I made mine using a 243 cal comparator with a small stack of washers in the bottom of the adapter so the bullets wouldn't bottom out. the middle of the washer must be big enough that the bullets don't touch and the outside fits snug inside the adapter so they don't move. stack the washers in the bottom until the setscrew just barely holds the comparator. the rimfire case is a nice fit inside the small end of the 243 comparator.