The dedicated 9mm magazines have a spacer in the back to set the shorter 9mms forward in the mag. I have Colt and Metalform 9mm, Metalform .38s and one Shooting Star 10-shot .38 magazine (I use it to get to a 9+1 load in IDPA without having to juggle a "Barney Bullet" mag.) All work fine with 9mm ball, cast RN or ogival JHPs like the bulk Remington 115 gr. I don't know about short or truncated cone bullets like Sierras. The only .38 magazine I have that won't handle 9mm is a Colt that came with a second hand gun. It won't feed .38 either, so I think it is just a lemon. I have heard other brands of .38 mags do not do as well with 9mm, but you should sure try the ones you have before you buy a bunch of new 9mm magazines.
Thanks, Jim. I checked out the bbl when I got home and noticed that the feed ramp was slightly concave, I flattened it with a swiss needle file then polished it. I haven't had a chance to test fire it but I fed a magazine through it by hand with no problems. It looked like the nose of the bullet was jamming right in the middle of the feed ramp. Makes sense.