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Rifles, Reloading, Optics, Equipment
Reloading
What is “powder bridging”?
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<blockquote data-quote="QuietTexan" data-source="post: 2912146" data-attributes="member: 116181"><p>IMO it's more of an issue with neck diameter vs grain size than the shoulder angle. </p><p></p><p>Powder bridging is an expansion on packing problems - 65% (random) to 75% (ideal) is the highest packing density range you can get for spheres in a sphere, so there's an upper limit on how much powder can be present in the neck at a given time and a thus a limit on how much powder can flow through the neck at any time. As neck caliber decreases wasted space increases with powder grain cylinder size being held constant. So internal case pressure has to spike because more powder converts to gas in the case (which is then has to flow out of) rather than converting in the expanding barrel volume which is not constricted. And will in fact expand to the point that pressure drops and burning ceases before the bullet ever leaves the muzzle (in most cases, not all).</p><p></p><p>SC and SSC powders are shorter extruded cylinders, so they have a higher packing density in a cylinder (the neck), than a larger cylinder would.</p><p></p><p>Steeper shoulders angles would exacerbate the problem because the case becomes essentially a nozzle with high converging angle (inside shoulder angle), a very long throat (neck length), and incredibly small diverging angle (case mouth into barrel leade, not the entire barrel length since we're looking at powder flow and not gas flow).</p><p></p><p>It's almost like half of a nozzle problem, so let me butcher fluid dynamics for a second. The double radius Weatherby design is more akin to an ASTAR nozzle with convex then concave convergent angles and longer throat, rather than a basic De Laval nozzle with a single angle convergent design but longer throat. The complex convergent section should reduce pressure loss, which is a long way of saying IIRC it flows better, for want of a more technical term.</p><p></p><p>The practical answer is don't put H50BMG in a 6mm and expect happy results - you're more likely (not certain, but likely) to see more sudden and larger pressure spikes as caliber is reduced with large powders kernels. But a Weatherby might be slightly more efficient in managing powder bridging by giving better flow out of the case.</p><p></p><p>Powder undoubtedly burns outside the case in standard rear ignition cases. 300 Blackout is a perfect example of being able to partially burned kernels come out of the barrel. Those were kernels that stopped burning when pressure dropped because pressure vessel volume (meaning case volume plus bore behind the bullet combined) got to the point there were no more exponential pressure increases to continue the burning reaction. Pressure curves look like parabolas for a reason - pressure grows exponentially and would otherwise continue until it stopped very suddenly with the full consumption of powder, but since the pressure chamber volume is increasing at it's own exponential rate as the bullet accelerates (distance per second per second) the curves drops down on an arc instead of dropping of a cliff.</p></blockquote><p></p>
[QUOTE="QuietTexan, post: 2912146, member: 116181"] IMO it's more of an issue with neck diameter vs grain size than the shoulder angle. Powder bridging is an expansion on packing problems - 65% (random) to 75% (ideal) is the highest packing density range you can get for spheres in a sphere, so there's an upper limit on how much powder can be present in the neck at a given time and a thus a limit on how much powder can flow through the neck at any time. As neck caliber decreases wasted space increases with powder grain cylinder size being held constant. So internal case pressure has to spike because more powder converts to gas in the case (which is then has to flow out of) rather than converting in the expanding barrel volume which is not constricted. And will in fact expand to the point that pressure drops and burning ceases before the bullet ever leaves the muzzle (in most cases, not all). SC and SSC powders are shorter extruded cylinders, so they have a higher packing density in a cylinder (the neck), than a larger cylinder would. Steeper shoulders angles would exacerbate the problem because the case becomes essentially a nozzle with high converging angle (inside shoulder angle), a very long throat (neck length), and incredibly small diverging angle (case mouth into barrel leade, not the entire barrel length since we're looking at powder flow and not gas flow). It's almost like half of a nozzle problem, so let me butcher fluid dynamics for a second. The double radius Weatherby design is more akin to an ASTAR nozzle with convex then concave convergent angles and longer throat, rather than a basic De Laval nozzle with a single angle convergent design but longer throat. The complex convergent section should reduce pressure loss, which is a long way of saying IIRC it flows better, for want of a more technical term. The practical answer is don't put H50BMG in a 6mm and expect happy results - you're more likely (not certain, but likely) to see more sudden and larger pressure spikes as caliber is reduced with large powders kernels. But a Weatherby might be slightly more efficient in managing powder bridging by giving better flow out of the case. Powder undoubtedly burns outside the case in standard rear ignition cases. 300 Blackout is a perfect example of being able to partially burned kernels come out of the barrel. Those were kernels that stopped burning when pressure dropped because pressure vessel volume (meaning case volume plus bore behind the bullet combined) got to the point there were no more exponential pressure increases to continue the burning reaction. Pressure curves look like parabolas for a reason - pressure grows exponentially and would otherwise continue until it stopped very suddenly with the full consumption of powder, but since the pressure chamber volume is increasing at it's own exponential rate as the bullet accelerates (distance per second per second) the curves drops down on an arc instead of dropping of a cliff. [/QUOTE]
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What is “powder bridging”?
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