My experience with pumps has been with the Win. 97s, 12s, 1200s and 1300s, Ithaca 37s, Hi-Stds, Rem. 10s, 31s and 870s, Stevens 520s and 620s, and probably more than I have forgotten about, NONE of which have ever blown the breechbolt halfway open upon firing.
I fully realize that the M3 is quite an unusual design, operating in both pump and semi-automatic modes.
So, once we get the sarcasm aside, is THIS normal operation for an M3 in pump mode? To blow the bolt halfway back when fired?
(Forgot that the Ithaca Roadblocker was a semi, so I removed it. It was a gun I had repaired and had to test fire and it was quite unpleasant.)