Boy! The list keeps growing!!
If your only going to build a GPR, you really don't need this but if you intend to stay in the black powder game you will be very glad you bought a Dial Caliper.
There are several on the market and any of them made of real steel or stainless, with a real dial on them are worth having.
I'm not saying their all the same. Some are much better than others, but for about $35 you can get one which is good enough for home hobby style gunsmithing.
Not only will it replace a micrometer an inside caliper and a depth gauge but it makes a good little hammer! :: Just kidding about the hammer. It makes a lousy hammer, but when your child or wife dumps all of your .530 and .535 balls into a paper sack a Dial Caliper is a super fast way to seperate them back into the boxes they belong in.
I would say in all honesty that my dial caliper is one of the most used tools I have.