need help with new construction

[flat top]

I am currently in the process of building an 300/221 upper for my AR and have a few questions.

1. adjustable gas tube/blocks- i have seen the adjustable gas "valves" from ssk and this is what i'm looking for (they won't put one in for me), i want to shoot both subsonic and supersonic loads. SSK's "valve" looks eaiser to go back and forth than Fulton Armory's. Not to mention it looks "cleaner" also. Any ideas?

2. barrels- i have read a 1:8 twist is what you want but what about length? I read 16" seems popular, would a longer barrel be more accurate, maybe a 20" barrel be better for subsonic and supersonic loads?

3. suppressors- i have been looking at the advanced arament but not sure. I have a gem-tech on my 22 and i'm very happy. Has anyone had good results with other brands?

once i get the upper up and going i'll be back for reloading info.

thanks to all who can help.

Robbie