Measuring mine front to rear:
The very end is turned down and threaded 5/8"x24TPI for the flash suppressor....Then the barrel itself has a slight taper which measures .760" from the rear of the front sight to .780" where it meets the gas block, then drops down to .5631" under most of the hand guard, then has step-ups of .750" for an inch and a half, and then .908" for the last 2" before it meets the receiver.....In other words, it is obvious to me that there is some sort of engineering going on there which is probably intended to help tame barrel harmonics. Not unlike Paul Mauser did with his Model 98 design....but whatever the reason(s), what Ruger did there works well. My 585-Series 5868 is the most accurate Mini-30 I've had, or used.
Anyhow, hope this helps.
DGW