I went out and put 200 downrange today, and I had about 10 or 12 failures. The slide was locking back mid-magazine. It's a sig, so I think mechanical problems are unlikely. It seemed like it always happened after the 5th round, but on at least 3 of 4 mags.
WTF is going on here? Weak mag springs?
"Always carry a small flagon of whiskey in case of snakebite. Furthermore, always carry a small snake." W.C. Fields
What kind of ammo was being used, new or reloads? I have found that if I have my thumb positioned incorrectly on a couple of my 1911a1's, the slide will lock when firing. Try to load 5 rounds, and please check your thumb position after each shot.
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x2 on thumb position. This is very common for thumbs forward shooters and Sigs. Takes a little practice but your thumbs should eventually find a spot that doesn't interfere with the slide lock.
Try shooting a couple of magazines strong hand only, see if the slide locks back then. Make sure to keep your thumb away from the slide lock, this should give you a clue whether it is your thumbs or a mechanical issue.