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