alluding to earlier with:

 

The difference between the 2 will lie in the rate of acceleration to that rpm......the bullet in the constant twist barrel started turning at 1:12 the second it engaged the rifling and accelerated rotationally as a product of its increasing forward vel ....whilst the gain twist bullet undergoes it's greatest rotational acceleration when it already has significant fwd vel

 

...intuitively, there could be an argument that the bullet screaming down a smoothbore and then, in effect, hitting rifling would experience greater stress than a bullet starting in a fixed thread at vel 0 and working its way forward as its vel increases.

 

I think it could be argued that the bullet that -rotationally speaking- gets the soft ride is the bullet accelerating from 0 in constant twist rifling. The issue of driving bands stripping on bullet solids but not on shells just reflects issues of material strength vs scaling.

[Ronin]

Just as an aside, there is one gunsmith who is using very over bore wildcats in the states that had removed throat erosion (well not quite removed - lessened) from the equation by altering the lead angle of the throat.

 

Standard leade is 1.5 degrees, the revised leade is sub 1 degree angle..

 

 

Long throats

 

 

Roy Weatherby's cases were so high pressure, he had to have long throats before the bullet engaged the rifleing to reduce the possibilities of bursts...or so I recall reading , somewhere!

[ds1]

Hi BD,

 

Sorry - now understand what you were saying - it was a bit complicated for me.

 

I think we are stuck with the bullet designs and materials we have, otherwise we would be talking about fins. Transition from free bore to riffling could be "eased" with the type of riffling - like a rotating polygon (polygonal rifling) and a gentle transition into it - could be done from hammer forged or button riffling machines.

 

David.