[Shrapnel[Op-For]]

I'm trying to find a 340mm tight bore for my VFC sr15 (10.5 inch combat master). Searching through google has proved not very useful, I was wondering if anyone has any suggestions?

Also wondering about what most peoples preferred hop up rubber/bucking brand is? I believe when my sr15 arrives it will be shooting somewhere between 330-400 fps, but each website and reviewer seems to provide a different fps rating.

Thanks