A number of years ago a stalking client I took out was a rifle maker from south Africa, and at the time I was using a Remi 700 with a heavy barrel in .223. He told me I could cut the barrel down to 16" and it would not effect the accuracy one bit. He said it is a one in nine twist, rifling and 16" is the desired lenth, ,one and a half turns, in fact he said a barrel any longer was actually not good for the bullet.
I listened to him, but was not brave enough to cut my rifle to 16", but cut it down to 18", and it had no effect on the bullet accuracy, at any range. When I had shot this rifle out, the next Remi that replaced it was cut down, and threaded before it came to me.