500 short vs 500 long

[JJ1911]

Hey all I have a mossberg 590, and I am installing a new forend. However the instructions tell me I need to use either a 500 short adaptor, or 500 long. Which size fits my mossberg 590? Thanks in advance I've been looking on the internet for an answer but figure the best place to ask is here!

Edit: This is an ATI forend, if that makes a difference.

[Rob454]

is it the 9 shot 590? it should have the long pump action on it. Thats what my 9 shot 500 has