Designing wildcats and leade?

[constructor]

Using the 6XC as an example. Is the leade on this really about .300?
I'm working on a new cat. It's max OL is 2.8. I took the length of a 139 Lapuas ogive of .595 and then another .015 just in case I want to shove the 139 into the lands and subtracted that from 2.8. and come up with 2.19". That should give me a good location for the leade right? If my case is 2.035 then my leade should be .155"?

Also has anyone tried a .250 jump to the lands and can you get good accuracy that way? I know a smith that swears he has all of his reamers made with a .250 leade at bore size and can load to higher velocities and gets good accuracy because the bullet aligns better.
1 last thing, How do you figure bolt thrust? I've heard several arguments about it so anyone have an opinion?

 

[tmbryant]

If I want to have a reamer made to fit a specific bullet, I'll seat a bullet to the depth that I want it. Then, for instance, with a 6mm size bullet, I'll take a 1" micrometer and set it to .001" under bullet diameter. In a 6mm, the mike would be set to .242". I then drop the mike down the length of the bullet from the point of the bullet. The mike will stop sliding down the bullet at the point where the bullet starts getting larger than .242". I'll measure from the top of the neck to where the edge of the micrometer stops closest to the neck with a dial or digital caliper. This measurement will be my freebore length for the new reamer. The freebore diameter will be .0005" over bullet diameter usually .2435" for a 6mm bullet. Making a freebore diameter .2430" would not be a good idea as a lot of bullets are over .2430" in diameter. If the freebore diameter is .2430 and you try to push a .2432" diameter bullet into the freebore, you would be scraping .0002" off the sides of the bullet which is not a good thing for accuracy. You have to have clearances in anything you specify for accurate rifle work. The trick is keeping the clearances to a minimum. But, in essence, trying to fit a plug into a hole that is smaller than the plug will do nothing for accuracy whether it is a cartridge neck to chamber neck clearance or a bullet to freebore clearance. I use 1.5 degree leades on all of my reamers. I like them better than other leade angles that I've used. A typical freebore for a 6mm bullet would be on the order of .2435" diameter x .100" long x 1degrees 30 minutes or 1.5 degrees which is the same leade angle.

Or a simple way to get the correct freebore length is to send three dummy rounds to your reamer manufacturer and have him match the freebore to the dummy rounds. Any of the reamer manufacturers can do this.

 

[RickB]

Most reamer manufacturers have an optical comparator which is much more accurate than any of the measurement techniques available to most of us. If you will set up a dummy cartridge with the bullet of your choice they can measure it and make recommendations to you to accomplish your objective.

The reamer manufacturers also have the benefit of having done this many times before.