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steve4102 · November 8, 2011, 08:05 AM

Semi-autos can be a little difficult to find that .003 shoulder bump. Ya see, when a round is fired in a bolt action the brass expands to fit the chamber then springs back just a bit to allow for easy extraction. Measuring the fired case and bumping the shoulder back is simple.
Things are different in an auto-loader. When the round is fired it expands just as it does in a bolt action, the difference is that the case is still under a little bit of pressure when it is extracted from the chamber. This tiny bit of pressure can cause the case to continue to grow after it has left the chamber. The case may be longer or even fatter than the chamber it was fired in. I believe this is why you are seeing a .008 increase.

Basically only two ways to set up an auto-loader for FL sizing. Trial and error, size and check, size and check until an empty case chambers with zero effort. A better way is to get a case gauge and set the die up so the case fits perfectly into the gauge. This will ensure that the sized case will fit and function in his chamber.

November 8, 2011, 08:05 AM	#2
steve4102 Senior Member Join Date: December 23, 2005 Location: Minnesota Posts: 2,952	Semi-autos can be a little difficult to find that .003 shoulder bump. Ya see, when a round is fired in a bolt action the brass expands to fit the chamber then springs back just a bit to allow for easy extraction. Measuring the fired case and bumping the shoulder back is simple. Things are different in an auto-loader. When the round is fired it expands just as it does in a bolt action, the difference is that the case is still under a little bit of pressure when it is extracted from the chamber. This tiny bit of pressure can cause the case to continue to grow after it has left the chamber. The case may be longer or even fatter than the chamber it was fired in. I believe this is why you are seeing a .008 increase. Basically only two ways to set up an auto-loader for FL sizing. Trial and error, size and check, size and check until an empty case chambers with zero effort. A better way is to get a case gauge and set the die up so the case fits perfectly into the gauge. This will ensure that the sized case will fit and function in his chamber.