Update SigSauer M400 jam.
Thanks for all for the input. Found out later that the DANGEROUS jam configuration during firing was reproducible with two different Sig polymer mags during successive manual chambering and eject down to about 5 or 6 rounds left. The PMAG tried later has no such problem. A Sig rep said that these polymer mags were discontinued a few years back. It appears everyone was right.... the mag. THANKS