I just went over the magazine engagment and cycled a bunch of dummy rounds through the midway Underwood.
FWIW the magazines seem to have only a little play when the bolt is closed.
That's becausae the follower is pressing on the bottom of the bolt. If you are checking them with the bolt open, yes there is a lot of play. Examine the fit with the stock removed and its easy to see.

On this carbine I also observed about 5 thousanths verticle play on the right side of the magazine catch. Just eyeball guess as I don't think I have any feeler gages at the house right now. I'll look in the AM. When this carbine has FTFs, its on the right side, 2nd to last round, in one of the new Korean mags. I can solve it by pushing up on the magazine while cycling. Later this wekend I'll compare with my SG to see if its the mag or the mag catch or just that combo.