the plot thickens!
so... a brief bit of context here: i almost always load my ammo slightly to moderately under-pressure. i very rarely load anything near max, or over. i remain confident the loads in the previous posts of this thread are a little hot. however, i've solved the mystery of the primer indentions.
when i shot those loads, i had roughly 1900 rnds through my upper and BCG.
after shooting those loads, i decided to get rid of some old stocks of ammo, which were my usual light 55g win FMJ target load (my aforementioned 24g of 2230c). i shot through them with no problems. mean velocity was a hair over 3000 fps.
now that those were out of the way, i started trying to mimic the M193 round, using M193 pulldowns and 26g of A2230-C. (by way of comparison, M193 is around 3150 fps)
with 10 rounds, I got a mean velocity of 3095 fps with SD of 37 and this:
(the other 5 looked the same, and the middle ones are also pierced, but rotated slightly around so that the opening is to the right and rear)
light bulbs started going off, and I started investigating my firing pin, which, by the way, is one of those fancy TiN plated jobs. sure, enough, it looked a lot different than it did when I bought it. so I swap it out with a regular aluminum one, and get this:
for those ten rounds, the chrony showed a mean of 3081 fps with an SD of 24. then I chrono'd another ten with the suppressor on and got a mean of 3097 fps with an SD of 25. all those primers looked perfectly normal.
here are some pics of the culprit. sorry i couldn't get my camera to focus any better on the tip.
and I'm pretty firmly in the "don't coat your firing pins with any crap" camp now. actually, i have been for some time, but it came with the BCG so i figured i'd use it.
as you can see, the end is very flat, with sharp corners (soup-can shaped), compared to a normal firing pin, which is rounded in the middle. this pretty well explains the shape of the primer indentions i was seeing. another interesting observation is that at 3000 fps, the pin was happily just making funny dents in the primers, but at 3095, it was consistently piercing them.
on the positive side, I finished the day by shooting another 250 rounds chasing cans around a berm and just generally amusing myself. in a couple weeks, I'll go back to working on those 100 grainers and see if i can't get the primers to stay in the pockets.
another interesting observation is that the plating seems to be coming off the pin further back, at the 2nd place the diameter increases. i've inspected the rest of the BCG and can't find any flaking anywhere else. of course, the rest of it is steel, not aluminum underneath the coating.
and yet another interesting observation is that the SD was much higher when i was piercing primers. and the max and min were higher and lower, respectively, than all the normal rounds.
MAX MIN
3137 3015 when piercing primers
3105 3029 normal, unsuppressed
3124 3053 normal, suppressed
). All the remaining loaded shells were well below the low datum line on the guage. We now have sinclair case guages on-the-way from midway.
