Bigfish 5R,
I didn't see your post earlier. Would have been worthy of starting a new thread.
I recommend using a variant of the
round robin Dan Newberry's uses for charge weight, substituting seating depth changes for powder charge changes, to find the best depth. The old procedure was to do this with a starting load. You can see why by looking at the pressure vs. seating depth graph below. Seating a bullet either too long or too short raises pressure. By using a low recoil, low pressure load you eliminate a lot of barrel vibration influence, which is why light loads are often surprisingly accurate.
I am going to conduct some experiments with Trail Boss for this purpose later this summer, but loads of SR4759 or even Rocky's general purpose .30 cal catsneeze load of 9 or 10 grains of Unique should do fine. You want something that is still accurate enough to see the difference in the seating depth. Once you have a best seating depth established, work your real load up to a sweet spot with it. It is not uncommon for more than one seating depth sweet spot to be found. One out near the throat and one back nearer to one caliber into the case neck.
GMFWoodchuck,
I understand Weatherby has stopped using the long freebores in their chambers. It wasn't jamming they were supposed to prevent, but rather they were supposed to allow a delay before engraving the bullet in the lands began raising pressure. The idea was that this would allow more powder to be used and a higher final velocity to be achieved because of that. The problem is that it didn't really do that very effectively. The reason can be seen in the graph below. The left edge is pressure at contact with the throat, and as you seat deeper, you first see it drop, then rise again as the bullet gets very deep into the case. This plot was testing with a round nose bullet, and pointed bullets produce a sharper difference at the left, but you get the general idea.
The cause of the pressure rise as the bullet approaches the lands is that the gas that normally bypasses a bullet after the case mouth opens and before the bullet plugs the bore reduces the rate of early pressure rise and thereby reduces peak pressure. But if you start with the bullet already at the lands, little gas bypasses it so early pressure rise is uninterrupted. If you seat the bullet too deeply, the space the powder starts burning in is reduced so much that it causes early pressure rise before the bullet moves far enough forward for the gas bypass to mitigate it. A freebore is usually only a couple thousandths over bullet diameter, so you can see that even just being in a freebore also greatly constricts gas bypass. I believe that's why the old long throat Weatherby's and the newer short throat ones don't realize much velocity and powder capacity difference.
You are probably surprised not to see a sharper pressure jump when the bullet is seated to touch the lands. Even though the higher static coefficient of friction is supposed to raise it there, and can with some bullet shapes or with those jammed really hard into the lands, most bullet ogives make a poor match to the taper of the leade in the rifling. This means that though the engraving force needed to start the bullet from stationary contact with the lands is greater than when the bullet makes contact in motion, the first few hundredths of engraving moves less bullet metal than when that leade is biting into the full diameter of the bullet bearing surface. So that pressure rise due to static coefficient of friction is less than is usually is assumed to be the case.
Linear Mode
