The amount of shoulder bump is the approximate head clearance when the cartridge is chambered. Meaning the distance the case must stretch when fired to contact the bolt face.
Below you can see that Lake City brass is made Ford Truck Tough
I buy once fired Lake City 5.56 cases because they are made of harder brass than commercial .223 cases. The brands of cases have different hardness and elastic limits, meaning their ability to stretch and spring back to their original size. The recommended shoulder bump amounts are .001 to .002 for a bolt action and .003 to .006 for a semi-auto.
How Hard is Your Brass? 5.56 and .223 Rem Base Hardness Tests
http://bulletin.accurateshooter.com/2014/05/how-hard-is-your-brass-5-56-and-223-rem-base-hardness-tests/
Below you can see that Lake City brass is made Ford Truck Tough
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