MWAG,
I hope Dennis will grace us with his photographs once again of his various barrels showing the chambers of each.

Basically it means this: "Any firearm that has a portion of the chamber removed." In recent times, and that is all I can speak to, it shows up most of the time in semi-automatic handguns where the need to have the rounds chamber automatically, a portion of that chamber is removed for part of the feed ramp.

In a lot of firearms, the feed ramp is part of the frame, in older 1911's as an illustration.

If Dennis posts the pictures I have spoken about it will be self evident. If not, I'll try to find some of my own.

In my opinion, and that ain't worth much, some of those "run all the time" semi-auto handguns have had the chamber attacked to make them so. Adjusting the feed ramp angle and polishing them will only take you so far. The shorter the firearm, the less the slide moves, the less the barrel tips, yada, yada, yada, the more one has to turn their attention to other areas to get them to run right. I could be WAY wrong.

Jessie,
Is that Sig one of those that is considered one of those with an unsupported chamber?