Two very precisely modeled tools for measuring the case length of 5.56 / .223 rounds to determine if they need trimming or are too short and should be disposed of. 

For the model I printed in orange, if your casing fits in the shorter slot, it is too short and should be disposed of. If it does not fit in the taller slot, it needs to be trimmed.

For the model I printed in red, your casing should not fall below the height of the model on the minus side, and should not stick above the height of the model on the plus side. 

Check your print with a pair of calipers after printing to ensure proper sizing, you may need to adjust the model size depending on your printer. Do not rely on these tools without calibrating to ensure they are the correct size.