Buy Oval Egg Sinkers Here
The best method I have found is to use oval egg sinkers (from fishing tackle department), otherwise known as slip-sinkers. These sinkers are perfect for slugging not only cylinder throats but barrel groove dimensions as well. The reasons for using them is four-fold
1. They are usually pure lead, and thus very soft, making them easy to shove through cylinder throats and bores.
2. Their egg shape leaves only a small percentage of their overall dimension which actually bears against the constrictions they are shoved through, unlike a solid lead bullet with lots of bearing surface. This also facilitates ease of shoving them through cylinder throats and rifled bores.
3. They are easy to start into the cylinder or bore since they are egg shaped they are simple to get started.
4. Lastly, and most importantly, they have a longitudinal hole running through them which allows a place for the lead to displace when put through a constriction, thus eliminating "spring-back" or residual memory of the sinker, thereby producing a slug of nearly perfect dimensional characteristics when finished.
Use these sinkers, especially in barrels with a VERY light lubricant. Although I recommend seldom for very many things, WD-40 is a great asset here. If starting down the bore of a barrel, I invert a fired cartridge case over the oval end of the sinker sticking out of the bore, give it a sharp rap with a hammer, and the slug is seated neatly into the muzzle of the firearm. The brass cartridge case will not mar the muzzle's crown, nor the bluing, and gets the job done with minimal deformation of the oval egg sinker.