Savage does not seem to trust their customers to use wrenches, and IMHO the smooth barrel nut is intended to be tamper proof like the rear bolt assembly nut. Both parts were on the Savage ‘sell to gunsmiths only’ parts list, but they now have a waiver for non-licenced gunsmith purchasers.
Sharp Shooter Supply makes special wrenches for both the notched and smooth Savage barrel nuts.
http://www.sharpshootersupply.com/
Wheeler Engineering (div of Battenfeld Technologies) also makes a Savage barrel nut wrench that supposedly / sometimes includes a wrench for the smooth nut, YMMV.
http://www.btibrands.com/product/savage-barrel-wrench/
The SSS wrenches are much more money, but supposedly are of better quality. Some have reported problems with the Wheeler square holes for attaching a torque wrench, but I suspect the failures were more likely from the use of a big breaker bar. I hope that the Wheeler wrenches work better than their website.
Good Luck, YMMV
PM sent