20 Extreme - AR15 Build

[Oso Polaris]

I have the dies and borrowed a reamer for 20 Extreme. I am all full on bolt guns, and thought I might turn it into an AR15 shooting +40 grain bullets.

I need to move relatively fast in lining up a barrel so that I can return the reamer.

Questions:

1. Twist: Any disadvantage to using a 1:9 twist barrel, instead of the traditional 1:10 recommended for mid-40 grain bullets. I know the jacket on most of the varmint bullets are thinner than larger calibers, but is there really any risk of de-laminating or destabilizing the bullet?

2) Length: I was thinking 20"...thoughts?

Thanks,
Edward

[cmatera]

1. Doubt it. I have a .204 Ruger 1-9" no problems.
2. Same gun above is 20". I like it.