Ball, Groove, & Land

[Elijah Gunn]

Just want to make sure I have my terms straight.
The groove diameter is the large diameter in  the bore.
The lands make up the small diameter in the bore, and impart spin to the ball as it travels through the bore.
The ball is sized to a diameter when it is rammed into a chamber in the cylinder.
The reason I'm curious is because awhile back at  another site I was reading that the ball dia. after being rammed into the cyl. should be about  .002 bigger than the groove diam.  Does that sound correct?
You would definitely have a good gas seal that way, and since the ball is dead soft lead it doesn't sound dangerous to me. However, if we are using a conical shape that has a lot more surface area in contact with the bore I begin to get concerened.
What are the basic  ball, groove, and land diameters for a .36 caliber?
I know about how to check the chamber and bore sizes but have not done it yet. When I do it, if I find the chamber dia. to be a few thousandths smaller than the groove dia. I'd like to enlarge them as long as it's a safe thing to do.
Thanks, Mark

[Flint]

Yes, groove diameter is the larger diameter of the bore.
Lands are the surface of the bore, diameter before rifling.
The ball SHOULD be sized upon seating in the cylinder chamber, or it will probably fall out again.

The ball, I would think, should be at least sized to the groove diameter, but it would seem that few are.

Conical bullets should match the bore, but I shoot Remington and Colt conversions in 45 Colt sized 452 and the bore is 454 or better on the Italian guns.

The 38 conversions (Kirst or R&D) to a cap & ball Navy, using hollow based bullets seem to open up to match the .375 (or better)bore exactly.

According to Cimarron's website, the 36 bore is .36 (9.144mm), the groove diameter is .379 (9.16mm) (WRONG) and the land width is .08.  There are 7 grooves in both the 36 and the 44 bore, so measuring a slugged bore can be difficult with odd numbers, even numbered grooves are much more easily measured.

WRONG, above is my comment, as .379 would be 9.267mm, not 9.16mm.

I will slug some of my bores 44 and 36 to see what they are.  I see by eye that they seem to differ by age of manufacture.  An older Navy Arms Navy bore rifling looks more like smokeless rifling, very shallow, while later bores have deeper grooves.

I will also measure chamber throats.

[longcaribiner]

In cap and ball revolvers, there is an additional sizing step that is often over looked.  First, you need a slightly over sized ball to (1) grip the sides of the chamber so it doesn't fall out and (2) help seal the chamber against chain fireings.  When the gun is fired, and the ball jumps forward to the breech of the barrel, there is a forcing cone which acts like a funnel to catch the ball if the chamber is a thousandth or two out of line and to again size the ball or bullet into the rifling.  Kind of like the taper in a breech loading rifle chamber that permits the bullet to squeeze into the rifling. 

Hopefully, if everything is engineered and machined properly, the ball will fill the rifling and not allow any blow by and or gas cutting.  Using pure lead balls/bullets also permits a certain amount of obturation.  That is basically when the explosive force of the charge "hammers" the soft ball and expands it into the space available.  So even if your chamber is slightly undersize, the ball can still expand to fill the rifling. 

In the early days of the Italian imports, some chambers were undersized, timing didn't line the chamber up with the barrel adequately, and a host of other problems.   Such guns often shaved and spit lead sideways often hitting and spraying folks standing to the side of the shooter.   

[Flint]

After measuring and checking ball sizes in the bore, I find that Uberti 44 bores show daylight up to .46 diameter ball.  A .457 ball still shows daylight, so opening up the chambers to that would still have blowby and no longer hold that ball tightly.

Measuring the actual diameter after slugging with a .465 ball was not possible as the bore has 7 grooves, but did show rub marks from the bottom of the grooves.

A .380 ball shows daylight in a 36 bore, Uberti as well.

Though balls like the .465 I have a box of could work with an enlarged chamber, I would hesitate to do all that work, but might try one spare cylinder sometime, as I have a reamer that might work..

Historically, rifling was first invented as an attempt to give fouling a place to go to allow more shots between cleanings.  By chance, it was found that accuracy improved, and in an attempt to make the grooves longer, the twist was tried, and accuracy improved even more.

In that light, perhaps the cap & ball bore is oversized for a similar reason, a fouling trap.  At some point, there would be no blowby as the grooves fill up with carbon.  There is also bullet upset to consider.

Seeing as how my Remington Revolving Rifle (with a 45-70 bore barrel, I built it years ago before Uberti made one) has a 458 bore and can hit a 200 yard 18X24 steel plate consistantly with a 45Colt conversion cylinder in it shooting 452 diameter hard cast bullets, the excess groove depth can't be hurting it too much.

Speaking of Remington, note that the Remington factory Conversion was .46 caliber.