Some Data Regarding Barrel Bores & Handloading Cast Bullets by Karl Bosselman
Slug that bore!
Slug that bore!
It’s hard to over emphasize the importance of knowing the exact rifling groove diameter of your barrel. Obtaining this measurement, usually done by slugging will provide an excellent opportunity to also check rifling twist and to make certain it is what you need for your intentions. First and foremost, a firearm to be used at all seriously must be reliable. Second in importance, it must be accurate... with different ammunition shooting differently in different rifles, tailored-to-the-rifle handloads are necessary to achieving topmost performance.
As is to be expected, ever since the beginning of firearms manufacturemany centuries ago, production tolerances have varied from specifications. Barrels vary; revolver chamber throats,cylinders, and chamber locations vary; barrel chamber throats and forcing conesvary; bullets vary ... everything varies since each firearm is a distinctindividual - this due to wearing of tooling, due to general workmen sloppiness due to poor quality controls, sometimesdue to varying product specification approaches by individual craftsmen and bymanufacturers, et al. It takesconcentrated work to obtain and retain consistently fine accuracy, more soaccuracy to the topmost levels so dedicated riflemen and pistoleros havecomplicated tasks ahead of them. In theend the last resort can prove to be the best one in the first place asadvocated by those most knowledgeable - a custom, finest quality barrel orderedto precise specifications and installed by an established custom gunsmith whoalso performs the other necessary work.Here’s an example. In a quick check of five .458 Win. Magnum chambered factory-produced rifles barrel groove diameters ran from .4586” to .4601”; a random check of eight different jacketed and solid bullets of assorted manufacturers run .4574” to .4589” so these figures mean that specifically chosen bullets must go to specific rifles for better accuracy results - it isn’t a situation of grab, load, and hunt.
Regarding cast lead/lead alloy bullets, generally speaking best accuracy levels are accomplished with custom moulds producing bullets designed for the specific rifle’s chamber and throat and the rifling groove diameter although, regarding the latter, some firearms demand an approximate variance of +.001” or more to achieve finest results. Knowledge, time-consuming methodical experimentation and trial-and-error testing are required for discoveries and top performance.
Revolvers not only require bore slugging (check for bore constriction at barrel breech from torquing into frame), but also precise measuring of all chamber throats which tend to vary but which ideally should be of the same diameter or slightly larger than the barrel rifling grooves. Throats must not be undersize as this is certainly detrimental to top
Regarding cast lead/lead alloy bullets, generally speaking best accuracy levels are accomplished with custom moulds producing bullets designed for the specific rifle’s chamber and throat and the rifling groove diameter although, regarding the latter, some firearms demand an approximate variance of +.001” or more to achieve finest results. Knowledge, time-consuming methodical experimentation and trial-and-error testing are required for discoveries and top performance.
Revolvers not only require bore slugging (check for bore constriction at barrel breech from torquing into frame), but also precise measuring of all chamber throats which tend to vary but which ideally should be of the same diameter or slightly larger than the barrel rifling grooves. Throats must not be undersize as this is certainly detrimental to top
accuracy especially as the bullets might not upset enough upon firing to fill the grooves for decent accuracy. Revolver barrel forcing cones must also be concentric and smooth.
As readers of this magazine realize cast bullets can produce absolutely superb results, either accuracy at the competition range or regarding terminal ballistics in the hunting fields. Custom moulds designed to the specific firearm, its bore and chamber(s) are of course, the ideal approach. But extreme care must be taken in their manufacture down to the maker’s knowing the exact alloy(s) that will be used. Not only do casting techniques between individuals vary, and bullet diameters change with some alloys when projectiles are stored even under ideal conditions but numerous other factors must be considered. From the current Redding Reloading Equipment 2002 Product Catalog, Page 39, here’s some valuable information:
“Bullet diameters and weights will vary considerably depending on the lead casting alloy used. This variation can be as much as 1/2% on the diameter, and 8% on the weight among the most commonly used casting alloys. For example, a .358-158 grain bullet might show a diameter variation of .002”, and a 13 grain difference in weight.
“Of the most commonly used alloys, wheel weights (.5% tin, 3% antimony, 96% lead) will produce bullets having the smallest diameter and heaviest weight, with such bullets running approximately .3% smaller in diameter and 3% heavier than bullets cast with Taracorp’s metal. Linotype will produce bullets with the largest diameter and lightest weights. This alloy will produce bullets approximately 1/10% larger and 3% lighter than Taracorp. Other alloys of tin and antimony, with antimony content above 5%, will produce bullets with diameters and weights falling between those cast from wheel weights and linotype.
As readers of this magazine realize cast bullets can produce absolutely superb results, either accuracy at the competition range or regarding terminal ballistics in the hunting fields. Custom moulds designed to the specific firearm, its bore and chamber(s) are of course, the ideal approach. But extreme care must be taken in their manufacture down to the maker’s knowing the exact alloy(s) that will be used. Not only do casting techniques between individuals vary, and bullet diameters change with some alloys when projectiles are stored even under ideal conditions but numerous other factors must be considered. From the current Redding Reloading Equipment 2002 Product Catalog, Page 39, here’s some valuable information:
Bullet Sizes & Weights & How to Vary Them
“The bullet diameters and weights presented in this list are based on the use of Taracorp’s Lawrence Magnum bullet alloy (2% tin, 6% antimony, 1/4% arsenic, 91.75% lead).“Bullet diameters and weights will vary considerably depending on the lead casting alloy used. This variation can be as much as 1/2% on the diameter, and 8% on the weight among the most commonly used casting alloys. For example, a .358-158 grain bullet might show a diameter variation of .002”, and a 13 grain difference in weight.
“Of the most commonly used alloys, wheel weights (.5% tin, 3% antimony, 96% lead) will produce bullets having the smallest diameter and heaviest weight, with such bullets running approximately .3% smaller in diameter and 3% heavier than bullets cast with Taracorp’s metal. Linotype will produce bullets with the largest diameter and lightest weights. This alloy will produce bullets approximately 1/10% larger and 3% lighter than Taracorp. Other alloys of tin and antimony, with antimony content above 5%, will produce bullets with diameters and weights falling between those cast from wheel weights and linotype.
“Alloys containing little or no antimony will cast considerably smaller than wheel weights and in some cases will produce bullets too small for adequate sizing. Within the limitations given above, the weight and diameter of a cast bullet can be adjusted by varying the alloy’s antimony content.
“The size and weight of bullets of a given alloy will also vary according to casting temperature. Higher temperatures will result in greater shrinkage as the bullet cools, thereby producing a slightly smaller and lighter bullet than one cast of the same alloy at a lower temperature.”
Accurate and effective use of cast bullets is a far more complex and exacting endeavor than is accurate jacketed, solid, and homogeneous solid bullets use. Casting of quality bullets is the toughest part of the normal handloading endeavor - with little difference it takes about the same amount of time to cast a bad bullet as it does to produce a fine one. Cast bullets most often demand more overall detailed loads engineering and development over jacketed since more variables exist upon which top levels of achievement of consistent accuracy and of consistent terminal ballistic performance levels exist.
Interestingly, rifles produced in the latter third of the 19th Century possessed some very significant barrel groove diameter variations commonly differing .004” and more in most given calibers including - .38-56 W.C.F., .40-60 W.C.F., .40-65 W.C.F., .40-70 Sharps Straight, and a host of others. For example, Connor Duncan, in the Nov./Dec. 1999 issue of Single Shot Rifle Journal states in his column, “Classic Cartridges”, that varying “...groove diameters for the .45-120 SS (Sharps Straight) are as small as .451” or as large as .461”...”
With currently-produced barrels “...fairly consistent .458” groovediameter especially if they’re made in the U.S.” But it’s wise to note that alot of these vintage barrels would/will produce acceptable accuracy if bulletswere cast quite soft so they would upset to
“The size and weight of bullets of a given alloy will also vary according to casting temperature. Higher temperatures will result in greater shrinkage as the bullet cools, thereby producing a slightly smaller and lighter bullet than one cast of the same alloy at a lower temperature.”
Accurate and effective use of cast bullets is a far more complex and exacting endeavor than is accurate jacketed, solid, and homogeneous solid bullets use. Casting of quality bullets is the toughest part of the normal handloading endeavor - with little difference it takes about the same amount of time to cast a bad bullet as it does to produce a fine one. Cast bullets most often demand more overall detailed loads engineering and development over jacketed since more variables exist upon which top levels of achievement of consistent accuracy and of consistent terminal ballistic performance levels exist.
Interestingly, rifles produced in the latter third of the 19th Century possessed some very significant barrel groove diameter variations commonly differing .004” and more in most given calibers including - .38-56 W.C.F., .40-60 W.C.F., .40-65 W.C.F., .40-70 Sharps Straight, and a host of others. For example, Connor Duncan, in the Nov./Dec. 1999 issue of Single Shot Rifle Journal states in his column, “Classic Cartridges”, that varying “...groove diameters for the .45-120 SS (Sharps Straight) are as small as .451” or as large as .461”...”
With currently-produced barrels “...fairly consistent .458” groovediameter especially if they’re made in the U.S.” But it’s wise to note that alot of these vintage barrels would/will produce acceptable accuracy if bulletswere cast quite soft so they would upset to
closely fill the grooves upon combustion of the black powder. However, when smokeless powder/jacketed bullet loads were issued at the end of that century, coupled with those varying bore diameters accuracy predictably went down the tubes. Personally the most extreme variation I’ve experienced has been with the .38-55 Ballard target cartridge issued in 1884 (subsequently adopted by Marlin then Winchester; in modern times called the .38-55
Winchester), which I’ve repeatedly slugged in vintage to modern firearms runs from .3752” to .3821” - a disparity of .0069”. I have some Winchester ammunition Lot #80HM41, with seated bullets averaging .3763”. Olinproduced Model 1894 Winchester Commemorative barrels in this caliber often have groove diameters exceeding these bullets by well over .0035”).
What are the usual causes of bore leading and inaccuracy?
- Bullet diameter incorrect for barrel groove diameter.
-Velocity too high for the chosen alloy (alloy too soft). Bullet hardness that is correct for your pursuit is absolutely critical to top accuracy and top terminal ballistics.
-Poorly dimensioned, or nonexistent or not concentric chamber throat or forcing cone.
-Poor quality lubricant or inappropriate lubricant for the velocity(ies) or the propellant. (There is NO allpurpose lubricant; formulated lubes exist for black powder and for smokeless powders and for velocity ranges regarding the latter. Investigate well).
-Poor barrel bore: tooling marks (especially lateral), erosion, pitting corrosion, rusted, and/or dimensionally inconsistent.
- Bullet design incorrect: too light (too short) or too heavy (too long) for the rifling pitch and type; front drive band too narrow and/or other drive bands too wide; poorly dimensioned grease grooves; nose diameter too small and/or too short for barrel throat; ogive not streamlined enough for long-distance shooting.
Winchester), which I’ve repeatedly slugged in vintage to modern firearms runs from .3752” to .3821” - a disparity of .0069”. I have some Winchester ammunition Lot #80HM41, with seated bullets averaging .3763”. Olinproduced Model 1894 Winchester Commemorative barrels in this caliber often have groove diameters exceeding these bullets by well over .0035”).
What are the usual causes of bore leading and inaccuracy?
- Bullet diameter incorrect for barrel groove diameter.
-Velocity too high for the chosen alloy (alloy too soft). Bullet hardness that is correct for your pursuit is absolutely critical to top accuracy and top terminal ballistics.
-Poorly dimensioned, or nonexistent or not concentric chamber throat or forcing cone.
-Poor quality lubricant or inappropriate lubricant for the velocity(ies) or the propellant. (There is NO allpurpose lubricant; formulated lubes exist for black powder and for smokeless powders and for velocity ranges regarding the latter. Investigate well).
-Poor barrel bore: tooling marks (especially lateral), erosion, pitting corrosion, rusted, and/or dimensionally inconsistent.
- Bullet design incorrect: too light (too short) or too heavy (too long) for the rifling pitch and type; front drive band too narrow and/or other drive bands too wide; poorly dimensioned grease grooves; nose diameter too small and/or too short for barrel throat; ogive not streamlined enough for long-distance shooting.
For those who realize the extreme importance of accurate bullet placement, the approach must be from that of absolute specifics. Every existing relevant factor to accuracy and of course, terminal ballistics excellence for hunting-category arms, requires a specific reason for its presence.
Here follow some basic essentials for fine accuracy and superb in-field performance:
1.Carefully investigate then methodically select the precise bullet style(s) and type(s) for your exact purpose(s) planned.
2.Select the appropriate preferably exact, alloy(s) for your activity(ies) and its/their behavior upon impact - if hunting game; study the properties. Some will harden with age but others will soften. There are the few that will remain quite stable as to Brinnel hardness.
If you have cast bullets that have been stored for months or even many years, especially if they’re secondhand, measure their hardness and their diameters before loading them.
Here follow some basic essentials for fine accuracy and superb in-field performance:
1.Carefully investigate then methodically select the precise bullet style(s) and type(s) for your exact purpose(s) planned.
2.Select the appropriate preferably exact, alloy(s) for your activity(ies) and its/their behavior upon impact - if hunting game; study the properties. Some will harden with age but others will soften. There are the few that will remain quite stable as to Brinnel hardness.
If you have cast bullets that have been stored for months or even many years, especially if they’re secondhand, measure their hardness and their diameters before loading them.
Double-cavity mould blocks of 2024-T-351 aluminum by LBT (Lead Bullets Technology) producing
445-grain Wide Flat Nose gas check bullets tailored to a .458 Win. Mag. chambered rifle. Accuracy
results of this heavy bore hunting rifle are impressive, hovering around the 3/4” category and less, center-to-center for 6 rounds at 100 yards under
ideal benchrest conditions. Custom moulds provide crucial exacting fit to the specific firearm for finest accuracy potentials.
445-grain Wide Flat Nose gas check bullets tailored to a .458 Win. Mag. chambered rifle. Accuracy
results of this heavy bore hunting rifle are impressive, hovering around the 3/4” category and less, center-to-center for 6 rounds at 100 yards under
ideal benchrest conditions. Custom moulds provide crucial exacting fit to the specific firearm for finest accuracy potentials.
Use a consistent alloy(s) from the same source if feasible to try to ensure the same composition batch-to-batch. The best approach is to melt a large amount of quality alloy at the same time, flux to mix well then pour into ingot moulds. The more suppliers or sources involved the more variance is to be expected. Linotype metal as an example, tends to vary substantially in composition. Count upon variances between virgin alloys and reclaimed metal especially if the latter has an unknown use history or source. Variations in alloy will create bullet weight and hardness differences, thus accuracy deviation.3.Maintain an ideal temperature for the alloy
used - LBT suggests about 750° (F) - and consistent pot, metal, and mould temperatures.
When casting large quantities of bullets, especially large bore heavyweights, it’s best to use a secondary melting pot for fluxing and for supplying liquid metal to the bottom-pour primary pot. This to ensure a good constant supply of heated metal so your rhythm is only minimally broken.
When casting large quantities of bullets, especially large bore heavyweights, it’s best to use a secondary melting pot for fluxing and for supplying liquid metal to the bottom-pour primary pot. This to ensure a good constant supply of heated metal so your rhythm is only minimally broken.
4. For top results a knowledgeable, proficient craftsman must do the casting. This ensures a proper, regular and consistent rhythm via a proven and accomplished technique. Longer-length casting sessions are best; shorter sessions can easily alter the workflow thus consistency in bullet quality occurs.
Different people cast differently. This variation is reflected in bullet quality, and sometimes in bullet weight and hardness depending upon the technique used, this despite the same equipment and alloy. The obvious results are variations in accuracy.
And count upon bullet quality varying if you cast one day when you feel good and another day when you feel badly: those batches will without doubt vary and pistons are usually cut to a slight concave radius which is utilized to keep lubricant off the bullet bases. However, I most always prefer mine cut to a flat surface. Some manufacturing mistakes occasionally release these pistons with mis-machined or poorly machined tops perhaps cut irregularly convexed, which is wrong. (Return these to the factory for correction or replacement).
Different people cast differently. This variation is reflected in bullet quality, and sometimes in bullet weight and hardness depending upon the technique used, this despite the same equipment and alloy. The obvious results are variations in accuracy.
And count upon bullet quality varying if you cast one day when you feel good and another day when you feel badly: those batches will without doubt vary and pistons are usually cut to a slight concave radius which is utilized to keep lubricant off the bullet bases. However, I most always prefer mine cut to a flat surface. Some manufacturing mistakes occasionally release these pistons with mis-machined or poorly machined tops perhaps cut irregularly convexed, which is wrong. (Return these to the factory for correction or replacement).
As has proven to be the usual approach, I most always seat bullets, cast and jacketed, out to solid chamber throat contact. Over decades this has most always proven to be the best for my rifles and given the best accuracy.
Micro-groove-type rifling can often shoot cast bullets magnificently but the bores must be mirrorsmooth, dimensionally consistent and bullet selection ideal ... no bore-ride or narrow-nosed styles. With this type, having shallower grooves and more numerous lands, bullet distortion during bore travel is minimized.
With narrow-land rifling bullets with short noses and long shanks generally shoot best. Wide-land barrels tend to promote inaccuracy with cast bullets because this type generally increases bullet distortion and compromise accuracy.
Micro-groove-type rifling can often shoot cast bullets magnificently but the bores must be mirrorsmooth, dimensionally consistent and bullet selection ideal ... no bore-ride or narrow-nosed styles. With this type, having shallower grooves and more numerous lands, bullet distortion during bore travel is minimized.
With narrow-land rifling bullets with short noses and long shanks generally shoot best. Wide-land barrels tend to promote inaccuracy with cast bullets because this type generally increases bullet distortion and compromise accuracy.
Properly tapered chamber throating aligns (i.e. centers) the bullet in the boreat the beginning of its travel, thus is critical to achievingconsistently high levels of accuracy.At left are two pure lead slugs formed in the forward chamber/bore breechareas of two Browning Model 71 lever-action rifles - no tapered throat, simplyan abrupt step-down into the rifling. This is one of many cost-cutting manufacturingtechniques of traditional type lever guns that translate into poor levelsof accuracy. Third from left is a lead slug formed in the same region of afalling block Ruger in .458 Win. Magnum chambering. Note the well-taperedgradual leade from the chamber-into the bore. This is as it should be. (A fired.458 case is at right). 5. Cast your chamberor slug your throat to see thelatter’s diameter vs. that of thebarrel groove diameter. Toachieve top accuracy youmight have to use a bulletmore closely fitting the throatand which tends to be overlylarge for the grooves. Mostalways the best for cast bulletsis a long-tapered throat: themore abrupt and shorter themore inaccurate it’s inclined tobe.
6.The more bulletdiameter is reduced in size ina sizer-lubricator the more distortionis created, thus morecompromising of accuracy, soyour mould with your chosenalloy should perhaps only castabout .0015” maximum overrequired diameter but this isvery individualistic as to yourfirearm’s preferences.
After making certainmy lubrisizer has beenmachined on-axis I’ll minimizethe tendency to bullet distortionby cutting the sizer diebevel to a more tapered 60-degree angle.Incidentally sizer die pistons are usually cut to a slight concave radius whichis utilized to keep lubricant off the bullet bases. However,I most always prefer mine cut to a flat surface. Somemanufacturing mistakes occasionally release these pistonswith mis-machined or poorly machined tops perhapscut irregularly convexed, which is wrong. (Return theseto the factory for correction or replacement).
6.The more bulletdiameter is reduced in size ina sizer-lubricator the more distortionis created, thus morecompromising of accuracy, soyour mould with your chosenalloy should perhaps only castabout .0015” maximum overrequired diameter but this isvery individualistic as to yourfirearm’s preferences.
After making certainmy lubrisizer has beenmachined on-axis I’ll minimizethe tendency to bullet distortionby cutting the sizer diebevel to a more tapered 60-degree angle.Incidentally sizer die pistons are usually cut to a slight concave radius whichis utilized to keep lubricant off the bullet bases. However,I most always prefer mine cut to a flat surface. Somemanufacturing mistakes occasionally release these pistonswith mis-machined or poorly machined tops perhapscut irregularly convexed, which is wrong. (Return theseto the factory for correction or replacement).
As has proven to be the usual approach, I mostalways seat bullets, cast and jacketed, out to solid chamberthroat contact. Over decades this has most alwaysproven to be the best for my rifles and given the bestaccuracy.
Micro-groove-type rifling can often shoot castbullets magnificently but the bores must be mirrorsmooth,dimensionally consistent and bullet selectionideal ... no bore-ride or narrow-nosed styles. With thistype, having shallower grooves and more numerouslands, bullet distortion during bore travel is minimized.
With narrow-land rifling bullets with short nosesand long shanks generally shoot best. Wide-land barrelstend to promote inaccuracy with cast bullets because thistype generally increases bullet distortion.
Remember the importance of consistencies - fromevery conceivable factor and at all times. Without it, all bullet performance suffers. Of course, impeccable tuningof rifles and exacting mechanics regarding cartridgesare a huge part of shooting successes; another is theeffect of fouling on your shot placements. Other ones ofmajor importance are accurate reading of the wind andaccurate gauging of distances, both of which can beextreme challenges indeed. •
Micro-groove-type rifling can often shoot castbullets magnificently but the bores must be mirrorsmooth,dimensionally consistent and bullet selectionideal ... no bore-ride or narrow-nosed styles. With thistype, having shallower grooves and more numerouslands, bullet distortion during bore travel is minimized.
With narrow-land rifling bullets with short nosesand long shanks generally shoot best. Wide-land barrelstend to promote inaccuracy with cast bullets because thistype generally increases bullet distortion.
Remember the importance of consistencies - fromevery conceivable factor and at all times. Without it, all bullet performance suffers. Of course, impeccable tuningof rifles and exacting mechanics regarding cartridgesare a huge part of shooting successes; another is theeffect of fouling on your shot placements. Other ones ofmajor importance are accurate reading of the wind andaccurate gauging of distances, both of which can beextreme challenges indeed. •
