ROCK ISLAND BASEPADS DON’T FIT THE FRAME
Your non-RIA mags will lock into place, but only because they're actually too long. (Sounds weird, but true!)
When you lock a stock mag with a stock basepad into your gun, the top edge of the folded-steel basepad almost hits the frame. It's very tightly fitted in order to prevent the mag from over-inserting into an empty, slide-locked pistol and potentially bending the ejector.
The RIA guns that ship with magwells ship with shortened frames -- shortened enough to allow room for the magwell and an extra-capacity basepad (which is thicker than the folded steel version).
Our RIA basepads are meant for the "magwell-length" short frames. It's actually pretty common for RIA owners to shorten their long frames to allow for our magwell and/or the RIA magwell to be used.
If you don't want to do that, we just recently released a magwell for the "tall" guns, but it only works with MBX, STI, or (interestingly) Remington R1/RP9 magazines. My personal RIA is an unmodified "tall" frame with replacement (short) grips, R1/RP9 mags, and TF R1/RP9 basepads.
American Tactical should have both the magwell and the shorter grips on their site.
Rock Island Competition +1 not fitting magazines
NOTE: Some Mec-Gar-marked magazines have a large, deep notch at the bottom-front of the mag tube (take the basepad off to check). If you have a Mec-Gar marked mag with a deep notch, this basepad won't work for you. If the tube is NOT marked Mec-Gar and has a shallow notch (like RIA factory mags with the one-piece hollow basepad), it will work great. (Thanks to customer Sebastien Richard for helping figure this out!)
NOTE 2: Rock Island recently changed the profile of the magwell. If you find your +1s won't fit, call us back and we'll exchange your +1s against a set of +4s!