Gain twist rifling question

[Riomouse911]

The Carcano thread got me to thinking, and I figured I would pose this question to those much more in the know than I about rifling/spin stabilization, twist rates, etc.

My Q is: With gain-twist rifling basically accelerating the spin of a bullet on its axis as it travels down the bore, does the bullet continue to increase in RPM as it travels downrange away from the rifle at the same rate as it did in the bore, or does it immediately reach it's "maximum spin zenith" at the muzzle and then start to slow down as centrifugal forces and drag begin to work on the bullet as it flies towards the target?

A secondary Q: what is the purpose of gain-twist rifling in standard velocity rounds? I can see it in a 4000 fps round possibly reducing deformation as the bullet slams into the lands and grooves and they work on the jacket material of the bullet, or in the .460 S&W with the sppeds and forces encountered there, but in a sedate rifle round like the 6.5 and 7.35 Carcano I just don't see the benefits. (I can't recall which caliber was said to have the gain-twist rifling.)

Just a wonderin' on a Sunday eve.

Stay safe!

 

[WestKentucky]

Max rotational velocity will be effectively at the muzzle. Air isn’t much of a drag, but it still is a drag force. The purpose is supposedly increased accuracy, but that’s questionable. The only thing I have heard praises sang about with gain twist is the Python, and I chalk that up to the brand following.

 

[Macchina]

Any change in velocity is acceleration. Acceleration requires energy to be put into the system. This counts for rotational acceleration too.

 

[Coal Dragger]

I know that very subtle gain twist barrels are becoming popular with precision rifle guys, particularly those running solid lathe turned bullets with driving bands. Bartlein offers gain twist barrels, and they have a very good reputation for obscene precision potential.

Probably find out more useful information on the Sniper’s Hide than on here.

 

[Varminterror]

Backing up what others have stated, the rotation rate is at its greatest when the bullet is at its highest speed and still in contact with the rifling. The instant it becomes free, it will begin decelerating both forward and rotationally.

Gain twist barrels are generally considered somewhere betweeen “a gimmick which does no harm” and “there might be some tiny opportunity there.” The technology has been around for a long time, so if it were significantly better, everyone would use ONLY gain twist barrels. But we don’t. They do cost more, usually, and they don’t really seem to last appreciably longer, so I expect a lot of guys just don’t bother with the small incremental POTENTIAL.

In theory, the advantage is a more gradual acceleration of the bullet which should cause less bullet damage and offer less resistance during the peak of the pressure curve. I’ve heard both sides to that argument - the pro side says less resistance means equal pressure yields greater speed. Whereas the con side says the resistance is what helps build the pressure, and in a progressive powder, less resistance means lower pressure peak (or later) and less overall energy efficiency - aka, lower speeds for the same powder charge. The difference is smaller than the difference between two individual barrels, from what I have seen, so it’s impossible to decisively say which side is right. I do think it let’s a guy pour more powder to a cartridge before over pressuring, and I do think it yields a little less burn efficiency because of it - so an inch of extra barrel might be wise to help utilize the extra powder mass.

I’ve had one gainer, and have shot another enough in a borrowed rifle to know I don’t shoot well enough to reliably reveal any difference.

 

[CZ9shooter]

Even a standard twist barrel gradually (if you can call it that) accelerates the spin of the bullet, from zero, to whatever the rate ends up being at the muzzle. It doesnt happen instantaneously.

The projectiles picks up speed, and RPM, as it travels the length of the barrel.

 

[taliv]

i got one a while back when litz was trying to convince everyone that we needed faster twists to achieve the actual BC. I ordered it starting at the standard twist everyone uses and ending with the faster twist litz said we needed.

it shot great and didn't cost any more to order it in a gain twist so i'd prob keep doing that if i were planning ahead but all my other bartleins shoot great too so normally i just buy what's in stock.

 

[someguy2800]

I would think a gain twist would upset the bullet more. The rifling will engrave the bullet at the twist rate at the muzzle and as the twist quickens it will need to also drag the rifle engraving on the bullet around to the faster twist. With a cup and core bullet I would guess that it would actually twist the entire bullet slightly, not sure how a solid copper would react to the twisting.

 

[Riomouse911]

I see your point. I also see gain-twist rifling causing a bit more friction than standard rifling since the lands and grooves are impressed into the bullet shank within the length of the bullet as it enters the rifled portion of the bore. Standard twist rifling doesn’t change, but since G-T has a continual tightening of the twist. I would also believe this is making the Land and groove impression change on the bullet as it moves towards the muzzle.

How much of an impact this extra friction may have on the possible maximum velocity of a given round in a G-T barrel I have zero idea (probably minimal).

I thought that the max RPM would mirror the max velocity right at the bullets exit from the muzzle, but thought I’d ask in case I was off...

There is so much about the science of firearms I would love to learn. Thanks for the input!

 

[Varminterror]

If you want to claim more or less resistance, respectively, to suit your side of the argument, then you have to consider if the angular inertia or the deformation is the greater contributor to resistance. As I stated above - I’m prone to believe the real-world evidence which proves gain twist barrels offer less resistance, as every guy I know who has shot them (myself included) has had lower pressure signs with similar loads.

 

[Coal Dragger]

So if cost and availability were the same, and you could choose between a gain twist barrel or a constant twist barrel which would you choose?

I’d probably give the gain twist a try myself. Specifically a Bartlein with a left hand twist since I’m right handed.

 

[LoonWulf]

I would think a gain twist would upset the bullet more. The rifling will engrave the bullet at the twist rate at the muzzle and as the twist quickens it will need to also drag the rifle engraving on the bullet around to the faster twist. With a cup and core bullet I would guess that it would actually twist the entire bullet slightly, not sure how a solid copper would react to the twisting.

I had wondered something similar when this thread got started.
I honestly do not have a good enough grasp of the physics involved to have any real input in this thread, but im enjoying it so far

 

[mcb]

The only place I know gain twist rifling is being commonly used in is 105mm howitzers and some 20mm and 30mm multi barreled aircraft guns (the A-10 GAU-8 is gain twist). Gain twist work better in medium and large caliber weapon systems since most of the projectile does not engage the rifling only the driving band. Thus as the rate of twist changes only the driving band must continually deform to this new angle. Gain twist should reduce throat erosion rate some but in practice the extended barrel life probably is not enough to offset to the increased cost of manufacturing (the multi barrel cannons might be the exception here due to the cost of replacing barrels in those systems compared to single barrel systems). As other have already stated. If there is any real world gain in accuracy it is small and in most applications hard to see in the rest of the noise of the system.

 

[Varminterror]

I sent a FB message to a gain-twist barrel manufacturer last night from whom I have bought a few non-gain barrels asking about info on these theories above. He mentioned several top level short range benchresters are using gain twist barrels which only gain 0.1” over the length of the barrel 1:13.85-13.75. The idea being - as @mcb noted above - the slight gain forces the bullet against the driving side of the lands throughout the length of the barrel. Gain twist barrels they manufacture only pick up less than 1/2” or more commonly less in twist over the length of the barrel. So we’re not talking about starting with straight flutes and ending with a 1:7”.

He also mentioned many try them, don’t see significant results and go back, but those who like them tend to LOVE them and use only gain-twist tubes.

 

[taliv]

wouldn't simple inertia of the bullet force it into the driving side of the lands?

many bbl mfg tighten the twist at the end. this is one of the reasons why i have cautioned for many years against chopping 24" factory barrels down to 16" or 18" as the 'tactical' crowd sometimes wants to do. Especially in button rifles, the twist varies and is not constant. if it happens to be slightly slowing down instead of gaining at the 16"-18" length, then your accuracy may stink. seems like it would be possible to measure before chopping but somewhat difficult.

my gain twist went from 8.5 to 7.2 or something.

 

[crestoncowboy]

The 460 x- frames use gain twist too. Ive never shot a more accurate revolver either.

 

[Gordon]

I have an old Arms Tech gas trap 9.5" barrel upper that is mounted on a Noveske Diplomat pistol lower that besides being stone reliable, has their own polygonal gain twist rifling. I have never used any thing heavier than 64 grain Winchester Power point ammo in it supposedly good for 77 graim ammo tho, but being only 9.5" in length it consistently shoots faster than any barrel I have under 14" and equals some of them. It is very accurate too, more so than the Noveske upper I took off the Diplomat. Maybe some Voodoo but I have heard rumors about those Arm Tech gain twist barrels being very fast and accurate .

http://www.armstechltd.com/products.php?id=compak16

 

[PapaG]

It all got started with muzzle loading rifles supposedly to keep the patch from stripping. Also, to gradually ease the soft lead ball or bullet up to speed without stripping.

 

[boom boom]

I have no high falutin ballistics, physics of deforming moving objects, nor benchrest experience to add about gain twist.

However, my 1930's vintage Fusil 91 Carcano long rifle which has an as issued gain twist barrel is quite accurate using the Hornady ammo with the right sized bullet. The long rifle Carcanos go from 1:19" at the throat to 1:8" at the muzzle which is pretty extreme compared with those above. It is at least as accurate as its peer military bolt action rifles in WWII (about 3-4 MOA @100 yards on benchrest using open sights combined with its long barrel.) It does have a slight warp in the stock forend which is why I have a spare stock to mount but haven't messed replacing the original because it shoots pretty well. I have not used cast bullets in it but I suspect that they might even shoot better especially after knowing what Papa G posted.

 

[MachIVshooter]

several top level short range benchresters are using gain twist barrels which only gain 0.1” over the length of the barrel 1:13.85-13.75. The idea being - as @mcb noted above - the slight gain forces the bullet against the driving side of the lands throughout the length of the barrel.

That makes a lot of sense, is actually an aspect I hadn't ever given much thought to. I suspect the overall quality of the barrel has orders of magnitude more impact on accuracy, and most shooters (myself included) wouldn't be consistent enough to see the difference, but when you're in that top tier, wringing out every little bit matters.

 

[LoonWulf]

What's the advantage to keeping the bullet forced against the driving side of the lands? Consistency?

 

[Demi-human]

I wonder if it keeps the gases contained better. A constant snug seal, instead of parts where the bullet could skid more easily, keeping the powder happy during its burn.

I, too, hold not enough higher education, nor experience, to know.