Beagle33,
My first interest in coating the base actually is not about velocity, but about maintaining symmetry of muzzle blast deflection of it at bullet exit. It's the same reason a uniform crown matters. It takes only a small asymmetry in the gas jet playing off the bullet base at exit to introduce an off-bore-axis drift that stays with the bullet all the way to the target. That opens groups up. Harry Pope said over a century ago, "the base steers the bullet", and that's what the uncoated bases first brought to mind.
You are obviously correct the bullet fit has to be good, but even a gas checked bullet that doesn't fit the bore well does not shoot well, either, IME, so I don't think it's about making up lost diameter. I think it's more about refining the seal. So, can a layer a thousandth of pliable material help make a better seal at the inside corners of the lands? I can't see why it would not. It's sort of like having a conformal O-ring. Will it have the mechanical strength of a real gas check? No. Obviously not. But I expect one reason liquid Alox works even when the lube grooves are not filled, is that it does help create an edge seal as it is dragged off the sides of the bullet as it enters the throat. I have no proof of that, though, so you've made me think up a couple of test experiments.
Accuracy testing is straight forward. Bullet base condition might take some forensic effort. I think the coating should come off recovered bullets sitting in methylene chloride for a time, making it possible to examine the lead underneath to see how well it has been protected. I've taken molded epoxy packaging off transistors and integrated circuits that way before. For velocity, I've got a 6.5 mm mold for my '96 Swede that I have yet to break in, so that might be a good test bed. The bore is very smooth.
Rottweiler,
It's counter-intuitive, but, ironically, the low melting point actually helps prevent heat from penetrating the coating too far. This is due to enthalpy of fusion. Polymers, like water ice, require that extra heat to break up the solid structure and render it liquid. So, even though heat starts to penetrate the surface, it can't raise the temperature above the melting point until after it has also put that extra heat in. Since how fast heat is driven into something depends on the temperature difference between where the heat is coming from and where it is going to, that melting point limit on temperature also limits how fast the heat can move from the surface of the melting thing down into it until enough heat to cause surface melting has been delivered by the heat source. This is what Stubbicatt is referring to.
Chrome-moly barrel steel has on the order of 20 times higher thermal effusivity than typical polyester, and has been shown by the military to be penetrated only a couple thousandths of an inch by the leading edge of the thermal transient during the time high heat and pressure are still in a barrel. That's why you get the differential surface expansion that causes the alligator cracking of a throat as it ages. So, figure you might get a twentieth of that, or a ten thousandth of an inch penetration of your powder coat layer before the bullet clears the muzzle. When you then take into account enthalpy of fusion, another factor or 30 or so is likely involved, so you might get three millionths of an inch of melting at the surface. Probably shallower than the surface texture is deep.
My first interest in coating the base actually is not about velocity, but about maintaining symmetry of muzzle blast deflection of it at bullet exit. It's the same reason a uniform crown matters. It takes only a small asymmetry in the gas jet playing off the bullet base at exit to introduce an off-bore-axis drift that stays with the bullet all the way to the target. That opens groups up. Harry Pope said over a century ago, "the base steers the bullet", and that's what the uncoated bases first brought to mind.
You are obviously correct the bullet fit has to be good, but even a gas checked bullet that doesn't fit the bore well does not shoot well, either, IME, so I don't think it's about making up lost diameter. I think it's more about refining the seal. So, can a layer a thousandth of pliable material help make a better seal at the inside corners of the lands? I can't see why it would not. It's sort of like having a conformal O-ring. Will it have the mechanical strength of a real gas check? No. Obviously not. But I expect one reason liquid Alox works even when the lube grooves are not filled, is that it does help create an edge seal as it is dragged off the sides of the bullet as it enters the throat. I have no proof of that, though, so you've made me think up a couple of test experiments.
Accuracy testing is straight forward. Bullet base condition might take some forensic effort. I think the coating should come off recovered bullets sitting in methylene chloride for a time, making it possible to examine the lead underneath to see how well it has been protected. I've taken molded epoxy packaging off transistors and integrated circuits that way before. For velocity, I've got a 6.5 mm mold for my '96 Swede that I have yet to break in, so that might be a good test bed. The bore is very smooth.
Rottweiler,
It's counter-intuitive, but, ironically, the low melting point actually helps prevent heat from penetrating the coating too far. This is due to enthalpy of fusion. Polymers, like water ice, require that extra heat to break up the solid structure and render it liquid. So, even though heat starts to penetrate the surface, it can't raise the temperature above the melting point until after it has also put that extra heat in. Since how fast heat is driven into something depends on the temperature difference between where the heat is coming from and where it is going to, that melting point limit on temperature also limits how fast the heat can move from the surface of the melting thing down into it until enough heat to cause surface melting has been delivered by the heat source. This is what Stubbicatt is referring to.
Chrome-moly barrel steel has on the order of 20 times higher thermal effusivity than typical polyester, and has been shown by the military to be penetrated only a couple thousandths of an inch by the leading edge of the thermal transient during the time high heat and pressure are still in a barrel. That's why you get the differential surface expansion that causes the alligator cracking of a throat as it ages. So, figure you might get a twentieth of that, or a ten thousandth of an inch penetration of your powder coat layer before the bullet clears the muzzle. When you then take into account enthalpy of fusion, another factor or 30 or so is likely involved, so you might get three millionths of an inch of melting at the surface. Probably shallower than the surface texture is deep.
