This cartridge comparator set functions like the Hornady Lock-n-Load Bullet Comparator Set and a functionally similar 3-D printed comparator set available on Etsy.  It attaches to your caliper jaws magnetically to allow measurement of the cartridge from the base to the point on the bullet ogive corresponding to the barrel bore at the lands.  This measurement, not overall cartridge length measurement, determines the bullet “jump” to the lands when fired, an important parameter for accuracy.

This design differs from the others in that the “head” which mates with the bullet nose (ogive) is not two pieces but a single piece, whereas the Hornady design is metal and using a set screw to hold two pieces together and the Etsy design threads them together.

I made the design this way because I don't reload 7 or 14 different calibers and thus don't need the flexibility of the two piece design.  By making it one piece, I print one for each caliber I reload and the only real penalty is I need a magnet for each caliber head instead of just one for the set.  Since I purchased a couple dozen magnets for another project, that is no disadvantage for me.

Included are .stl's for the head for several calibers plus an anvil that makes it easier to hold the cartridge in the calipers.  I designed it so that the distance between the caliper jaws with the comparator parts installed is one inch, as near as the accuracy of the printer can make it.  This simplifies measurements when using analog dial calipers instead of electronic calipers that can be zeroed with the comparator in place.

I designed the parts so that the magnets are installed after the print is finished instead of inserting them while printing because I use a tungsten carbide nozzle that is magnetic.  I had a bunch of 12x2mm magnets on hand so that is what I used.  Off my printer, they are a tight fit in the hole but I still use a dab of super glue when I install them.  The slots for the caliper jaws are a tight fit off my printer and I used a flat file to just open them enough for a snug fit that helps with alignment. This may not be necessary for your printer.

I have included the Fusion 360 design file to allow you to make other caliber comparators.  It is a parametric design so you just need to change the bullet diameter in the table to update the design.  You can also change the magnet size, go to a print-in-place magnet design if you wish or make any other changes you feel would work for you.

Print the parts with slots oriented on top so no supports are required.  If your print results in elephant foot in the hole, just break the edge to get the correct diameter.

Printables Tags: cartridgereloading; reloading; reloadingtools