410 GNR question

of your cylinder - it should be snug- --since you know the size of the cast bullet - you know now what to use.. likely a .410 will work but you can try .411
normally a .411 work fine also -- the bullet if over sized will be sized to the mouth of the cylinder upon firing in any case
a JHP will also work just insert it also and again you will know the dia go from there
I normally size mine to .410 with a gc because of the pressure involved the .410 GNR