JimGnitecki
Member
- Joined
- Mar 28, 2010
- Messages
- 1,258
Forum Admin HSO has suggested that those of us interested in consistent superior handgun accuracy start a thread dedicated to gathering up ideas to improve consistency of Base-to-Ogive ("BTO") for handgun loads, to optimize both performance and safety.
WHY
Apparently, a lot of handgun shooters, and especially handgun competitors, are realizing that accepting large BTO variations (.010" or even larger) is no longer the thing to do, since modern manufacturing and tuning improvements have made some handguns remarkably accurate even with just factory ammunition. Just as 1 MOA rifle performance is now a seriously outdated performance standard, 3 -inch handgun groups at 25 yards and .010" BTO variances are no longer "good enough", and should not be passively accepted as "good enough" or "the best we can practically do”.
HOW
However, everyone seems to be working in their own "bubble" in efforts to improve BTO consistency. This thread's purpose is to try to jumpstart a "brainstorming" teamwork approach, where we all contribute any single idea or multiple ideas to experiment with, and then each individually try some of those ideas and report back on how effective or ineffective they turn out to be when executed with some care in the real world.
We can probably expect some ideas to flop when actually tested, but others may prove to be the "breakthrough" that gets us at least close to what precision rifle reloaders are actually achieving.
So, this thread invites you to post potentially beneficial ideas to improve BTO consistency.
AND, in the spirit of true brainstorming, we should not say "it'll never work" to ANY idea suggested here, unless someone can truthfully say that they have "tried that' with the proper degree of detail, precision, and true measurment and recordkeeping, and can give hard data about why the idea did not work or worked only marginally.
FUNDAMENTALS FOR THE DISCUSSION & IDEAS
As a starting point, let's do some basic intro to this whole concept of BTO and why it is important:
First, let's discuss the difference bewteen BTO and COAL.
Cartridge Overall Length ("COAL") measures the length and consistency of finished loaded cartridges, measured from the bottom of the cartridge case to the tip of the bullet.
BTO on the other hand measures the length and consistency of finished loaded cartridges, measured from the bottom of the cartridge case to a specific point on the ogive (the curved or angled side surface of the bullet, located between the "shank" (the cylindrical portion of the bullet that actually contacts the rifling) and the "meplat" (the "top" or "tip" of the bullet). However, the “specific point” selected on the ogive varies from cartridge to cartridge and from shooter to shooter, for reasons that will be explained below!
Why have TWO measures? Because they are each important for reasons that are simultaneously similar but also significantly different!
COAL is important because overall length of a cartridge can be a “fit” problem if too long or too short. First, every caliber specifies the acceptable COAL and the acceptable variance for that COAL. If a cartridge is too long it might not fit into a properly constructed firearm chamber of that caliber. Also, if too long, a cartridge might not fit into, or feed acceptably from, a magazine. A COAL that is too short might result in too long a “jump” for its bullet to get from the case mouth to the rifling, and thus hurt accuracy. A COAL that is too short can also result in a smaller “combustion chamber” inside the cartridge case, which can dramatically raise the peak pressure in the case and cause a catastrophic failure that can damage or destroy the firearm and injure or kill the shooter and any bystanders.
So, COAL is important for fit, accuracy, and safety reasons.
And, if all bullets in the same caliber and shape and production batch were truly “identical”, COAL would be sufficient” as a measure of important consistency.
But, bullets in the same caliber and shape and production batch are nevertheless not “identical”. They have production tolerances. And, specifically in the case of hollowpoint jacketed bulllets, the LARGEST tolerance turns out to be in the overall length of the bullet. This variation in overall length of the bullet means that if you measure COAL (only) in one box of ammo, individual bullets might be too long or too short for ultimate accuracy, too short to make the combustion chamber pressure high enough to attain the desired velocity, or too long to keep peak pressure inside the cartridge case at a safe level.
In a mild load, and in a firearm designed and built to accept some variance in ammunition, and for use by a shooter with moderate expectations, production bullets are adequate.
But in a “hot” load, or in a firearm built for precision accuracy, in the hands of a shooter who needs or wants ultimate attainable accuracy, the variability of the bullet length, despite COAL being consistent, is potentially a big problem.
BTO is important for the same reasons as COAL, but when discussing accuracy and safety, it recognizes that:
- Bullets in the same caliber and shape and production batch are nevertheless not “identical”
- COAL is not a precise enough measure to control bullet jump or pressure safety
- In ADDITION to measuring COAL, a reloader aiming for hot loads or superior accuracy needs to measure BTO because BTO actually measures what directly affects both bullet jump consistency and combustion pressure consistency, and thus both accuracy and safety.
This is because while the COAL is important for the reasons stated earlier, it does not actually measure one key measurement that determines the consistency of both combustion pressure and bullet jump: the distance from the bottom of the cartridge case to the point at which the bullet actually engages the rifling. That point is critical because:
- Its distance from the base of the bullet is far more consistent than the distance from the base of the bullet to the meplat. Therefore, it is a way superior way to control combustion pressure.
- Its distance from the rifling is a far superior way to measure and control jump. The meplat position relative to the rifling has NO effect on bullet jump. The BTO point on the other hand is PRECISELY, by definition, the first point of contact with the rifling. You cannot get a better measure of jump!
THE THEORETICAL PROBLEM AND ITS SOLUTION
There is however a problem in trying to measure from the base of the cartridge to the exact point where the shank ends and the ogive begins.
The theoretical way such a measurement is imagined to occur would be to use a hollowed out cylinder, whose open-end inside-diameter is exactly the bullet shank diameter. You would secure this cylinder to one jaw in a precision caliper, zero the caliper with the cylinder captured between the jaws, and then insert a finished cartridge into the caliper jaws with its bullet end pushed into the cylinder, and its base held by the 2nd caliper jaw, and take the BTO measurement.
But, this theoretical measurement won’t work for a couple of reasons:
1. On an actual production bullet, the intersection of the shank with the ogive is NOT a “sharp edge” intersection. On a bullet with a curved ogive, it is a “region” in which the shank transitions from shank to ogive. On a bullet with a straight angular ogive (picture a shank with a cone on one end), the intersection of stright shank and straight ogive STILL has a small “chamfer” to it, so again, it is more a “region” than a point.
2. Even if you somehow believe you can accurately “estimate” the correct “point’ within the region, you have no “positive stop” for the hollow cylinder to engage. If you keep pushing the caliper jaws closed, the bullet’s meplat and ogive and then actual shank will simply keep sliding into the cylinder!
This huge problem is solved by making an important realization: To measure the distance from base of cartridge to the point at which the bullet engages the rifling, with surprisingly good accuracy, you need not measure to precisely that point, but rather measure to a point CLOSE to that point (FAR better than measuring to the meplat which has the absolute worst variability in the entire bullet).
So, for example, on a 9mm bullet whose very-high-tolerance shank diameter is .3556”, if you use a measuring cylinder with an ID of say .350”, you will be measuring BTO at a point VERY close to the true intersection of shank and ogive. And, the variability you discover in measuring a reasonably large sampling of your finished cartridges will be a pretty good indicator of the true consistency of your BTO.
Where do you easily and inexpensively get a cylinder of appropriate ID that will work for whichever caliber you are reloading? Just buy one of the “comparator” sets from Hornady or elsewhere. A typical set contains several to many cylinders intended to cover a large number of individual calibers. For each caliber you load, just select the largest ID cylinder that is still smaller than your bullet diameter. And don’t worry if that largest but still smaller ID cylinder is a bit smaller than you’d like. It is STILL going to be a much better measure of BTO consistency than any COAL measurement technique.
WHERE BTO IS MOST IMPORTANT
Where is using BTO consistency measurement most critical in a handgun environment?
1. On a high power load where combustion pressure is high enough to be approaching the published safe limit for the caliber you are loading for. For example, 9 Major or 357 Magnum hot loads. NOT measuring BTO here can cause catastrophic firearm damage, and shooter and nearby spectator injury or death.
2. In a competitive bullseye match, where small differences in on-target accuracy and consistency will win or lose the match
SPECIFIC STARTER QUESTIONS FOR DISCUSSION
- How CAN WE counter THE adverse features of progressive presses that hurt consistency of BTO? WE should ask this because most handgun shooters need progressive presses to make the volume of cartridges they need for competition and practice, despite the at least theoretical precision shortcomings of progressives.
- What bullet SHAPES are particularly good or particularly bad for BTO precision? e.g. Can BTO be more accurately measured on bullets with straight (uncurved) ogives versus curved ogives?
- What TYPES of bullets are particularly good or particularly bad for BTO precision? For example, relative consistency of cast bullets, plated bullets, jacketed bullets, hollowpoint jacketed bullets, etc?
- Which Die makers’ bullet seater designs are particularly good or particularly bad for BTO precision
- Which die bullet seating inserts are easiest and/or cheapest to modify, and how can they most easily and cheaply be modified? Example: Dillon pistol seating inserts are simply basically a hollowed out cylinder with a cross-pin to secure it in a die body, so no fancy machining required
- Are "open tube" seating inserts better than "pocket" inserts, at least for jacketed bullets, because they positively prevent meplat contact with the seating insert before the ogive can engage the seating insert?
I hope that my quickly assembled introduction, and the few starter questions I came up with this morning while on an out-of-town trip migt spark some starting ideas and testing.
I’m hoping that a team effort that focuses on BTO and proposes and actually tests ideas and proposals might get us all closer to the accuracy and safety we wish for in our reloading efforts.
Jim G
WHY
Apparently, a lot of handgun shooters, and especially handgun competitors, are realizing that accepting large BTO variations (.010" or even larger) is no longer the thing to do, since modern manufacturing and tuning improvements have made some handguns remarkably accurate even with just factory ammunition. Just as 1 MOA rifle performance is now a seriously outdated performance standard, 3 -inch handgun groups at 25 yards and .010" BTO variances are no longer "good enough", and should not be passively accepted as "good enough" or "the best we can practically do”.
HOW
However, everyone seems to be working in their own "bubble" in efforts to improve BTO consistency. This thread's purpose is to try to jumpstart a "brainstorming" teamwork approach, where we all contribute any single idea or multiple ideas to experiment with, and then each individually try some of those ideas and report back on how effective or ineffective they turn out to be when executed with some care in the real world.
We can probably expect some ideas to flop when actually tested, but others may prove to be the "breakthrough" that gets us at least close to what precision rifle reloaders are actually achieving.
So, this thread invites you to post potentially beneficial ideas to improve BTO consistency.
AND, in the spirit of true brainstorming, we should not say "it'll never work" to ANY idea suggested here, unless someone can truthfully say that they have "tried that' with the proper degree of detail, precision, and true measurment and recordkeeping, and can give hard data about why the idea did not work or worked only marginally.
FUNDAMENTALS FOR THE DISCUSSION & IDEAS
As a starting point, let's do some basic intro to this whole concept of BTO and why it is important:
First, let's discuss the difference bewteen BTO and COAL.
Cartridge Overall Length ("COAL") measures the length and consistency of finished loaded cartridges, measured from the bottom of the cartridge case to the tip of the bullet.
BTO on the other hand measures the length and consistency of finished loaded cartridges, measured from the bottom of the cartridge case to a specific point on the ogive (the curved or angled side surface of the bullet, located between the "shank" (the cylindrical portion of the bullet that actually contacts the rifling) and the "meplat" (the "top" or "tip" of the bullet). However, the “specific point” selected on the ogive varies from cartridge to cartridge and from shooter to shooter, for reasons that will be explained below!
Why have TWO measures? Because they are each important for reasons that are simultaneously similar but also significantly different!
COAL is important because overall length of a cartridge can be a “fit” problem if too long or too short. First, every caliber specifies the acceptable COAL and the acceptable variance for that COAL. If a cartridge is too long it might not fit into a properly constructed firearm chamber of that caliber. Also, if too long, a cartridge might not fit into, or feed acceptably from, a magazine. A COAL that is too short might result in too long a “jump” for its bullet to get from the case mouth to the rifling, and thus hurt accuracy. A COAL that is too short can also result in a smaller “combustion chamber” inside the cartridge case, which can dramatically raise the peak pressure in the case and cause a catastrophic failure that can damage or destroy the firearm and injure or kill the shooter and any bystanders.
So, COAL is important for fit, accuracy, and safety reasons.
And, if all bullets in the same caliber and shape and production batch were truly “identical”, COAL would be sufficient” as a measure of important consistency.
But, bullets in the same caliber and shape and production batch are nevertheless not “identical”. They have production tolerances. And, specifically in the case of hollowpoint jacketed bulllets, the LARGEST tolerance turns out to be in the overall length of the bullet. This variation in overall length of the bullet means that if you measure COAL (only) in one box of ammo, individual bullets might be too long or too short for ultimate accuracy, too short to make the combustion chamber pressure high enough to attain the desired velocity, or too long to keep peak pressure inside the cartridge case at a safe level.
In a mild load, and in a firearm designed and built to accept some variance in ammunition, and for use by a shooter with moderate expectations, production bullets are adequate.
But in a “hot” load, or in a firearm built for precision accuracy, in the hands of a shooter who needs or wants ultimate attainable accuracy, the variability of the bullet length, despite COAL being consistent, is potentially a big problem.
BTO is important for the same reasons as COAL, but when discussing accuracy and safety, it recognizes that:
- Bullets in the same caliber and shape and production batch are nevertheless not “identical”
- COAL is not a precise enough measure to control bullet jump or pressure safety
- In ADDITION to measuring COAL, a reloader aiming for hot loads or superior accuracy needs to measure BTO because BTO actually measures what directly affects both bullet jump consistency and combustion pressure consistency, and thus both accuracy and safety.
This is because while the COAL is important for the reasons stated earlier, it does not actually measure one key measurement that determines the consistency of both combustion pressure and bullet jump: the distance from the bottom of the cartridge case to the point at which the bullet actually engages the rifling. That point is critical because:
- Its distance from the base of the bullet is far more consistent than the distance from the base of the bullet to the meplat. Therefore, it is a way superior way to control combustion pressure.
- Its distance from the rifling is a far superior way to measure and control jump. The meplat position relative to the rifling has NO effect on bullet jump. The BTO point on the other hand is PRECISELY, by definition, the first point of contact with the rifling. You cannot get a better measure of jump!
THE THEORETICAL PROBLEM AND ITS SOLUTION
There is however a problem in trying to measure from the base of the cartridge to the exact point where the shank ends and the ogive begins.
The theoretical way such a measurement is imagined to occur would be to use a hollowed out cylinder, whose open-end inside-diameter is exactly the bullet shank diameter. You would secure this cylinder to one jaw in a precision caliper, zero the caliper with the cylinder captured between the jaws, and then insert a finished cartridge into the caliper jaws with its bullet end pushed into the cylinder, and its base held by the 2nd caliper jaw, and take the BTO measurement.
But, this theoretical measurement won’t work for a couple of reasons:
1. On an actual production bullet, the intersection of the shank with the ogive is NOT a “sharp edge” intersection. On a bullet with a curved ogive, it is a “region” in which the shank transitions from shank to ogive. On a bullet with a straight angular ogive (picture a shank with a cone on one end), the intersection of stright shank and straight ogive STILL has a small “chamfer” to it, so again, it is more a “region” than a point.
2. Even if you somehow believe you can accurately “estimate” the correct “point’ within the region, you have no “positive stop” for the hollow cylinder to engage. If you keep pushing the caliper jaws closed, the bullet’s meplat and ogive and then actual shank will simply keep sliding into the cylinder!
This huge problem is solved by making an important realization: To measure the distance from base of cartridge to the point at which the bullet engages the rifling, with surprisingly good accuracy, you need not measure to precisely that point, but rather measure to a point CLOSE to that point (FAR better than measuring to the meplat which has the absolute worst variability in the entire bullet).
So, for example, on a 9mm bullet whose very-high-tolerance shank diameter is .3556”, if you use a measuring cylinder with an ID of say .350”, you will be measuring BTO at a point VERY close to the true intersection of shank and ogive. And, the variability you discover in measuring a reasonably large sampling of your finished cartridges will be a pretty good indicator of the true consistency of your BTO.
Where do you easily and inexpensively get a cylinder of appropriate ID that will work for whichever caliber you are reloading? Just buy one of the “comparator” sets from Hornady or elsewhere. A typical set contains several to many cylinders intended to cover a large number of individual calibers. For each caliber you load, just select the largest ID cylinder that is still smaller than your bullet diameter. And don’t worry if that largest but still smaller ID cylinder is a bit smaller than you’d like. It is STILL going to be a much better measure of BTO consistency than any COAL measurement technique.
WHERE BTO IS MOST IMPORTANT
Where is using BTO consistency measurement most critical in a handgun environment?
1. On a high power load where combustion pressure is high enough to be approaching the published safe limit for the caliber you are loading for. For example, 9 Major or 357 Magnum hot loads. NOT measuring BTO here can cause catastrophic firearm damage, and shooter and nearby spectator injury or death.
2. In a competitive bullseye match, where small differences in on-target accuracy and consistency will win or lose the match
SPECIFIC STARTER QUESTIONS FOR DISCUSSION
- How CAN WE counter THE adverse features of progressive presses that hurt consistency of BTO? WE should ask this because most handgun shooters need progressive presses to make the volume of cartridges they need for competition and practice, despite the at least theoretical precision shortcomings of progressives.
- What bullet SHAPES are particularly good or particularly bad for BTO precision? e.g. Can BTO be more accurately measured on bullets with straight (uncurved) ogives versus curved ogives?
- What TYPES of bullets are particularly good or particularly bad for BTO precision? For example, relative consistency of cast bullets, plated bullets, jacketed bullets, hollowpoint jacketed bullets, etc?
- Which Die makers’ bullet seater designs are particularly good or particularly bad for BTO precision
- Which die bullet seating inserts are easiest and/or cheapest to modify, and how can they most easily and cheaply be modified? Example: Dillon pistol seating inserts are simply basically a hollowed out cylinder with a cross-pin to secure it in a die body, so no fancy machining required
- Are "open tube" seating inserts better than "pocket" inserts, at least for jacketed bullets, because they positively prevent meplat contact with the seating insert before the ogive can engage the seating insert?
I hope that my quickly assembled introduction, and the few starter questions I came up with this morning while on an out-of-town trip migt spark some starting ideas and testing.
I’m hoping that a team effort that focuses on BTO and proposes and actually tests ideas and proposals might get us all closer to the accuracy and safety we wish for in our reloading efforts.
Jim G
