Written by Norman F. Johnson, this article appeared in the May/June 1996 #121 issue of the Fouling Shot.
In a revolver, the throats are the areas in each cylinder chamber immediately ahead of the portion of the chamber where the brass case rests and into which the bullet projects. If the bullet is sized so that it is a gentle force fit in the throat, all else being equal your accuracy potential will increase greatly. When a bullet properly fits the throat, two factors come into play to improve accuracy:
a) The bullet axis is more nearly coaxial with the barrel axis.
b) Gas cutting, which will positively ruin accuracy, is virtually
Gas cutting is the jetting of hot propellant gases past the bullet, causing erosion of the outer periphery of the bullet and plating some of that eroded lead onto the bore surface where the bullet runs over it, further increasing bullet degradation. When the cylinder throats are more than about .000511 over bullet diameter, gas cutting damage is severe even before the bullet hits the forcing cone, making good accuracy improbable. This is true of cast bullets, more so than jacketed, because they are far more subject to gas cutting.
The first thing I do with a new revolver is to shoot it with some of my "standard" loads and if it does well I accept the gun the way it is. For those that do not shoot as well as expected, I swage bullets, both cast and jacketed, to fit the revolver's cylinder throats. As a number of Cast Bullet Association members have discovered, fitting the bullets to a revolver's throat is probably the single most important step that one can take to improve a revolver's accuracy. Most of the revolvers I have owned in the past few years have throats that are oversize.
Despite what the gun writers say, my tests show that there are no observable pressure increases when using lead bullets up to .006" over groove diameter, and such bullets do not increase leading if other loading practices are reasonable. My Ruger Blackhawk has .4545" throats and a .449" slugged bore. Even when loaded with my 350 gr., 1300 fps ''boomers", the Blackhawk shoots .45411, 13 BHN bullets better than any other.
Using an old C-H Swage-0-Matic bullet swaging press and appropriately modified ( opened) dies I am able to reshape and resize most bullets, including most of my cast bullet rejects, to usable and usually very accurate projectiles. If the bullet is not too hard and the increase in diameter does not exceed a couple of thousandths, a sizer-lubricator with the appropriate die can be used to bump up a bullet to proper size. Bumping in a sizer-lubricator does not provide the same control and consistency as bullet swaging equipment. Be careful, it is easy to break the sizer-lubricator's handle.
I have improved the performance of a Redhawk .44 Magnum from a best of 2" at 25 yards, to a constant . 7511 at the same distance, by changing bullet diameter alone. Now that is satisfying!
In a revolver, the throats are the areas in each cylinder chamber immediately ahead of the portion of the chamber where the brass case rests and into which the bullet projects. If the bullet is sized so that it is a gentle force fit in the throat, all else being equal your accuracy potential will increase greatly. When a bullet properly fits the throat, two factors come into play to improve accuracy:
a) The bullet axis is more nearly coaxial with the barrel axis.
b) Gas cutting, which will positively ruin accuracy, is virtually
Gas cutting is the jetting of hot propellant gases past the bullet, causing erosion of the outer periphery of the bullet and plating some of that eroded lead onto the bore surface where the bullet runs over it, further increasing bullet degradation. When the cylinder throats are more than about .000511 over bullet diameter, gas cutting damage is severe even before the bullet hits the forcing cone, making good accuracy improbable. This is true of cast bullets, more so than jacketed, because they are far more subject to gas cutting.
The first thing I do with a new revolver is to shoot it with some of my "standard" loads and if it does well I accept the gun the way it is. For those that do not shoot as well as expected, I swage bullets, both cast and jacketed, to fit the revolver's cylinder throats. As a number of Cast Bullet Association members have discovered, fitting the bullets to a revolver's throat is probably the single most important step that one can take to improve a revolver's accuracy. Most of the revolvers I have owned in the past few years have throats that are oversize.
Despite what the gun writers say, my tests show that there are no observable pressure increases when using lead bullets up to .006" over groove diameter, and such bullets do not increase leading if other loading practices are reasonable. My Ruger Blackhawk has .4545" throats and a .449" slugged bore. Even when loaded with my 350 gr., 1300 fps ''boomers", the Blackhawk shoots .45411, 13 BHN bullets better than any other.
Using an old C-H Swage-0-Matic bullet swaging press and appropriately modified ( opened) dies I am able to reshape and resize most bullets, including most of my cast bullet rejects, to usable and usually very accurate projectiles. If the bullet is not too hard and the increase in diameter does not exceed a couple of thousandths, a sizer-lubricator with the appropriate die can be used to bump up a bullet to proper size. Bumping in a sizer-lubricator does not provide the same control and consistency as bullet swaging equipment. Be careful, it is easy to break the sizer-lubricator's handle.
I have improved the performance of a Redhawk .44 Magnum from a best of 2" at 25 yards, to a constant . 7511 at the same distance, by changing bullet diameter alone. Now that is satisfying!
It seems that many cast bullet shooters move to a harder alloy if their reloads lead the barrel This usually compounds the problem because leading is often the result of poor bullet fit. It sometimes pays to try a softer alloy so that the undersized bullet will bump up upon powder ignition. This fills the revolver's throat and precludes gas cutting. All of my target loads, 1000 fps and below, are soft alloys made up of just about any alloy of 8 BHN or less. Harder alloys will work but must fit the throat more closely.
A more common and practical approach to obtaining proper bullet fit is to have a mould cut for your throat and bullet alloy. Veral Smith of Lead Bullet Technology, does this work for little more than the price of a production mould. Ask him to cut the mould to cast bullets about .0005" under your throat size. Minor bullet diameter adjustments can be made by varying the bullet alloy content.
To obtain the diameter of a bullet that will shoot well in your revolver, take a soft lead bullet and squeeze it gently in a vise until it is about .01011 larger than nominal bullet diameter. Attempt to push this slug through each throat of your cylinder using hand pressure only. If it will not pass through the cylinder's throats, your slug is large enough. If it will pass easily, you must increase the diameter of the slug slightly.
To prepare the cylinder for slugging, remove it from the gun, clean it of all lead and copper fouling, lubricate it lightly and make sure that it is solidly supported on a suitable wooden block. With an appropriate size brass drift, carefully drive the prepared slug through each throat from the rear of the chamber. What you are attempting here is to find the diameter of the smallest throat, so handle the soft lead slug carefully and drive it straight to avoid distorting it. This may take a couple of practice runs because the transition, between the area of the chamber where the brass case resides and the area into which the bullet projects, varies from cylinder to cylinder. Some cylinders have a sharp step, while others have a relatively long, smooth transition. If the revolver has a sharp step, it may be easier to avoid distorting the slug by driving it from the front of the cylinder.
The diameter of the slug should be the ideal bullet diameter to be fired in you gun. If the slug is the nominal diameter for the caliber, you will be very fortunate; usually you will find it is somewhat larger. Don't worry about the size difference in the throats in a particular cylinder, I find that even though they are sometimes badly oversize, they usually do not vary much more than .0005". If the barrel slugs larger than throats you have a problem and two options; either open the throats to . 00 I" or .002" larger than the bore or get a new gun. Fortunately this condition is rare.
You may encounter a condition where a throat size bullet is so large that a loaded cartridge cannot be chambered. In cases where this condition is marginal a taper crimp will usually solve the problem. Where it is more extreme, the chamber, but not the throat, will need to be enlarged to accommodate the cartridge. The only revolver in which I have had this experience was a stainless steel Virginian Dragoon .44 Magnum Interarms fitted a new cylinder to the gun. Use only enough taper crimp to hold the bullet securely, otherwise you will size the bullet as it enters the taper crimp die and ruin all of your careful efforts.
Remember that when shooting cast bullets the size of the expander plug is very important. If you use the standard size plug normally supplied by the manufacturer, the oversize bullet may be sized down when it is seated in the cartridge case, again ruining your efforts. The expander plug for cast bullets should be .001" to .002" smaller than your new bullet diameter.
I keep a supply of various size Lyman M die expander plugs. When I need a particular size, I con a machinist friend to turn one down to the proper diameter. The Lyman M die is an excellent design because it applies a step at the case mouth that makes for an easy way to start the bullet into the case with the bullet axis concentric with the case axis; a very important characteristic for pistol and rifle ammunition.
Another problem is one that few shooters know exists. In revolvers, the area of the bore adjacent to the forcing cone, where the barrel is screwed into the frame, is sometimes smaller than the rest of the bore. This damages the bullet the same as an undersize throat and unfortunately, is a very common condition. The solution is to remove the constriction with an abrasive charged lead lap. This is not an easy job for the inexperienced. An excellent alternative is to shoot several hundred rounds, 200 to 500, depending on the amount of constriction, charged with a suitable lapping compound. This has the added advantages of polishing and very slightly tapering the bore. Lapping improves cast bullet performance and minimizes leading. It is important that the proper lapping compound be used, Clover's Valve Grinding Compound is much too coarse. LBT can supply the proper stuff for about $10. 00.
When shooting the longer, fitted bullets, load them as far into the throat as is practically possible. This assures true concentricity of the bullet and bore axes for precise launch into the bore. It makes the cartridge fit independent of a sloppy chamber because the bullet supports the case rather than the reverse. I load my heavy bullet .45 Colt, .44 Magnum, and .357 loads to clear the front of the chamber by about only .10". Of course, short bullets do not allow seating out very far.
I would conclude with one more tip that applies not only to revolvers, but to cast bullet shooting in general Slow powders seem to be much kinder to cast bullets than faster ones. I have developed the practice of using the slowest powder that will provide the desired velocity. Most of my practice shooting is done with cartridges loaded with some of the very slow, low priced swplus powders. Slow powders sometimes makes a .308 Win. into a .30-30, but paper and the appropriate game do not know the difference. Characteristically unburned powder is left in the bore but blowing it out between shots does not change accuracy. Lead .45-70 slugs with 2% tin shoot well in my Marlin and Contender, with no leading at 1,300 fps when loaded with a case full of MR-8600 or MR-8700.
A more common and practical approach to obtaining proper bullet fit is to have a mould cut for your throat and bullet alloy. Veral Smith of Lead Bullet Technology, does this work for little more than the price of a production mould. Ask him to cut the mould to cast bullets about .0005" under your throat size. Minor bullet diameter adjustments can be made by varying the bullet alloy content.
To obtain the diameter of a bullet that will shoot well in your revolver, take a soft lead bullet and squeeze it gently in a vise until it is about .01011 larger than nominal bullet diameter. Attempt to push this slug through each throat of your cylinder using hand pressure only. If it will not pass through the cylinder's throats, your slug is large enough. If it will pass easily, you must increase the diameter of the slug slightly.
To prepare the cylinder for slugging, remove it from the gun, clean it of all lead and copper fouling, lubricate it lightly and make sure that it is solidly supported on a suitable wooden block. With an appropriate size brass drift, carefully drive the prepared slug through each throat from the rear of the chamber. What you are attempting here is to find the diameter of the smallest throat, so handle the soft lead slug carefully and drive it straight to avoid distorting it. This may take a couple of practice runs because the transition, between the area of the chamber where the brass case resides and the area into which the bullet projects, varies from cylinder to cylinder. Some cylinders have a sharp step, while others have a relatively long, smooth transition. If the revolver has a sharp step, it may be easier to avoid distorting the slug by driving it from the front of the cylinder.
The diameter of the slug should be the ideal bullet diameter to be fired in you gun. If the slug is the nominal diameter for the caliber, you will be very fortunate; usually you will find it is somewhat larger. Don't worry about the size difference in the throats in a particular cylinder, I find that even though they are sometimes badly oversize, they usually do not vary much more than .0005". If the barrel slugs larger than throats you have a problem and two options; either open the throats to . 00 I" or .002" larger than the bore or get a new gun. Fortunately this condition is rare.
You may encounter a condition where a throat size bullet is so large that a loaded cartridge cannot be chambered. In cases where this condition is marginal a taper crimp will usually solve the problem. Where it is more extreme, the chamber, but not the throat, will need to be enlarged to accommodate the cartridge. The only revolver in which I have had this experience was a stainless steel Virginian Dragoon .44 Magnum Interarms fitted a new cylinder to the gun. Use only enough taper crimp to hold the bullet securely, otherwise you will size the bullet as it enters the taper crimp die and ruin all of your careful efforts.
Remember that when shooting cast bullets the size of the expander plug is very important. If you use the standard size plug normally supplied by the manufacturer, the oversize bullet may be sized down when it is seated in the cartridge case, again ruining your efforts. The expander plug for cast bullets should be .001" to .002" smaller than your new bullet diameter.
I keep a supply of various size Lyman M die expander plugs. When I need a particular size, I con a machinist friend to turn one down to the proper diameter. The Lyman M die is an excellent design because it applies a step at the case mouth that makes for an easy way to start the bullet into the case with the bullet axis concentric with the case axis; a very important characteristic for pistol and rifle ammunition.
Another problem is one that few shooters know exists. In revolvers, the area of the bore adjacent to the forcing cone, where the barrel is screwed into the frame, is sometimes smaller than the rest of the bore. This damages the bullet the same as an undersize throat and unfortunately, is a very common condition. The solution is to remove the constriction with an abrasive charged lead lap. This is not an easy job for the inexperienced. An excellent alternative is to shoot several hundred rounds, 200 to 500, depending on the amount of constriction, charged with a suitable lapping compound. This has the added advantages of polishing and very slightly tapering the bore. Lapping improves cast bullet performance and minimizes leading. It is important that the proper lapping compound be used, Clover's Valve Grinding Compound is much too coarse. LBT can supply the proper stuff for about $10. 00.
When shooting the longer, fitted bullets, load them as far into the throat as is practically possible. This assures true concentricity of the bullet and bore axes for precise launch into the bore. It makes the cartridge fit independent of a sloppy chamber because the bullet supports the case rather than the reverse. I load my heavy bullet .45 Colt, .44 Magnum, and .357 loads to clear the front of the chamber by about only .10". Of course, short bullets do not allow seating out very far.
I would conclude with one more tip that applies not only to revolvers, but to cast bullet shooting in general Slow powders seem to be much kinder to cast bullets than faster ones. I have developed the practice of using the slowest powder that will provide the desired velocity. Most of my practice shooting is done with cartridges loaded with some of the very slow, low priced swplus powders. Slow powders sometimes makes a .308 Win. into a .30-30, but paper and the appropriate game do not know the difference. Characteristically unburned powder is left in the bore but blowing it out between shots does not change accuracy. Lead .45-70 slugs with 2% tin shoot well in my Marlin and Contender, with no leading at 1,300 fps when loaded with a case full of MR-8600 or MR-8700.