Primer Depth Indicator

  • I usually measure primer depth with a caliper, either a Mitutoyo digital or Starret dial and did have a set up using a dial indicator with a magnetic base. The indicator was a pain to set up so I decided to make an a adapter for the indicator. I'm sure someone sells these and it took me two modifications on this one to get it where I like it. There is also a base that will attach to the beam of your calipers if they have the right dimensions. Don't know if they would help steady it to do the measuring. Tilting the beam throws you way off when trying to use the stinger. The first specimen I made I had drilled the 3/8 hole the tube of the indicator is inserted in to mount it end to end so it was hard to get the case centered and would tilt if you were on the edge of the hole so I made a stainless steel bushing with a small clearance hole for the plunger and pressed it in. That was better and zeroing the dial was easy. Just set it on a flat surface and rotate the face to zero. I took it one step further and counter bored the face of the adapter a couple of thousands bigger than the case head of a 308 base cartridge so when you inserted the case it centered the primer right in the plunger. It took a little measuring to get the overall length right so when I was at zero with a .200 preload on the plunger I was at six o clock which makes it easy to see. Only catch is I have to use a dummy plug to set my zero which was no big deal just an aluminum shaft turned to .473 and inserted into the counter bore. I haven't tried a smaller case head like a .223 but it should be good. While making this fixture I noticed a lot about case heads. They're not all real flat and the head stamps can have high spots and unless you mechanically true the primer pockets with a cutter they can vary in depth. Just for grins I pulled a box of reloads I know I used a Lee hand cramper to seat the primers with and measured them with my new tool and also with a digital scale. The results were some were as much as three thousand's deeper a couple out of twenty was even more but none were much less than an average of about .004 deep. I did the same test with some recent loads primed with a CPS Lite and the error was .0005 to .001 deeper with a few wild ones going .002 over. I blame the wild ones on the pockets themselves or me pulling extra hard on the press. I guess what I'm trying to get across is if you don't do any treatment to the pockets it's not worth losing sleep if you get some wild measurements. You certainly can't blame the CPS, It's doing exactly what it is supposed to do. The other types relying on feel can't hang.

  • Good stuff. I had something similar made and will be offering it as a kit with digital indicator in the future.

  • @orkan
    Digital would be nice with a finer resolution. That cheap Shars will change if you push the plunger side to side any and that's why I did the counter bore to limit movement. The ball shaped tip of the plunger is too large to go inside the primer pocket to measure its depth but I have seen a digital with a small pointed attachment that would work for that purpose. The set screw if over tightened will bind the plunger so finger tight is all you can go on that, something to consider when mounting. I plan on making one with a knurled head later.

  • Is all figured out. :)

  • Made another one today for a .223 case or similar. Other than make the smaller counter bore I did change up the way I made this one. I drilled the hole in the body about .040 undersized and finished it with a tiny boring bar and got a nice slip fit with no movement. Don't need a set screw. May wear though and need one later but not for now. The first one I drilled with an under size bit also and like always a drill bit drills a larger hole and it came out loose so the set screw. Wish I had a DRO on this old dog. Hell, I wish I had a new lathe with one sitting on it.

  • I went digital with the primer tool last week. I ordered a new indicator with some skinny probes to get inside the primer pockets to measure their depth and also measure the actual primer depth after seating. Of course the probes are smaller than the flash holes so I made a 4mm sleeve for the pointed one to measure pocket depth and made it just long enough for the point to barely enter the hole. A slight modification had to made for the zeroing tool with a center drill and turned the other end to the correct diameter to enter the recess of the base for corresponding types of brass. To measure the primer depth I install the non pointed small probe. I also made a stand for it but it can still be used without it and actually seems easier to work that way. The slightest movement of the case makes the display shift and using both hands helps keep the case tight against the base. I have been measuring pocket depth on new brass and hardly any are showing much error. Sometimes the half thousands will flash and and at the worst maybe a thousands. Like I said any movement or tilting of the brass will show up on the digital readout even though it's a relatively inexpensive one. Fired brass is a different story. It gets bent up regardless of what it's fired in. This is about as far as I'm taking this project.
    Pocket Probe
    Primer Probe