Welcome to the forum trekvana
Jim and Bubba7 are correct. A gauge is a good investment, but if you're in a pinch an M2 round may prevent you from buying a rifle with a worn out barrel.
Jim also made a good point about gun shows -
Many gun shows frown on carrying live ammo for obvious safety reasons. You may even be bounced right out at some shows.
The bullet test is only used as a rough guide, but it works ok as a quick tool such as if you come across a nice looking rifle but want to know how worn the bore is. If the muzzle swallows the bullet down to the shell casing you know the bore is probably worn out.
You’re only using the taper of the bullet as a relative guide as to how far it goes into the muzzle, and it’s not exact. Also the taper of bullets from different manufacturers may vary.
You asked why an M1 carbine round wouldn't work -
The bullet tip for an M1 carbine is short and round, so it doesn’t work very well because the relative differences in muzzle size is hard to see. (Even on a new barrel, the muzzle is very close to the front of the shell casing)
The taper of an M2 bullet is long, so you can see the small differences of how far it will go into the muzzle better.
The photos below might help to better understand. These are measurements using the same M1 Garand with an almost new barrel.
(It works the same for an M1 carbine)
http://www.milsurps.com/images/imported/2011/01/Bullettest1-1.jpg