I made up a new tool to measure my throat free bore on my .300 win mag. I drilled out the back of a 240gr SMK, soldered in a 1/16 copper welding rod that then sticks through the primer hole of a sized cartridge. I turned the 240gr SMK down slightly so it fit freely in the cartridge but didn't touch the ogive. I then inserted it into the throat with a spacer on the back that which allowed me to push the cartridge in snuggly. Using the welding rod, I can push and pull the attached 240gr SMK back and forth to feel exactly how it fits into the rifling lands.
Anyway, pulled it out and measured the COAL and it comes out to 3.5415". This seems long to me. A lot of books set the COAL to 3.34 for 168SMK thru 190. I had found that by setting the COAL (and I am referring to the absolute length, not to the ogive) on 190s to 3.44" gave me much better groups and had measured the COAL throat with another, less accurate, method at 3.475 about 200 rounds ago, so at 3.44" I was about .030 off the lands. Pressures always looked good as long as I kept the loads a little lower considering the magnum primers.
This barrel has about 600 rounds thru it, mostly lightly loaded 168's and 190's as I kept the former to 2650 and the later to 2550 only sometimes pushing both to the limits. I don't think the freebore would be burned out that much to date.
Long way of just asking if 3.5415" for an SMK seems long. I tried seating a bullet at 3.5415 and it seated and ejected cleanly so that looks like the number.
Thanks for the help.
Jerry
Anyway, pulled it out and measured the COAL and it comes out to 3.5415". This seems long to me. A lot of books set the COAL to 3.34 for 168SMK thru 190. I had found that by setting the COAL (and I am referring to the absolute length, not to the ogive) on 190s to 3.44" gave me much better groups and had measured the COAL throat with another, less accurate, method at 3.475 about 200 rounds ago, so at 3.44" I was about .030 off the lands. Pressures always looked good as long as I kept the loads a little lower considering the magnum primers.
This barrel has about 600 rounds thru it, mostly lightly loaded 168's and 190's as I kept the former to 2650 and the later to 2550 only sometimes pushing both to the limits. I don't think the freebore would be burned out that much to date.
Long way of just asking if 3.5415" for an SMK seems long. I tried seating a bullet at 3.5415 and it seated and ejected cleanly so that looks like the number.
Thanks for the help.
Jerry
