Progressive Rifling

[StonewallSharpeson]

Hello, can anyone tell give me some specifics on the progressive used in Civil War era muskets? Firstly, was the progressive rifling used have both progressive depth (deep to shallow) and progressive rate of twist (slow to fast), or just one or the other? Did the specs vary based on manufacturer or model?

Does anyone know the progressive depths and rates of twist (progressive or not) for these common civil war era firearms: M1855, M1861, rifled .69" muskets (eg M1842, percussion conversions, and flintlocks), P53 (which models?), Lorenz

Was the rifling different in the rifles of the muskets (aka "two banders") or other carbines, including breech loaders, which used solid conicals rather than minies?

Thanks for your help!

[Carolina Reb]

Progressive depth (or decreasing depth) rifling was used in Springfield and Enfield muskets. In Springfields, the grooves are 0.015" deep at the breech and taper to 0.005" at the muzzle. The rifling twist rate is a constant 1 in 72 for Springfields and 1 in 78 for Enfields. Spencer rifles also have decreasing depth grooves. They taper 0.003" to 0.005" from breech to muzzle. Other CW arms probably have this feature, but I haven't measured them.

The French actually discovered the advantages of progressive depth grooves while rifling old smoothbores. Some of them were too thin at the muzzle to rifle, so the crafty armorers made a machine to cut decreasing depth grooves. It turned out that muskets so rifled shot a lot better than the ones with constant depth grooves. And the rest is history, as they say.

Gain twist is where the rifling twist rate starts off slow at the breech and speeds up towards the muzzle. The grooves may or may not decrease in depth at the same time. Burnsides and Ballards have this feature, as well as most CW era revolvers.

[Kevin Tinny]

Hello, Carolina Reb:

Something else to think about with gain twist is that the width of the lands and grooves narrow as the twist rate accelerates because the cutter changes its cutting angle, slightly. So tension is created both by the increasing rate of twist (angle) and the narrowing of the lands and grooves.

Some may disagree with the following:

Gain twist works relatively well in breech loaders, but creates looseness of the bullet if muzzle loaded. Still, everything else being in mechanical harmony, it is generally agreed that a gain twist rifled bore will not produce better accuracy than conventional, constant twist rifling. Yes, I know of all the fine gunsmiths of the late 1800's and first 35 years of the 1900's that favored gain twist - for breech loaded bullets.

I have shot against some fine rifleman over the years, a few of which used well made gain twist rifled barrels. They were.determined to prove the old timers were correct in espousing gain twist, but for unknown reasons, these especially capable riflemen were less consistent in their scores and never managed to win a championship with them. I do not know of any modern bench rest records set with gain twist rifling. The only thing they could point to was the difficulty of lapping the grooves in gain twist bores. Several present era barrel makers I am aware of have the equipment to cut gain twist, but determining the ending twist and its duration has been a big challenge. One is working on gain with a section of constant twist near the muzzle.

Regards,

Kevin Tinny

[Kevin Tinny]

Hello, again, Carolina Reb:

Thank you for the Spencer rifling info. Roy Marcot makes no mention of it in his book.

I need your help on one aspect.
Would you please share where you learned of progressive depth rifling in Spencers and if it applies to both Spencer rifles and carbines.

Also, anyone that is more interested in rifling types and how they are machined can Google Clifford LaBounty or his exceptional book, Rifling Machines and Methods, for authoritative info.

Most respectfully,

Kevin Tinny

[dbm]

Gain twist works relatively well in breech loaders, but creates looseness of the bullet if muzzle loaded........

But surely only if the bullet is groove dia. and being forced down the barrel? If bore dia. the bullet when muzzle loaded slips snugly and smoothly down the barrel irrespective of the rifling. It's only on firing when it expands to fill the rifling. William Metford's gain twist rifling on the muzzle loading match rifles made by George Gibbs in the 19thC had a fine reputation for accuracy and were used in international competition.

David

[Jim Wimbish, 10395]

Gain twist was supposed to allow the bullet to more gradually come up to the desired twist rate. This was supposed to produce less fouling in the lower part of the barrel and I suppose that was where the increased accuracy was supposed to come from. I have read that gain twist barrels have a smaller load window that you can use to work up charges. The Civil War Burnside had a gain twist barrel but very few of us shoot them. Years ago, John Bly was making up a barrel for my 36 Maynard and he gave me a choice of a gain twist or a regular barrel. I decided to go with the gain twist and it has worked out fine for me. This is not to say that the non gain twist wouldn't have worked out just as well. My 36 Maynard shoots quite well and I don't have fouling issues with it, so I guess that I made a good choice. I also have a revolver that has a gain twist, the Pietta Remington Shooters Model, which is also an excellent shooter.

[Kevin Tinny]

Thanks for sharing:

I totally accept the Metford and Jim' experiences. I just haven't found a gain twist that will out perform a constant twist bore, if both are of similar quality. I don't know anyone that has lapped gain twist grooves. In frustration, my buddies tried to fire lap theirs, but that's difficult with a tight lead bullet. They also tried leather washers. Nothing worked for the grooves.

My question about a source for the Spencers having progressive depth rifling remains open. I have texted some experts, but haven't heard back. Will share what I hear. Thanks.

Respectfully,
Kevin Tinny

[Greg Ogdan 110th OVI]

Kevin, you seem to think that rifling grooves need to be lapped. You have also referred to some issues in modern barrels. Check me, but every borescope picture I see of modern barrels shows unlapped grooves. This is because lapping of the grooves is not necessary; all the tool marks are coincident with the direction of bullet travel.

[Kevin Tinny]

Hello, Gregg:

I borescope also. Have seen lots of crazy stuff including helical blemishes in both grooves and on lands in a super quality barrel. It was buttoned over the spiral (reamer?) blemish and then lapped. The maker would not sell me a new one except on the condition that I surrender the blemished one. Guess he didn't want that one loose, saying that it was a "MONDAY BARREL". He routinely provides barrels that win Olympic gold medals. So much for his "inspection/quality control".

Anyway, yes, from what I have seen, many barrel makers lap grooves with expanding lead laps. Have seen it, and had it done in high-end custom shops. We also lapped with expanding cast iron tubing slotted via dividing heads. But the was for reamed holes PRIOR to rifling.

Every one of my record setting slug gun and ball benchrest barrels and those of most of the other equally successful shooters that I competed with for over 20 years at Friendship were groove lapped.

IF THE BORE IS PROPERLY MADE, a lapper CAN/WILL push the lap completely out and rotate it one groove and repeat the process. This indexing requires that the grooves are precisely oriented.

Check with BENCHMARK barrels. Their website's FAQ'Ss indicate WHY they fully lap an otherwise fine barrel. A friend made some of their rifling machinery and just delivered an impressive CNC rifling rig last week. He told me they groove lap. My Shilen and Hart barrels were also groove lapped, but that was in the 1990''s.

Regards,

Kevin Tinny

[Carolina Reb]

Kevin,
The way I found the Spencer rifle decreasing depth grooves was slugging one. The slug started tight from the muzzle and then went very easily towards the breech. So, out of curiosity, I started a slug at the breech (not easy on a Spencer) and then tapped it back out the chamber. Sure enough, the groove diameter was 0.005" bigger than the one that went all the way down the barrel. I happened to have two other rifles available at the time, so I checked those. One was 0.003" bigger at the breech and the other was 0.005". I have not seen this mentioned in any references, but going 3 for 3, I doubt it was random chance. Unfortunately, I haven't been able to check any Spencer carbines. It wouldn't be a surprise if they have this feature as well.