Post by Bullshop on May 22, 2019 17:45:55 GMT -7
Duplex loading with smokeless powder seems to be an overlooked area of reloading that most folks reloading ammo are content to remain ignorant of. This may be a good thing too because there is so little published load data or information of any kind related to this subject that without guidance could lead to serious trouble.
Duplex loading using a small primer charge of smokeless powder and a case full of black powder seems to be a broadly accepted practice but tracking down load data or helpful information on the safe procedure of developing duplex smokeless powder loads it is safe to say has gaps in availability.
Its been maybe 30 or so years since I got the bug and headed down this road. In that time I have learned a few things on when and how to make the practice a helpful addition to my bag of tricks to develop accurate cast bullet rifle loads. Since so few people use duplex smokeless loading and even fewer discuss it it seems my duty to share some of the things I have learned and to maybe shed some light on a safe procedure for others to develop their own duplex smokeless loads.
My first experience with duplex smokeless loading was with the 45-70. Starting on the learning curve with this type of loading straight wall rifle cases are probably the easiest and safest to get started with. The absence of a shoulder on a case changes the way the cartridge builds pressure and as a general rule straight wall cases will require faster powder burn rates for optimum and maximum performance than for shouldered counterparts.. There in lies a clew (optimum) to the application and safe development of duplex loads. Duplex loads are seldom developed for optimum or maximum performance. In fact the safety feature inherent in duplex loading is that the powder to be used for the bulk charge has far too slow a burn rate to be even close to optimum for conventional application.
Some available load data that helped start me on my way to duplexing is available from the Accurate Arms company. The AA data book does not offer duplex load data but what they do is offer data for most rifle cartridges with their full range of rifle powders. In practice what they are offering is data starting with powders that are too fast burning to be optimum through powders that are of optimum burn rate and on to powders that are far too slow to be anywhere near optimum in burn rate.
By optimum what I mean is for instance lets look at the 30-06 and starting with fastest to slowest powders what you will see is starting with too fast powders pressures peak before top achievable velocities are reached. Then moving to progressively slower burning powder we move progressively up in velocity until optimum burn rate is reached and highest velocities are recorded. Progressively moving on to still slower burning powders we will not a corresponding reduction in velocity from optimum and as powder burn gets progressively slower velocity is correspondingly lower. At some point in powder burn rate when at 100% load density and velocity is somewhere near 50% of optimum we are in the powder burn rate that is well suited to duplex loading. These are going to be the powders on the very slow end of published burn rat charts powders such as H-870 IMR -5010 AA-8700 and several of the surplus powders such as WC-870 and most powders intended for the 50 BMG.
I mentioned earlier how the AA data book helped me get started so I will explain that. Being something of a scrounger and delighting in powder prices far removed from retail I came into some AA-8700 at a price I could not refuse. Going to the AA data book gave loads for the 45-70 with their AA 8700 powder. In use what I found was that even with 100% load density even with 500+gn bullets in the 45-70 that there was a lot of unburned powder leaf in the barrel and even in the case. This created a problem in that if the powder was not cleared from the chamber in some way that it was very difficult or impossible to chamber another cartridge. If another cartridge was chambered and fired without first clearing the chamber of unburned powder the powder kernels left in the chamber would make dents in the case on being fired in the dirty chamber.
Accuracy could be good with this loading producing black powder equivalent velocities but loading is slow because the chamber must be cleared between shots. Pondering this situation and trying to apply what little I could find on duplex loading I set out to develop a safe procedure to develop a duplex load that would stay in the low chamber pressure range producing black powder equivalent velocity but eliminate the need to clear the chamber between shots.
My idea was to start out with a full 100% density powder charge of the main charge powder then reduce that charge weight by 1 grain at a time and replacing the same volume not weight with a powder to be used as a priming charge. Basically what we are after here is to add a priming charge to give the primer a longer duration of spark to continue pushing on the main charge until it is past the chamber and out of the barrel. In effect the primer charge or as I sometimes call it the kicker charge is burning the main charge somewhat more efficiently because it will increase velocity by about 15% but the main purpose is not increased velocity but is a clean barrel and chamber.
In developing my loads I continue reducing the main charge by 1 grain at a time and replacing an equal volume not weight with the kicker. The reason I stress volume not weight of the kicker powder is that it is imperative that you maintain the 100% load density in the case so that the powder has no room for movement in the case so can not mix. You want and need for the powders to remain stacked in the case not mixed. The goal here is to replace some of the volume of the main charge with the kicker powder until you get to the point that all the unburned powder is blown out and the barrel is free from unburned powder. I refer to unburned powder being blown out the barrel which becomes quite obvious quite quickly when shooting over snow. Typically the weight of the kicker charge will end up being from 5 to 10% of the main charge depending of the burn rate of the kicker charge. The faster the burn rate of the kicker charge the lower the percentage will be of the main charge and visa versa .
Through experience in determining about what burn rate for kicker powder will give the desired results I have found that powder burn rate from about 2400 on the fast side to about 4198 on the slow side give the desired results without raising chamber pressures too much. Kicker powders faster than 2400 will raise chamber pressure and velocity so beware if loading for vintage arms. Kicker powders slower burning than 4198 may not give the desired results of clearing the barrel of all unburned powder until substantially larger volume has been exchanged but at that point chamber pressure as well as velocity will also be elevated.
As I mentioned earlier most of what I have said here pertains to the 45-70 and is a good safe place to start learning duplexing. I have since developed good accurate duplex loads in bottle neck cases such as the 30-06 and will try in the future to expand on that a bit. I also hope to ramble a bit on the WHY of duplexing but for this sitting I have accessed all the brain cells related to memory available at the moment. Will be happy to within my ability answer any related questions. This is something I have been wanting to talk about but cant cover everything I would like in one sitting but now have at least gotten started. Every journey begins with a first step.
Duplex loading using a small primer charge of smokeless powder and a case full of black powder seems to be a broadly accepted practice but tracking down load data or helpful information on the safe procedure of developing duplex smokeless powder loads it is safe to say has gaps in availability.
Its been maybe 30 or so years since I got the bug and headed down this road. In that time I have learned a few things on when and how to make the practice a helpful addition to my bag of tricks to develop accurate cast bullet rifle loads. Since so few people use duplex smokeless loading and even fewer discuss it it seems my duty to share some of the things I have learned and to maybe shed some light on a safe procedure for others to develop their own duplex smokeless loads.
My first experience with duplex smokeless loading was with the 45-70. Starting on the learning curve with this type of loading straight wall rifle cases are probably the easiest and safest to get started with. The absence of a shoulder on a case changes the way the cartridge builds pressure and as a general rule straight wall cases will require faster powder burn rates for optimum and maximum performance than for shouldered counterparts.. There in lies a clew (optimum) to the application and safe development of duplex loads. Duplex loads are seldom developed for optimum or maximum performance. In fact the safety feature inherent in duplex loading is that the powder to be used for the bulk charge has far too slow a burn rate to be even close to optimum for conventional application.
Some available load data that helped start me on my way to duplexing is available from the Accurate Arms company. The AA data book does not offer duplex load data but what they do is offer data for most rifle cartridges with their full range of rifle powders. In practice what they are offering is data starting with powders that are too fast burning to be optimum through powders that are of optimum burn rate and on to powders that are far too slow to be anywhere near optimum in burn rate.
By optimum what I mean is for instance lets look at the 30-06 and starting with fastest to slowest powders what you will see is starting with too fast powders pressures peak before top achievable velocities are reached. Then moving to progressively slower burning powder we move progressively up in velocity until optimum burn rate is reached and highest velocities are recorded. Progressively moving on to still slower burning powders we will not a corresponding reduction in velocity from optimum and as powder burn gets progressively slower velocity is correspondingly lower. At some point in powder burn rate when at 100% load density and velocity is somewhere near 50% of optimum we are in the powder burn rate that is well suited to duplex loading. These are going to be the powders on the very slow end of published burn rat charts powders such as H-870 IMR -5010 AA-8700 and several of the surplus powders such as WC-870 and most powders intended for the 50 BMG.
I mentioned earlier how the AA data book helped me get started so I will explain that. Being something of a scrounger and delighting in powder prices far removed from retail I came into some AA-8700 at a price I could not refuse. Going to the AA data book gave loads for the 45-70 with their AA 8700 powder. In use what I found was that even with 100% load density even with 500+gn bullets in the 45-70 that there was a lot of unburned powder leaf in the barrel and even in the case. This created a problem in that if the powder was not cleared from the chamber in some way that it was very difficult or impossible to chamber another cartridge. If another cartridge was chambered and fired without first clearing the chamber of unburned powder the powder kernels left in the chamber would make dents in the case on being fired in the dirty chamber.
Accuracy could be good with this loading producing black powder equivalent velocities but loading is slow because the chamber must be cleared between shots. Pondering this situation and trying to apply what little I could find on duplex loading I set out to develop a safe procedure to develop a duplex load that would stay in the low chamber pressure range producing black powder equivalent velocity but eliminate the need to clear the chamber between shots.
My idea was to start out with a full 100% density powder charge of the main charge powder then reduce that charge weight by 1 grain at a time and replacing the same volume not weight with a powder to be used as a priming charge. Basically what we are after here is to add a priming charge to give the primer a longer duration of spark to continue pushing on the main charge until it is past the chamber and out of the barrel. In effect the primer charge or as I sometimes call it the kicker charge is burning the main charge somewhat more efficiently because it will increase velocity by about 15% but the main purpose is not increased velocity but is a clean barrel and chamber.
In developing my loads I continue reducing the main charge by 1 grain at a time and replacing an equal volume not weight with the kicker. The reason I stress volume not weight of the kicker powder is that it is imperative that you maintain the 100% load density in the case so that the powder has no room for movement in the case so can not mix. You want and need for the powders to remain stacked in the case not mixed. The goal here is to replace some of the volume of the main charge with the kicker powder until you get to the point that all the unburned powder is blown out and the barrel is free from unburned powder. I refer to unburned powder being blown out the barrel which becomes quite obvious quite quickly when shooting over snow. Typically the weight of the kicker charge will end up being from 5 to 10% of the main charge depending of the burn rate of the kicker charge. The faster the burn rate of the kicker charge the lower the percentage will be of the main charge and visa versa .
Through experience in determining about what burn rate for kicker powder will give the desired results I have found that powder burn rate from about 2400 on the fast side to about 4198 on the slow side give the desired results without raising chamber pressures too much. Kicker powders faster than 2400 will raise chamber pressure and velocity so beware if loading for vintage arms. Kicker powders slower burning than 4198 may not give the desired results of clearing the barrel of all unburned powder until substantially larger volume has been exchanged but at that point chamber pressure as well as velocity will also be elevated.
As I mentioned earlier most of what I have said here pertains to the 45-70 and is a good safe place to start learning duplexing. I have since developed good accurate duplex loads in bottle neck cases such as the 30-06 and will try in the future to expand on that a bit. I also hope to ramble a bit on the WHY of duplexing but for this sitting I have accessed all the brain cells related to memory available at the moment. Will be happy to within my ability answer any related questions. This is something I have been wanting to talk about but cant cover everything I would like in one sitting but now have at least gotten started. Every journey begins with a first step.
youse got me thinkin again!!!!