TL,
Yes, you can see more than 30% change with some common cartridges and powders. 20% is a worst-case minimum for powders with lower fps change with each % powder charge. I suspect that's one reason 10% is the default starting load number for a lot of powders. There is concern the shooter might start with the bullet out too far, so it guarantees that is not going to go over pressure.
This is bullet shape dependent. Mentally simplified the bullet and bore throat into a conical taper and hole. As you back the taper away from the hole, the annular opening between the taper and hold gets wider. If the taper is gradual, you have to move it back further than you would with a more abrupt taper. Round-nose rifle bullets typically do not have a hemispherical tip sitting on a groove-diameter cylindrical section (the bearing surface). Rather, they are mostly made with a bearing surface length similar to that of a Spitzer bullet, followed by a very gradual taper that leads to the bottom of the hemisphere of the tip. This taper is even more gradual than the sides of a VLD bullet, so the round-nose bullet that Brownell was using had to move back a lot to achieve a given area of the annular gap between the throat and bullet. That's why the depth numbers are so big on the first graph, but the shorter 0.030" graph does enough on the second pressure plot.
I think it is interesting to note that the bottom of that first curve with a bullet jump of a quarter of an inch is a pressure inflection point that can cause a true velocity flat spot to occur. In contrast, powder charge and velocity are quite linear at more common seating depths closer to the throat. Most shooters never try for that much jump, though a few of the name shooters, Scott Satterly for one, have started loading very long jumps and finding they get better results with them. Obviously, the location of that pressure inflection will change with loading density as well as bullet design and case capacity, so it has to be determined by trial and error. I'm sorry that Jim Ristow had to retire, and his Pressure Trace instrument is off the market. It is a great way to find these nodes by measurement rather than having to shoot a hundred rounds to get close and be sure statistical variance isn't fooling you.
Yes, you can see more than 30% change with some common cartridges and powders. 20% is a worst-case minimum for powders with lower fps change with each % powder charge. I suspect that's one reason 10% is the default starting load number for a lot of powders. There is concern the shooter might start with the bullet out too far, so it guarantees that is not going to go over pressure.
95GTSpeedDemon said:
This is bullet shape dependent. Mentally simplified the bullet and bore throat into a conical taper and hole. As you back the taper away from the hole, the annular opening between the taper and hold gets wider. If the taper is gradual, you have to move it back further than you would with a more abrupt taper. Round-nose rifle bullets typically do not have a hemispherical tip sitting on a groove-diameter cylindrical section (the bearing surface). Rather, they are mostly made with a bearing surface length similar to that of a Spitzer bullet, followed by a very gradual taper that leads to the bottom of the hemisphere of the tip. This taper is even more gradual than the sides of a VLD bullet, so the round-nose bullet that Brownell was using had to move back a lot to achieve a given area of the annular gap between the throat and bullet. That's why the depth numbers are so big on the first graph, but the shorter 0.030" graph does enough on the second pressure plot.
I think it is interesting to note that the bottom of that first curve with a bullet jump of a quarter of an inch is a pressure inflection point that can cause a true velocity flat spot to occur. In contrast, powder charge and velocity are quite linear at more common seating depths closer to the throat. Most shooters never try for that much jump, though a few of the name shooters, Scott Satterly for one, have started loading very long jumps and finding they get better results with them. Obviously, the location of that pressure inflection will change with loading density as well as bullet design and case capacity, so it has to be determined by trial and error. I'm sorry that Jim Ristow had to retire, and his Pressure Trace instrument is off the market. It is a great way to find these nodes by measurement rather than having to shoot a hundred rounds to get close and be sure statistical variance isn't fooling you.