Jim,
I have been shooting my STI framed .9mm open gun for a year now.
Have never had a problem with it. Stay with a one sided style scope
mount, with a cmore. Mine is of the Dawson type, there better for
the raeson's of ejection. "I" have found the style that mounts on both
sides of the frame over the ejection port, to be a little more tricky on
getting that case out. I have seen them work fine, but just a little more
work!? Best of luck on the new open project!