Just got back from the range and cycled about 40 rounds through this modified mag w/ extension and it held up beautifully. The mag seated fine, didn't work itself loose or show any signs of coming apart. And it did help me control this little 'firebreather' better than naked mags. My hands are large and I can't even get my ring finger to fully wrap around the grip, much less my small finger so it was a big help - it kept me on target out to 25 yards so I was pleased.
One more thing - damn! - this is a great little gun! This was my first extended range experience, apart from my first get-acquainted session. My shots at 40' were all well within 8", and at 25 yards I couldn't believe I was within 12" on all of my shots. Next time, I'll slow down, take my time and see what she's really capable of. This R9S is just an amazing pocket-full!!!