you can use a field gauge, but your accuracy may suffer, as well as your brass..
just suck up the $25 and buy both. if you are not going to make a savage switchbarrel(like me) you can re sell both for 2/3-3/4 the price you paid.
you dont need to hammer the barrel nut wrench on, hand pressure is fine.
use red loctite on the barrel threads if a permanent change, or never-seize if making a switchbarrel.
a lot of tips on
http://switchbarrel.com/