For all brands I'm aware of except for Carbon Express, an arrow's model number is equal to its static spine deflection measured in inches times 1,000. For example, a Gold Tip Hunter 300 has a static spine deflection value of 0.300". Carbon Express's model numbers don't directly correlate to actual spine deflection, and their numbering system is effectively opposite of all other manufacturers: higher model numbers for stiffer (lower spine deflection) arrows and lower model numbers for weaker (higher spine deflection) arrows.