The gas tube isn't going to have anything to do with accuracy, unless... You can run it too short (think carbine gas tube length in a 20" barrel), and have the rifle cycling extemely hard. That would have the bolt unlocking very, very fast, and it would kick like crazy (which would wipe out chances of accurate follow-up shots). In addition, cycling that hard will affect the life of the moving/interfacing hard parts. It's pretty important on the AR-15s, but the .308s will cycle just about any combination. I'd go as long as you can on the gas tube length (rifle-length tube), and have it shooting as soft as possible. Even then, you might be needing a heavy buffer, or Tubb's CWS, in order to soften it up as much as you can. If you can give your bolt the longest lock time possible, that'll have the greatest effect on follow-up shot accuracy. Mechanical accuracy of the weapon will depend on other things.