Kevin,
Try:
http://www.silencertalk.com/forum/index.php
I use a hotmail account there
Check out the Silencersmithing forum. .400" is plenty big enough for the bore diameter. Remember - the more accurate the threads on the barrel and the mount on the suppressor, the tighter the bore can be. The tighter the bore is, generally speaking, the quieter the suppressor will be.
A rule of thumb for Form 1 suppressor builders is that there is a trade-off of sorts. You can have a small lightweight can, if you know what you are doing. Or if you are an 07/SOT and you can experiment. But a Form 1 does not allow you to try, destroy and try again. So you really have one chance to get it right. It is much easier to get something you will be happy with if you have a larger volume. The more volume, the quieter the can.
6061 AL is fine for a dedicated 300 Whisper can. A wall thickness of .065" is fine. You can probably get by with a .048" wall, but I wouldn't go much thinner.
If it were me, I would build a 30 caliber can out of stainless that you use for a .308 or 30.06 and still use on a Whisper. Much more versatile suppressor. Even if you don't have a .308 you want to suppress, you may want to sometime in the future.
In stainless, I like 304 for the tube (it welds nice) and 303 for the internals, it machines better. .065" will still handle a full house center-fire rifle but it will be good for a Whisper too.
Threaded Mode