You'll want to use the magazine release pin to replace that roll pin (just trade them out). You really shouldn't re-use a roll pin in the slide.
I doubt it's an extractor issue, or you'd be having problems with more than just the last round, though. Exactly what happens on that last round? Does it move out of the magazine at all or does the slide lock back? If you depress the slide release will it go into battery? If you push in on the magazine will it go. What condition is the magazine catch in?
Like you said, it's not a magazine issue, because it isn't happening with just one mag - like wise I don't think it's an extractor issue, because it's only happening on the last round.
I doubt it's an extractor issue, or you'd be having problems with more than just the last round, though. Exactly what happens on that last round? Does it move out of the magazine at all or does the slide lock back? If you depress the slide release will it go into battery? If you push in on the magazine will it go. What condition is the magazine catch in?
Like you said, it's not a magazine issue, because it isn't happening with just one mag - like wise I don't think it's an extractor issue, because it's only happening on the last round.
