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How does a Double Stack Magazine Work?
I have noticed many comments on this forum by people who have various luck with 25 round magazines in the 6.5 Grendel caliber. My comment is there are no manufacturers paying attention to the basic double stack rules.
For many many years it has been known that in order to get any long double stack to work, any side thrust to the cartridges must be eliminated to the greatest degree possible. How is this done? The internal width dimensions of the box have to be no more than a few thousands of an inch greater than the measurement of three cartridges strapped together tightly. If the width of the column is narrower than the minimum by only a few thousands of an inch, the magazine will exhibit excess friction to cartridge movement. If the cartridges are touching both vertically, and laterally, the cartridges will travel vertically without rotation. Once a gap exist between either the vertical columns, or between the columns, the cartridges become roller bearings, and rotate as they rise of fall. The rotation, and spring pressure will impart side thrust to each cartridge producing drag against the sides of the magazine walls. Additionally, those cartridges that are touching (those touching across the stacks) will be transferring energy between themselves and the side walls as they rotate. This rotational energy goes somewhere, and that is it transfers as more side thrust towards the walls of the magazine. Pretty soon everything comes to a halt until another force (vibration etc.) shakes things loose. The magazines swell in width as many have noted.
Sample measurements from three magazine makers show that none are paying close enough attention to the internal width measurements. The base measurements are varying from the minimum required to as much as 40 thousands too narrow. My latest C-Products Defense mag measures in overall width as measured from the outside rear spine as .896 top, .879 mag catch, .868 middle, .867 end of spring compress, and .889 at the floor plate. This of course makes the profile like a coke bottle, spreading the gaps between the verticle gaps, and increasing the side thrusts. I can only get 14 or 15 rounds in it without hurting my thumb. This mag works well with 6.8 SPC as the width dimensions are greater than the minimum required. My best mags are C-Products LLC prior to the demise. The measurements on the spine are .900 top, .902 mag catch, .905 middle, .913 end of spring compress, and .920 at the floor plate. This gradual taper spine is easy to load as the lower 15 cartridges do not rotate at all. This mag is sightly too narrow at the top, and vertical gaps start to appear toward the top of the mag. I have some ASC mags that measure .895 top. .896 mag catch, .897 middle, .891 end of spring compress, and .882 at the floor plate. They will load 18 easily, and 25 if I strain my thumb. I start to get vertical gaps at the bottom of the mag.
In the case of the 6.5 G, the measurement is .820" at the base, .800 at the end of the shoulder, and these measurements should be the minimum internal magazine wall widths where the mag touches the cartridge. Add 5 thousands to each measurement for tolerances for various cartridge makers.
Most makers are attempting to use internal vertical ribs to reduce friction, and stiffen the magazines side wall. However, they are not paying close enough attention to the width between the ribs internally, and none are providing .820 base of cartridge, and .800 shoulder width on 6.5 G mags. The best magazine I have seen is the 5.56 Magpul PMAG. It uses flat vertical sidewalls with a fore and aft extraction grove ridge to locate the cartridges in the body of the magazine. It runs well even though you might think there would be excess friction due to the flat inside walls. There is not due to the cartridges not rotation at all except for the top five. Magpul either was very smart, or they read the Army study written about 1905. I do not think there is enough width in a AR15 mag well to use plastic mags with 6.5G, just not enough wall thickness available even on a flat side mag. A stiff Aluminum alloy could be used without ribs at all (.035), or external ribs could be used to provide the correct internal dimensions. The look would not be what we are used to, and overall width would be greater that what we have now, but the thing would work I think. Just no money to put where my mouth is.
I am trying to get my son to cut a mag out of sheet, and TIG it together over a form so the internal measurements are uniform. No internal ribs, one out of Aluminum, and one out of Stainless. If I ever get him to give me some of his time, I may be able to show it on the forum, but do not hold your breadth.
For many many years it has been known that in order to get any long double stack to work, any side thrust to the cartridges must be eliminated to the greatest degree possible. How is this done? The internal width dimensions of the box have to be no more than a few thousands of an inch greater than the measurement of three cartridges strapped together tightly. If the width of the column is narrower than the minimum by only a few thousands of an inch, the magazine will exhibit excess friction to cartridge movement. If the cartridges are touching both vertically, and laterally, the cartridges will travel vertically without rotation. Once a gap exist between either the vertical columns, or between the columns, the cartridges become roller bearings, and rotate as they rise of fall. The rotation, and spring pressure will impart side thrust to each cartridge producing drag against the sides of the magazine walls. Additionally, those cartridges that are touching (those touching across the stacks) will be transferring energy between themselves and the side walls as they rotate. This rotational energy goes somewhere, and that is it transfers as more side thrust towards the walls of the magazine. Pretty soon everything comes to a halt until another force (vibration etc.) shakes things loose. The magazines swell in width as many have noted.
Sample measurements from three magazine makers show that none are paying close enough attention to the internal width measurements. The base measurements are varying from the minimum required to as much as 40 thousands too narrow. My latest C-Products Defense mag measures in overall width as measured from the outside rear spine as .896 top, .879 mag catch, .868 middle, .867 end of spring compress, and .889 at the floor plate. This of course makes the profile like a coke bottle, spreading the gaps between the verticle gaps, and increasing the side thrusts. I can only get 14 or 15 rounds in it without hurting my thumb. This mag works well with 6.8 SPC as the width dimensions are greater than the minimum required. My best mags are C-Products LLC prior to the demise. The measurements on the spine are .900 top, .902 mag catch, .905 middle, .913 end of spring compress, and .920 at the floor plate. This gradual taper spine is easy to load as the lower 15 cartridges do not rotate at all. This mag is sightly too narrow at the top, and vertical gaps start to appear toward the top of the mag. I have some ASC mags that measure .895 top. .896 mag catch, .897 middle, .891 end of spring compress, and .882 at the floor plate. They will load 18 easily, and 25 if I strain my thumb. I start to get vertical gaps at the bottom of the mag.
In the case of the 6.5 G, the measurement is .820" at the base, .800 at the end of the shoulder, and these measurements should be the minimum internal magazine wall widths where the mag touches the cartridge. Add 5 thousands to each measurement for tolerances for various cartridge makers.
Most makers are attempting to use internal vertical ribs to reduce friction, and stiffen the magazines side wall. However, they are not paying close enough attention to the width between the ribs internally, and none are providing .820 base of cartridge, and .800 shoulder width on 6.5 G mags. The best magazine I have seen is the 5.56 Magpul PMAG. It uses flat vertical sidewalls with a fore and aft extraction grove ridge to locate the cartridges in the body of the magazine. It runs well even though you might think there would be excess friction due to the flat inside walls. There is not due to the cartridges not rotation at all except for the top five. Magpul either was very smart, or they read the Army study written about 1905. I do not think there is enough width in a AR15 mag well to use plastic mags with 6.5G, just not enough wall thickness available even on a flat side mag. A stiff Aluminum alloy could be used without ribs at all (.035), or external ribs could be used to provide the correct internal dimensions. The look would not be what we are used to, and overall width would be greater that what we have now, but the thing would work I think. Just no money to put where my mouth is.
I am trying to get my son to cut a mag out of sheet, and TIG it together over a form so the internal measurements are uniform. No internal ribs, one out of Aluminum, and one out of Stainless. If I ever get him to give me some of his time, I may be able to show it on the forum, but do not hold your breadth.
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