Thats what I was thinking too ehntr... The numbers arrows are sold as are spine numbers, not weights.
Depending upon how much weight you are pulling back you need a different spine to correctly fly out of your bow. Most 50 pound bows can shoot arrows in the .500 range dependign upon arrow length but check the arrow manufacturers guidelines to be sure. here is eastons guide:
http://www.eastonarchery.com/products/selection
The weight of the arrow is usually inversly proportional to the spine so a .300 would be heavier than a .500.
Brett