Bullet JUMP Distance for My RAR Predator Loads

Discussion in 'Ammo & Reloading' started by roninwsnc, Aug 19, 2017.

  1. roninwsnc

    roninwsnc Member

    Hi Folks,

    High velocity .243 Winchester cartridges have a reputation of being “barrel burners”. I hope to extend my RAR Predator’s barrel life of by using a slower burning powder like IMR-4350 and by reducing the recommended starting load. I have been hand loading .243 Winchester brass (more off than on) for about a year. I have been exploring reduced velocity loads in the 2500 feet per second neighborhood assembled with CCI LR #200 Primers, IMR-4350 powder, and Sierra #1520 85 grain bullets seated to a Cartridge Overall Length (COAL) of 2.650 inches.

    Recently, I found an article called “Determining Bullet Seating Depth”, by the Sinclair folks. It was very interesting. I decided to order Hornady’s “Lock-N-Load Overall Length Gauge”, a “Modified .243 Winchester Case”, and their “Bullet Comparator Basic Set with 6 Inserts”. These tools along with my digital caliper are supposed to allow me to estimate my rifle’s JAM distance. The JAM distance is the distance between my rifle’s closed bolt face and the start of my rifle’s lands and grooves. In addition, the bullet comparator along with its 0.240" insert and anvil attached to my caliper should let me estimate the distance from a cartridge’s case head to the approximate start of the bullet’s ogive (CHTO). The CHTO distance is similar to the COAL, but shorter. I should be able to estimate a bullet’s JUMP distance for my rifle by subtracting the cartridge's observed CHTO distance from my rifle’s JAM distance. For a chambered round, the JUMP distance is the distance between a bullet’s ogive to the start of a rifle’s lands and grooves. I believe this summary explanation is correct. Is it correct?

    By using the bullet comparator, I should be able to adjust my seating die so it will produce cartridges with a desired bullet JUMP distance. I think that I more or less understand the basics. Now it becomes a matter of practice and developing a good technique for using these tools so that eventually I will be able to make reliable and repeatable estimates. So far, the estimates of my rifle’s JAM distance are not very repeatable. My estimates vary as much as 0.005 of an inch. Is this typical?

    Your comments and suggestions will be deeply appreciated.

    Thank You,