I am assuming that you want to slug your Enfield revolver but this will work with any revolver.
Get a pure soft lead (or close to it) bullet that is close to the size that you need and squeeze it in a vise end for end so that it squishes the bullet out and thus making it fatter than the size you need. Don't need much just enough so it is a bit fatter than the end of the bore.
Oil the barrel really good and you can also put a bit of oil on the fat bullet.
Place the revolver in a well padded vise with the barrel up. It is easier to just remover the cylinder on a revolver to get it out of the way.
Now with a plastic hammer or a piece of wood taped to the end of a hammer to protect the barrel, just lightly tap the bullet into the barrel till it is about flush with the end. Use a piece of brass rod about 3/8" in diameter (buy it at home depot, only need a foot or so) and tap the bullet thru the barrel until it comes out the other end.
Some barrels are a pain in the back side to try and use a micrometer/caliper on because they have an odd number of lands/grooves. What you are trying to do is measure across the lands (the fat parts) which are actually the grooves in the barrel to get the correct size.
Now take that slug that you just drove thru the barrel and drop it in the loading end of the cylinder and see if it comes out the throat of the cylinder. Try it in every cylinder and see what you get. Ultimately what you want is a barrel that is 1 or 2 thou. smaller that the throats on your cylinder. If the bullet will pass thru all 6 cylinders with no resistance or just a slight amount than you can smile. If the slug will not go thru a cylinder or all 6 then we have a problem because the cylinders are smaller than the barrel and you will do good to hit a 8"x11" target at 20 yards. What it means is the bullet will be squeezed upon ignition of the round thru the throat and will bounce around down the barrel before exiting. This was a common problem on Ruger Blackhawks/Vaquero's/Redhawks made in the 90's and some Italian 1873 clones, the cylinder throats were smaller than the barrel. Have a gunsmith open up the throats to at least 2 thou over the barrel size and you will be amazed at the accuracy you will get from it.
I bought a Ruger Super Blackhawk that had been traded at least 4 times and was officially considered a dud, couldn't make it hit a milk jug at 25 yards twice in a row. I bought it for parts because i wanted the grip handle. When I slugged the barrel it measured .430" and when I took the slug it wouldn't go thru any of the cylinders. When I measured each of the throats they were all over the place 2-.427, 2-.428 and the last two were .429. Well no wonder it was a dog at the range. I had the cylinder bored out to .432 and using .432 bullets it will hold all six shots inside 2 inches at 25 yards any day of the week.
I size all my revolvers to the size of the throat for best accuracy.
