I was loading some 556 last night and found 15 of 100 loaded rounds had a bulge right at the bottom of the shoulder. Seemed to happen in the bullet seating proses, checked all of them with a case gauge before priming and after seating. Looks like some of the 15 might chamber and most wont. Any idea why the shoulder collapsed when seating bullet? Brass worn out? All cases loaded are mixed head stamp once fired(?) that I bought from local indoor range. First time ive seen this in the 2000 or so rounds ive loaded in this cal with same dies and methods.
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