need gas port specs and barrel extension specs for a 338 Whisper / 338 BR

[ohnomrbillk]

Hello all.  This is my first post.  I'm thinking I need an AR308 of undetermined brand in 338 Whisper. 

I have built several AR15s, but this will be my first big gun.  I'm in need of a few specifications I have not been able to locate. 

1.  Gas port size and location for a 338 Whisper (338/7mmBR).  This will be a likely subsonic shooter only, although I'm open to flexibility in my build. 

2.  If purchasing a barrel blank, I use a 1" for the AR15 and turn to the profile I desire.  Is that sufficient in a 308AR, or do I need something larger?

I greatly appreciate any input. 

[Robocop1051]

Subsonic gets tricky with gas. I would stick to a carbine length gas system with an adjustable gas block. Are you planning on shooting with a silencer too?

I see Sierra is making a .338 300gr bullet now.  <thumbsup>

It shouldn't be too large of a task. The head space is the same/similar to the .308 and the bolt should be the same too. That 300gr might be too long for most mags though.

For the barrel you'll need a 308 specific blank and turn it to the profile you wish. I'm pretty sure the AR15 blanks are different and will be too narrow. Another route would be to just bore out a .308 barrel to the .338. That has been an option given to me if I planned on switching to the .338 Fed.

Question: Why not the .338 Federal? It's not a wildcat round and it's already proven to work.