What size barrel

[LazyMcCrazy]

Im making a DMR and im debating what would be best between a 455mm and a 509mm inner TB barrel.

Iv decided on the madbul python V2 as that's what the shop stocks where im getting all my other internals from.

Just want to know what size would do me best. Iv heard rumors that accuracy drops once you get longer than 455, is this true?

Advice please