Get the 500mm one, if it sticks 10mm out of the barrel then grab a power saw... If you want more info PM me about the cutting.
The reason I say go for the 500mm is because sometimes companies will have the barrel listed as just under a certain length(l96's, M14 Soc clones), so you don't put the wrong barrel in them. Sometimes its just an agreement between two manufactures so one gets more money for making a specialty barrel.
For reference 10mm is only 1 1/2 BB's.
sticks