Bore/groove versus cylinder dia

[bigted]

Here is a question we all have pondered I am sure. MAYBE somebody has the answer and MAYBE some theory that is useful info.

THE question is ;

WHY DO MANUFACTURERS MAKE THEIR CYLINDERS DIAMETERS SMALLER THEN THE BORE/GROOVE OF THEIR BARRELS?

In modern revolvers, we know that our cylinder/bullet in cast lead bullets require the bullet entering the barrel forcing cone to be .001 to .003 inch larger then the barrel GROOVE diameter. This helps the bullet to grip the rifling more successfully AND to seal the barrel for a "no gas cutting" condition therefore creating every chance for accurate shooting.

Why then do our beloved C&B revolvers have such a small cylinder chamber diameter compared to the barrel groove diameter?

Idea's ... Theory's ... fact's

[ShotgunDave]

Maybe the manufacturers figure that the soft lead bullet will obturate and "jump up" to groove diameter. Maybe they figure it's less stress on the gun, than stuffing a slightly oversized ball down the barrel. Just a guess though.

[Hewy]

Could be the need to create a tight fit of lead bullet is to seal the chamber ,allowing the gas expansion created that propels the bullet.
Metal cartridges ie modern , the sealing is performed  by crimping the bullet in the metal case.
Hewy

[Hawg]

I don't know why they do it but the lead balls do obturate nicely. I have never recovered a fired ball that didn't have good rifling marks.

[bigted]

I agree that the cartridge get's it's umff from being crimped ... however ... the seal I refer to needs to happen in the forcing cone and just the begining of the rifling ... otherwise you get gas blowby and leading in the barrel AND a inaccurate condition.

Also I see rifling on my projectiles ... however ... if you magnify the bore and groove marks ... most of the time you can see the groove marks do not have the longitudinal marks as do the bore marks. Therefor in my opinion, there is gas blowby through the groove marks caused by a non sealing condition. Also in this condition there is a fair possibility of poorer than possible accuracy.

I do know that matching bullet to groove is to improve accuracy in any rifled arm of any kind. 

So I can understand the merit of creating a condition of lessened pressure by making a condition of blowby in the barrel sorta like Ruger does with their long throat in their tropical chamberings.

However ... did Senior Colt think of this and build a safety valve in the barrel inadvertently by boring the cylinder smaller then the groove diameter ... or is this a modern IDEA to build in safety in a revolver lookalike of Col. Colt's invention?

Ha ... do not know

[Captainkirk]

Obturation is the answer, but as to why...

[Yolla Bolly Brad]

Here is a question we all have pondered I am sure. MAYBE somebody has the answer and MAYBE some theory that is useful info.

Why then do our beloved C&B revolvers have such a small cylinder chamber diameter compared to the barrel groove diameter?

Idea's ... Theory's ... fact's

Pietta Remington 1858's seem to be one of the worst offenders in this regard with a chamber mouth diameter of .446" and barrel groove diameter of .451". My theory is they do this to prevent a person from ramming a 45 cal jacketed bullet into the cylinder.

[G Dog]

Obturation is the answer, but as to why...

Also, base expansion (or the rearward hemisphere) as it begins acceleration from rest when hit by the ignited powder’s gas blast.  Bullet “upset”.

https://flicense.blogspot.com/2017/10/bullet-expansion-by-upset.html

[Doak]

OK ~ this is my can o' bullowney: I think, based on the history of "rifling", "smaller chambers" is left over from the old days, when deep grooves in a barrel were meant to give fowling a place to get outa da way. Seems, historically, early deep grooves were straight, and much narrower than the lands. The lands supported the ball, so the ball only had to be .001" to .003" larger than land diameter.

Then along came a twisted thinker. Why not spin the ball for stability, the gyroscopic effect, keeping the ball on a straighter path. Spiral rifling was born, but the "get the fowling outa da way", narrow, deep grooves,  prevailed for awhile. Old traditions & mfg'g habits die hard.

The "patched" ball, in front stuffers, relies on the "patch" to fill the deep grooves. Can't patch revolver balls. They cause trouble at the flash gap.

Trial & error, considering all the negative effects of blow-by, gas cutting, & leading, gradually lead barrel makers to close up the deep groove gap, & make the lands & grooves the same width, allowing the ball\bullet to make full contact allaway 'round the inside of the bore. Improvements in bullet lubes, and especially grease cookies, keep fowling under control. This is still in the BP era, before high velocity smokeless powder.

Enter smokeless powder. Little or no fowling. Now the lands are 1/2 the width of the grooves, and the grooves support the bullet. The main purpose of the lands, is to spin the bullet. Now it's important to have bullets equal to or larger than groove diameter.

OK, so here is the rub: Why have Pietta & Uberti gone to modern rifling w/narrow lands & wide grooves, and kept the old school system of smaller than groove diameter chambers?

The ROA doesn't do this. It's chambers are larger than groove diameter. ROA webbing between chambers is thicker.

Maybe this is why Piett's & Ubert's chambers remain smaller. They've modernized their barrels, but original design dimensions demand original chamber dimensions? Bah! I ream the chambers out to .454".

Factory webbing is .052". After reaming, webbing is .042". And I case harden the cylinder & frame. Hardening the case reduces the reamed chamber diameter by .0005". They shoot great.

I'm not a fan of "obturation", to make a bullet larger than chamber diameter, to fill the grooves of a barrel. The bullet's long gone before that.

Oops. Forgot to mention, the above pertains to 1858 Rem's regarding the reaming & case hardening.

Kindest Regards,
Doak

[Hawg]

I'm not a fan of "obturation", to make a bullet larger than chamber diameter, to fill the grooves of a barrel. The bullet's long gone before that.

If that were true you wouldn't get good rifling marks. I've recovered a lot of fired balls and they all had good rifling marks.

[ssb73q]

I'm not a fan of "obturation", to make a bullet larger than chamber diameter, to fill the grooves of a barrel. The bullet's long gone before that.

If that were true you wouldn't get good rifling marks. I've recovered a lot of fired balls and they all had good rifling marks.

Hi Hawg, I agree with you. Pure lead has a maximum compression strength value of 1740 psi. Typical BP chamber pressure is ~10,000 psi. There is plenty of pressure to pressure swage the lead projectile into the rifling. Pure lead anneals at room temperature so that work hardening changing compression strength isn't an issue.

IMO the people that ream out their cylinder chambers when using pure lead projectiles is simply wasting their time. Even chambers with different diameters from one another isn't an issue when shooting pure lead projectiles.

Obturation is supported in theory and as you prove by observation of rifling on the projectile after firing.

Regards,
Richard

[G Dog]

The bullet's long gone before that.

You make interesting points, Doak and I was with you until that.  ^^^ 

My first thought was “long gone” before what? 

On Piettas I see rifling marks on recovered balls (that have stayed intact) even when using .451/.375 although with .454/.380 the marks are a bit more distinct.

[Captainkirk]

Every RB out of my revolvers I've recovered has rifling engraved. Jus' sayin'...

[Doak]

Long gone, G Dog, before the diameter of the bullet/ball surface seals the space (surface) at the bottom of the grooves.

Yes, everyone sees rifling engraved marks on their recovered projectiles. And yes, those engraved marks on those projectiles are produced by "obturation". That's  compression obturation, not expansion obturation. The soft lead obturates upon contact w/the lands, which compresses the imprint of the lands onto the soft lead. This happens because the diameter of the chamber is larger than the diameter of the lands in the barrel. But the diameter of the chamber is not larger than the diameter of the grooves.

So, the diameter of the chamber is not large enough to allow the groove diameter, of the barrel, to compress (obturate) the undersize bullet diameter, that exits the undersize chamber, down to groove diameter of the barrel.

To do that, ya need an oversize chamber that expels a projectile larger than groove diameter, to allow the barrel to compress (obturate) the projectile down to groove diameter.

A recovered projectile may look like it made contact w/the bottoms of the grooves. That's because the sides of the projectile were swaged smooth when pressed into the chamber. And the lands imprinted on that surface.

Conventional wisdom of today's lead bullet casters/makers recommends bullets be .001" to .003" larger than groove diameter, when they enter the barrel. This requires chambers of like diameters.

Kindest Regards,
Doak

[Hawg]

Whether it's true or not as long as I can hit a Coke can at 25 yards more often than not this is all moot.