I don't have a Sig but I have had a spent round jam up a live round feed before in a Glock 20

Going through the list of possibilities:

I assume you have shot similar ammo through this gun before? it would be an easy fix to try...unless new/ different rounds duplicate problem in prone.
my money would be on the magazine... no facts, just guessing.
limp wristed...not likely a problem unless you ONLY masterbate with one hand. HA!
you'd think dirty would cause a fail to feed WAY before it grabbed two.