Did you say that you never cleaned it before firing it? If so, anything could have been in the chamber from the manufacturer. The ammunition you chose could also be dirty, although I doubt if it would do that by itself.
I'm no AR guy, but I do remember the mess back in the Vietnam era.
The chamber could be a little too tight, too....
I had a Springfield XD9SC, which is a small pistol, that occasionally, with range ammo, would about half extract a spent case, and then just sit there. If I backed the slide up a bit, and then let it go into battery in an attempt to get the extractor to do the work, the slide would seize up.... Took a hammer and cleaning rod to clear it.... Locking the slide back and using a fingertip to flip the round out worked all the time.... I don't think you have same problem, but a tight chamber and a bit of crud from the ammunition can cause all kinds of issues.... WWB or virgin factory loads never had a problem....
Enjoy the new Sig!
(Why write a quick note when you can write a novel?)
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