I've loaded for many, many different rifle calibers and have never needed a case guage.  They have their place in high-power semi-autos and I am sure they could be used to make competition-type ammo.  As I understand it, the guage primarily ensures that your "bottleneck" cartridge headspaces correctly.  The .30-40 is a "rimmed" cartridge and it headspaces on the rim.  Everyone has their own opinion, but don't make this harder than it should be...you are looking to have fun punching paper at 100 yds....same thing I do.

My advice is to "neck size" only.  When the cartridge is fired the first time, it will form to the size of your individual chamber (which is slightly different from any other guns chamber).  Why squeeze that cartridge back down to original size after each firing if it will only be used again in that particular rifle?  The only part that needs to be reformed is the neck.  So, take your once-fired brass and mark the neck and shoulder area with a coating of soot (from a candle).  Put your resizing die into your press, but do not run it down very far.  Run the smoked cartridge into the die and keep turning down the die until you can see where it is knocking the soot off the neck.  Keep removing the cartridge and turning down the die until you can see that the entire neck is being squeezed down, but not so far that the shoulder is being compressed.  Lock your die at that position and never change it. 

Your brass will last much longer and it is much easier to resize.  Plus, in theory, there is some advantage in accuracy to this since the cartridge exactly fits the chamber it is fired in. 

I have used IMR 4895 in my Krags with 180 grain jacketed bullets and had excellent results.  Remember, since you have a carbine, it is a relatively lightweight rifle.  As such, if you touch off those big, 220 grain slugs in front of anything approaching a max powder load, it might be a little uncomfortable.  Those old soldiers were tough!

fred