When I first started loading for my Glk 27 I had a similar issue, I was loading 180grhp-xtp @ 1.135 using mixed brass.. I was shooting and my lastfew2 rounds acted up. 2 rounds chambered but shortened my c.o.l 1.118, I pulled em got the t crimp dialed in and re used the brass and remembered they was federal and they did cycle but the last round made a thud followed by a wired cycle found the bullet a few yarda and the primer (cci) was in my mag! I too said wtf!