Slugging your bore is actually pretty easy to do. All that's needed is a stout cleaning rod, or wooden dowel. Measure the slug first. Ideally the bullet or slug will start out a bit larger than the actual bore, so it will compress and form into the rifling. Since the 38-40 has a nominal .400" bore.....you want it to be at least several thou. over that. In a pinch, you can use an ordinary bell sinker for a slug.
First, clean the rifle's bore thoroughly. Then, very lightly oil it. That will help the bullet slip down the bore easier. Ideally, you want to use a pure lead slug, as it forms easier into the rifling grooves than wheelweight lead or other harder lead alloys. Secure the rifle in a vice (with padded jaws so as not to damage the rifle), or other method that holds it firmly in place. You can even set it upright on its butt...on a padded surface...and do it that way.
First: Insert the lead bullet into the rifle's muzzle. Nose first, or base first...either way works. Use whichever way is easiest for you. I prefer to do it base first, but opinions vary as to which is best. Start the bullet down the bore with a few light taps on a short piece of wooden dowel, or short section of cleaning rod. Ideally, you want to use a rubber mallet, brass headed hammer, even a piece of wood will do. Something that won't damage the rifle if it strikes it. Then, using only enough force to drive the slug through the bore, tap it all the way through until it drops out in the receiver. Ideally, you want to have a soft piece of cloth, paper towel or something of the sort inside the receiver to catch the bullet.
Now its time to measure the bullet. The grooves and lands graved into the bullet are an exact representation of your rifle's actual bore diameter. Ideally, you want your cast bullet diameter to be .001" to .002" larger than actual bore diameter. This will ensure good accuracy and prevent or reduce bore leading. So, if your slug measures .400", for example...a cast bullet of .401" to .402" should be just about right.
To measure the lands and grooves, a micrometer is ideal and most exact. Though a good caliper will work almost as well. If all goes well, your bullet mould will cast a bullet of proper diameter for your rifle, and you are set to go. As a matter of interest, wheelweight lead and other harder alloys...Lyman #2 alloy, for example... will cast a bullet slightly larger than pure lead. This will also help in determining good bullet fit. Hope this helps.