This is RCBS Trim Die for 38-55 Winchester calibers doubles as a visual case length gauge. Run a case into the Trim Die and if the case sticks out above the top of the die it's too long. File the case down until it is flush with the top of the die, then deburr the case mouth with a deburring tool. The RCBS Trim Die is precision machined, case hardened, hand polished andheat treated to withstand a file bearing against the top surface without damage to the die. Standard 7/8"-14 thread. This die is quite useful when a considerable amount of material needs to be trimmed off in your reforming operations and should be used after you use any forming dies.