Jam
Weird...
I've had pistols that hang up at random places in the magazine, but never
seen one that does it consistently on the same round every time with every magazine tried except the top or bottom rounds unless there was a
problem with the gun itself.
(Thinkin' out loud)
Well...Since most mag-related problems occur on the top or bottom round, this one about has to be the gun...and since it happens on the same round, it's gotta be a combination of the slide speed and the mag spring tension comin' together at just the right time...or at just the wrong time
to let it happen.
Check a couple things...
Field-strip the gun and lay the barrel in the frame. Push the barrel down
and back. Look at the junction of the top of the feed ramp and the bottom of the barrel throat. The leading edge of the throat should sit forward of the top of the ramp just slightly. About a 32nd inch is good. It can be more, but not less. if the throat is flush at the back with the top of the ramp, or if it overhangs even a tiny bit, that'll hang one up. If the nose of the bullet stops right at the junction instead of farther down onto the ramp...that's the likely place to look. (Do the bullet noses on the jammed rounds have a gough mark?)
If the gap isn't there, the barrel can be set forward at the bottom and the throat reshaped. Not a task for a first-timer. It's easy to screw up the throat if you're not careful or don't have the tools to do it with...ad it could void your warranty even if you do it perfectly. (If you own a Dremel, take it out in the driveway and hit it with a large hammer about 5 or 6 times should do the trick. Might as well eliminate any temptation to use it)
Repeat the check, but this time put the slidestop pin through the frame and the link. Does the barrel lay flush in the bed..or does it stand off the bed even a tiny bit? Leave the arm of the stop hanging vertically and push the barrel down and back with a little pressure against the edge of a table.
Will the stop arm swing freely...or does it get into a hard bind? It should swing freely. If it doesn't, you have a long link. Another shot in the dark.
That condition CAN cause an occasional feeding problem, but probably not as consistently as yours.
There's a small chance that it could be that the extractor has a little too much tension on it...but this is a long shot.
Standin' by...
Tuner