I've had some mags where the rivet used was way too long and it curled like a banana, using a properly sized high quality rivet was all it took to get a little extra space. Sometimes a long rivet can grab a coil of the follower spring too which also jams things up. Not scratching up the Magwell is just a little bonus since the job is being done already.This only helps if it's the rivet head interfering with the magwell that's causing the issue. If it's the fact that the 5th round can't be pushed down far enough into the mag for the mag catch to engage (Which is what it seems like is happening here), this won't help.
Are the mags pinned in the spine or the side?




















































