510mm inner barrel size. The hop up rubber is VSR-10 type.
Because of a couple reasons, unless you get a thread adapter and silencer, using a different inner barrel length will result in inner barrel wobbling inside the outer barrel. So you have to have exactly 510mm.
But they don't sell VSR-10 barrels in 510mm length, only custom makers do.
Such as EdGI from phillipines, his inner barrel is made to your exact needs.
It's sold in 6.00mm or 6.01mm.
He doesn't have a website, you have to email him.
eicban@smg.sanmiguel.com.ph
other than that, if you get a thread adapter + silencer, then you will need this thread adapter.
http://www.wgcshop.com/pcart/shopper...R1002_srch_VSR
There is a 14mm positive threaded version sold on Redwolf.
Hop up rubbers:
http://www.wgcshop.com/pcart/shopper...GCY55_srch_VSR
http://www.wgcshop.com/pcart/shopper...SRHBH_srch_VSR
http://www.wgcshop.com/pcart/shopper...TMSCP_srch_VSR
Firefly seems to be the most popular. Nineball gives great airseal, and I'm really not sure about the KM.