Reloading: Trimming rifle cases
Trimming rifle cases
With multiple reloading of a cartridge case the brass tends to migrate from the base of the case to the case mouth, making the case too long. If left too long the case may get crimped down on the bullet and held crimped by the end of the chamber. That makes release of the bullet from the cartridge case too difficult and the pressures build up to an extreme. For this reason the overall length of the case of the cartridge must be kept in limits. It is trimmed back to the trim-to length and allowed to grow to the maximum case overall length before trimming again. The difference between max and min is typically 0.010 inches. But when should you trim in the processes of reloading?
When a case is resized it is reformed and its overall length changes in the sizing operation. So trimming should be done after the cases are sized. The inside and outside of the case mouth should be chamfered with the appropriate de-burring tool. The outside, so that it will not catch in the bullet seating die and the inside, to assure an easy entry for the bullet into the case during bullet seating.
There is nothing to be gained in trimming before the case overall length is close to exceeding the maximum overall length. And the case should not be trimmed shorter than the trim-to length noted in reloading manuals.
As an aside, there is a die made by RCBS that keeps the cases from growing in length. The cases are trimmed once and may never have to be trimmed again. The instruction for its use are too complicated to repeat here but are packed with the die and must be followed to the letter for the die to be effective.