Silver Vendor
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I've seen plenty of HKs that leave those longitudinal marks.I don't see the deep case striations from an H&K.
Never saw an extractor mark like that, but OK.
Ding on side indicates hitting frame on ejection.
It's the missing primer that really is scary.
MP5s (and all other guns in the same family of roller-delayed blowback firearms suck as HK G3, HK33/53, G41, etc) have fluted chambers to equalize the pressure. This helps keep the case from sticking in the chamber. I.e. the gases inside the bore will run back around the case inside these fluted grooves.
This is necessary as the roller-delayed blowback mechanism starts to cycle much earlier than other typical designs, and without the flutes, the pressure inside the casing would be so high that the casing walls would stick to the chamber walls while the case head is ripped off.
I've seen plenty of missing primers from fired rounds...mostly come from Lake City brass. Some people think Lake City LC brass is great...it's not. The primer pockets are all over the place for spec, and some don't have the tension to hold a primer very well...thus they will blow out upon firing.