I use a R.I.A TAC ULTRA FS HC 9mm that uses Para P18 mags. I bought Dawson Precision mac extension (that they don't seem to sell anymore...why not?) Which are great quality BUT allowed the follower to roll to one side and get stuck rendering the mag useless. These inserts solved the issue and my once 17 round mags are now 20 rounders. Success!
I should have known from the get that these were needed... Live and learn.