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Rifling, Bore and Groove Sizes, Twist

    Today, most people take it for granted that a rifle or pistol will have those little grooves cut inside the barrel. Other than that, they seem to know little about the subject. Even self described experts stumble and mumble when asked pointed questions about the function of those little grooves.

    Here, I will outline the long process our Ancestors went through, to get a Reloaded bullet to hit what they were shooting at. This is a simple summary, which will bring the basics from many sources, together in one place. Everyone should know the basics. If you are confused, I hope this helps.

    From here you can search the internet further and learn more about rifling types.

    This process has no end point and I can not predict what our next generations will come up with. If you know where this all started, you may come up with some new ideas!

    Keep in mind that a combination of bore size and rifling, priming and powder, then a projectile, work through mathematical principles. The Mechanics, ( Early form of Engineers), figured all this out through trial and error. They did a great job and brought us to where we are today. 

The Beginning

    The story of Gunpowder is easy to find on the internet. Mixing of Charcoal, Sulfur and Saltpeter into a paste makes the right mixture. Dry it and break it up into granular form.

    A Thoroughly Dangerous Process!

    Most powder mills blew up numerous times. But, when complete, if you survive, you have a pretty stable mixture. Black Powder has lasted intact for centuries, in old castles in Europe.


    At first, our Ancestors were using a smoothbore metal barrel to fire a projectile. The barrels had a small hole at the rear for some sort of ignition source. Burning Wicks worked, then more elaborate Wheel Locks and simple Flint Locks were developed.

    You could use a Smoothbore to fire a bunch of small round pellets, a single ball, or both at the same time. Pour in a powder charge, then push down some wadding over the powder. Drop in a shot charge or a loose fitting ball, then tamp in some wadding on top so the pellets or ball would not fall out! Ignite the powder at your convenience. This worked well for centuries. ( But, was not accurate!)

    Even during the American Revolution, The British Army used Smoothbore, Flintlock Muskets. They had learned to mass produce Smoothbore musket barrels with a fairly uniform BORE size. BORE meaning a hole drilled in a piece of metal, drilled and reamed to a particular size. Just a plain, round, smooth hole. The smoother the better.

    They also learned to make round lead balls of a fairly uniform size, somewhat smaller than the inside of the Musket BORE diameter.


    The British were issued Paper Cartridges, carried in a pouch. Basically, a paper tube with a powder charge and ball inside.

    Bite the end of the paper tube off and pour the powder down the barrel. The paper tube would then act as wadding around the round ball. Push the round ball, with paper still on it, down the barrel. Prime the pan and fire the rifle with a flint on steel lock.

    The average American Soldier was also stuck with these obsolete smoothbore muskets. The point was to shoot at a large column of soldiers walking toward you. It did not matter which one you hit. Just throw a ball over that direction and hope for a good hit!

    But, not all Americans were stuck with these Smoothbores!

    Some had Rifles!  


    You are already familiar with Bore Size. You probably use Bore Size Guns fairly often. Now, we call them Shotguns. Ten Bore, Twelve Bore, Twenty Bore, etc. The British still cling to this Bore Concept. They will say; "I took out my 12 Bore Gun today." Notice they say Gun, not rifle, because they are talking about a smooth bore Shotgun. They love tradition, and their statement is correct. The BORE of the Gun is Smooth with no rifling.

    Today we call these 10 Guage, 12 Guage and 20 Guage. The only exception to all this is the .410 guage shotgun. That is a decimal size. That is also deceptive, because some firearms are designed to shoot either the .410 shotgun shell or the 45 colt in the same chamber! 


    In the early days of Rifled Barrels, both the British and Americans used the Bore Size designations for their rifled barrels. Some of the early British African Hunters used rifles of 6 bore, 4 bore and even 2 bore caliber.

    A 6 bore rifle would mean SIX round balls of that caliber would weigh One Pound!

    A 2 bore rifle would mean TWO round balls of that caliber would weight One pound!

    This is really confusing because you often do not know if those rifles used a bare round ball, a cloth patched ball or a cylindrical bullet. Often, you have to experiment to determine what size ball you would use.

    The following list will give you a idea of the number of balls per pound you might expect from round balls, cast of pure lead, that we use today. This is just an approximation. That is why this method lost favor.

     Early bullet molds were often marked 38, 52 or 98, and that is the only marking.

A 38 bore mold would mean 38 balls to the pound. This corresponds to approximately a 50 caliber rifle in todays terms.


36 caliber- (.360) = 98 balls per pound

40 caliber- (.395) = 76 balls per pound

45 caliber- (.445) = 52 balls per pound

50 caliber- (.498) = 38 balls per pound

54 caliber- (.535) = 32 balls per pound

58 caliber- (.560) = 25 balls per pound

62 caliber- (.610) = 20 balls per pound

69 caliber- (.678) = 15 balls per pound

75 caliber- (.715) = 13 balls per pound


    You must realize these numbers are often for Muzzle-Loading Rifles that use a cloth patch around the ball. It does not indicate bore size or the depth and diameter of the Rifling Grooves. This is just the diameter of the ball and approximately how many it will take to weigh a pound.

    I have not taken the time to try to figure out Bore Size Designations for other antique guns because it is a broad, complex subject. Are you using a grooved, lubricated bullet? A smooth bullet with a double wrap of paper around it? If you think this looks like a Mess, you are right.


    In America, Pratt and Whitney introduced the hand held, micrometer caliper to the general public in 1867. You could operate in with one hand and it divided one inch into 1000 parts. Below you will see my every day, 0 to 1 inch micrometer. There are some horizontal lines on top of the barrel. Those will allow you to divide an inch into 10,000 parts. (I keep a pretty, new one in a box if I need to get really accurate!)

    The year 1867 was an important time. Remington and Sharps were making their first rifles with metallic cartridge cases, centerfire primers and grooved lubricated or paper patched bullets.

     The 0 to 1 inch micrometer and the Metallic, Centerfire Cartridges showed up at about the same time and Rifle Makers embraced the idea immediately. 


    After these little micrometers were introduced, the Bore or Guage size designation faded away quickly. If you wanted a 50 caliber, or 1/2 inch Internal Bore Diameter rifle barrel, you would say just that! A 50 caliber ( 1/2 inch bore diameter) rifle, shooting a 50 caliber bullet!

    But, the bullet was slightly larger, because the grooves, cut in the bore, made the rifling diameter bigger.

    Before this, you would say you wanted a 38 BORE rifle, meaning 38 balls to the pound, if you check the chart above. Even this is confusing, because you had to know what type bullet you were talking about, but 38 BORE meant a 50 caliber rifle in todays terms. Mostly you had to depend on the manufacturer and your knowledge of the muzzleloader or cartridge rifle you wanted.

    Today, if you say you have a 50 caliber rifle, you mean you have a rifle that measures a particular SIZE in the smallest diameter of the bore and grooves. This Size is determined with the micrometer above and is determined by dividing one inch into 1000 parts.

    When they made the rifle barrel, they drilled and reamed a 1/2 inch hole. The hole should measure 1/2 inch or .500 almost exactly! ( As measured with your micrometer.) This is the CALIBER of the rifle, which you see marked in red in the image below.

    Then the rifling was cut.

    The rifling was BIGGER in diameter than the BORE size. 


    Check out the GROOVES above. The GROOVES are a bigger diameter than The Caliber or Bore. The grooves are also cut with a ROUND or RADIUSED bottom. In a 50 caliber rifle, the cutter is ground to an exact radius to create a round shape that is a circle. You can see this in the image above 

    Grooves in the Old Days were anywhere from .005 to .010 deep. They varied a lot! That was OK, because Black Powder Slams! the base of the lead bullet and makes it expand into the grooves. ( Smokeless does not work this way and more accuracy for bore, groove and rifling diameters is necessary.)

    Most of the Old 50 caliber rifles by Springfield, Sharps and Remington had a .515 GROOVE Diameter. (Or Larger.) Lyman made a 515141 bullet mold just for the 50-70. They also offered molds that were .503 up to .520 in the same bullet shape and size. They are a little harder to find, but they are out there if you look.


    Example:  A .500 bore, plus a .0075 deep groove, plus another .0075 groove, would give you a .515 Groove Diameter. (There are two grooves, one on each side.) But like I said before, the old guns varied a lot! Remington's and Springfield's in particular.

    The reason for making a rounded or radiused shape on the cutter is simple.

    (1) You are shooting a ROUND bullet. Either lead or jacketed.

    (2) Square or flat bottom grooves would be very deep and would deform the bullet.

    (3) The bullet would not expand to fill the groove corners. There would be gas leakage around the bullet, destroying accuracy.

    (4) They would also be Extremely deep in the corners. They would be hard to clean.

    (5) The deep corners would be a fracture or breaking point. This would be a place for a high pressure barrel to crack or explode.


    A modern example would be 25 caliber rifles.

    The CALIBER or BORE is 25, (.250), in modern, accurately made rifles. ( 25-35), (25-20), (257 Roberts).

    Bullet diameter would be .257 in modern rifles.

    The GROOVES are .0035 thousands of and inch deep. (On each side)

    ( .250 + .0035 + .0035 = .257. This is the common bullet diameter of most modern, smokeless powder, 25 caliber rifles today.

The Great Deception

    You would think the invention of the Micrometer would make everyone honest, but that was not the case! Most people did not have a Micrometer and had no idea how to use them!

   When Winchester brought out the Model 1873 Rifle, they called the new cartridge for it, a 44-40. This was a Dishonest Marketing Ploy! 44-40 should mean a 44 caliber, (.447) diameter bullet and a cartridge case that would hold 40 grains of powder below a fully seated bullet.

    The First Lie. 

    44 caliber means you drill and ream a .440 hole in the barrel. Then you Rifle the barrel. This makes a Goove diameter of about .447. 

    All 44-40 bullets are .427, or 42 caliber.

    Second Lie. 

    Even the early, thin walled, folded head cartridges would only hold about 28 grains of 2F Black Powder under the bullet. Modern, thick walled and thick head cartridges, might hold 25 grains of 2F Black Powder under the bullet.

    The marketing guys at Winchester probably thought 44-40 sounded Big and Powerful. They managed to sell 720,000 of their Model 1873 Rifles. They probably would not have sold so many if they called it what it was. 

The Model 1873, a 42-28 caliber Rifle! That does not sound so great.

    Sharps and Remington were much more honest. They actually cut a 44 caliber, (.440) hole in the barrel and rifled it to .447. (Or often much larger.) The thin walled, folded head cartridges they marketed, would actually hold the Black Powder charges they described in their catalogs.

    Real Riflemen actually knew what was going on. All the famous Hunters and Guides used Sharps Rifles and some used the Remington Rolling Block.

    That is where the old saying came from:


" Sharps, The Rifle That made the West Safe for Winchester."

Those Magical Little Grooves

    I mentioned those early Smooth BORE rifles earlier. Our Ancestors found that Buckshot or a Loose Fitting Ball worked OK. These were better than a Spear, but not always better than a Bow and Arrow. You could fire a bow much faster. A good Archer could shoot you with two or three arrows while you were reloading your musket!

    A longer range, more accurate weapon was needed, to push those archers out of accurate range.

    Over time, they found that a tight fitting ball, in a smooth bore barrel was more accurate. This was not a complete solution, but the idea was adopted as the best possible solution of the time. 

    There was a big problem with this tight fitting ball. Black Powder Fouling!

    Fire a couple of shots and Fouling would build up. It became hard to force in a tight bullet. This is not good if you are in a hurry!

    Old books about Guns, mention that some fellow had a great idea. Cut some grooves in that smooth bore barrel and it might be easier to hammer or push a tight fitting round ball down the barrel. The grooves would fill with powder residue, but it might still be easier to hammer the ball in and get off a few more shots without cleaning the Bore.

    Most Authors think these first rifled Guns had straight grooves, parallel with the axis of the bore. This seems like a logical idea, but I have no idea if it is true.


    At some point lost in time, some gunsmith had the idea of cutting spiral grooves instead of straight grooves. There are mentions of rifling in Austria as early as 1498. August Kotter was supposed to have improved on the rifling idea in 1520. Details are scarce and vague. Apparently the process of developing spiral rifling took a long time!

    In the end, Gunsmiths and Riflemen found that a round ball was more accurate if it was spinning on it's axis.


    Austrian and Swiss Gunsmiths brought this idea to early America. These new Rifles were expensive. Not everyone could afford them. These Gunsmiths kept experimenting. They extended the range of Rifles to a very effective 125 yards. These accurate Rifles could keep the bow and arrow out of range.

    This was the point when the Pennsylvania/Kentucky rifle was born. A small caliber rifle that used less powder and lead. A Spiral Grooved barrel, loaded with a round ball. The round ball was encased in a thick cloth patch. The ball and patch fit tight in the Rifled barrel. Gunsmiths found just the right spin for the best accuracy through experimentation. (There were not many Physicists around in the sixteen hundreds.)

    This was also the beginning or the term "Sharpshooter", then the "Sniper".

    Early American Military units were usually armed with smoothbore muskets. A few Kentucky Riflemen were always handy to have around. With their Rifles, they could target British Officers. They could "Cut Off The Head of the SNAKE!" They could put a bullet were they needed it. When the need arose!

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