Ian
Notorious member
I think the two most important grooves on a cast bullet are the front one (usually a crimp groove) and the space in front of the gas check.
All that metal in the "shoulder" area of the bullet that we use for alignment and guidance for the nose tends to need to be bigger than groove diameter because the throat is bigger than groove diameter, and as that part squeezes through the throat that metal needs to be directed somewhere on purpose or it will cause the whole cross-section of the bullet at that point to start to flow (usually in ways we don't want, like the nose extruding in a curl like sausage coming out of a grinder). The crimp groove is often generous and lets the shoulder flow back into it with relatively low resistance, thus keeping engraving resistance low (remember that is good for preserving the parts of the bullet that are pushing it) and keeping the metal displacement limited to the outer layer of the bullet rather than squeezing and choking the bullet all the way through.
Veral Smith and others state that the space above the gas check is a reservoir for the lube, powder residue, and metal that the gas check scrapes off the bore. There needs to be enough space there, but not too much. Yes, you fill that space with lube, that lube is generally blown out as soon as the primer is lit, along with a little from the next groove or two forward. 45 2.1 mentioned one time that he thought a little lube being blown off the top of the grooves and forward before the bullet got fully corked up in the bore was one of the mechanisms by which bullet lube works. I tend to agree.
If you look at some photos of the NOE XCB bullet recovered after being shot at very high velocity that Larry Gibson has posted, you will notice some bad flame cutting on the back two driving bands. He used a rifle with a matching taper and a very tough, hard alloy (heat treated #2). The pressure to move that bullet against full shoulder contact with the throat, and alloy being very tough, causes the gas pressure to build very fast and high before the bullet can move and that gas is trying every which way to get by the obstruction. The bullet made of such alloy does not exhibit nearly as much gas damage if it is fired from a .308 that has a stepped throat because the steps cut into the bullet's shoulder gradually as the bullet moves forward out of the case neck. The XCB bullet also lacks a particularly deep front groove that would also ease engraving pressure. This bullet seems to like being sized .310" most if the time, less metal to move.
All that metal in the "shoulder" area of the bullet that we use for alignment and guidance for the nose tends to need to be bigger than groove diameter because the throat is bigger than groove diameter, and as that part squeezes through the throat that metal needs to be directed somewhere on purpose or it will cause the whole cross-section of the bullet at that point to start to flow (usually in ways we don't want, like the nose extruding in a curl like sausage coming out of a grinder). The crimp groove is often generous and lets the shoulder flow back into it with relatively low resistance, thus keeping engraving resistance low (remember that is good for preserving the parts of the bullet that are pushing it) and keeping the metal displacement limited to the outer layer of the bullet rather than squeezing and choking the bullet all the way through.
Veral Smith and others state that the space above the gas check is a reservoir for the lube, powder residue, and metal that the gas check scrapes off the bore. There needs to be enough space there, but not too much. Yes, you fill that space with lube, that lube is generally blown out as soon as the primer is lit, along with a little from the next groove or two forward. 45 2.1 mentioned one time that he thought a little lube being blown off the top of the grooves and forward before the bullet got fully corked up in the bore was one of the mechanisms by which bullet lube works. I tend to agree.
If you look at some photos of the NOE XCB bullet recovered after being shot at very high velocity that Larry Gibson has posted, you will notice some bad flame cutting on the back two driving bands. He used a rifle with a matching taper and a very tough, hard alloy (heat treated #2). The pressure to move that bullet against full shoulder contact with the throat, and alloy being very tough, causes the gas pressure to build very fast and high before the bullet can move and that gas is trying every which way to get by the obstruction. The bullet made of such alloy does not exhibit nearly as much gas damage if it is fired from a .308 that has a stepped throat because the steps cut into the bullet's shoulder gradually as the bullet moves forward out of the case neck. The XCB bullet also lacks a particularly deep front groove that would also ease engraving pressure. This bullet seems to like being sized .310" most if the time, less metal to move.
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