Where do I look on an AR barrel to find the twist rate. Im looking at buying a Bushmaster pre-ban upper and I want make sure it's 1in9. TIA.

I don't know about 20" barrels, but on the A2 Sporter from Colt it is printed on the top of the barrel between the front sight and the flash hider.  "5.56 NATO 1/7"  I think that is what it says.

Chida66

Stamped on top of barrel ahead of front sight like this: www.biggerhammer.net/ar15/barrelsbolts/

Mike

Maybe you already know this, but if the barrel’s not marked, you can always determine the twist with a cleaning rod.

Simply run a cleaning rod with a snug patch or bore brush through the barrel.  (Note that the rod rotates due to the rifling.)  Then reverse direction and pull the patch or bore brush back into the barrel a few inches and stop.  Use a felt tip to put a small mark on the rod where it projects from either the muzzle or end of the upper, depending on which way you’re going.  Continue pulling the rod out.  When the mark on the rod has made one complete revolution, stop.  Then measure the distance in inches from the mark’s present location to the muzzle or upper (wherever the mark was first made).  That’s the twist.

In other words, you’re simply measuring how far the cleaning rod travels down the bore when it makes one revolution.

Note:  While it’s not a big deal for a one-time test, an AR should generally be cleaned from the breech end.  Also, while you can do it with a patch, never try to reverse a bore brush in a barrel – always run it completely through and then reverse it.  

Thanks guys, I really appiciate your help. That's the last piece in the puzzle hopefully now I'll have my first AR soon.