Yes , you start at the muzzle . I push the lead slug in about 1/4" , then pull it back out and measure it , best I can to get the groove diameter . Before driving it in I had marked with a felt pen on the end indexing it with the front sight . When I put it back in the barrel , the felt mark aligns it correctly in the muzzle . Then I push it through the barrel , seeing if I feel any tight spots or machine marks in the barrel . When I reach near the end of the barrel , where the barrel joins the frame I am curious to see if there is a tight spot in the barrel stopping the lead slug from going any further . When it happens , you will feel it stop , " abruptly " !!
It's not unusual to find that it takes tapping on the rod pushing the slug through the barrel with a hammer to get it on through to the forcing cone . I measure the slug again as that will tell me the size of the " choke " at the tight spot, how many thousandths undersize the barrel is right there . Firelapping the barrel , then finishing hand lapping the barrel is what it takes to correct those problems . It sounds lengthy but it is what it takes to know the size of cast bullets you need , removes tight spots and machine marks in the barrel . With proper size , proper hardness of alloy and good lube you should enjoy a clean shooting barrel free of leading and increased accuracy . Regards, Paul