Ok, so I have a clip from a video capture program that's 15.?? frame rate (it's a WMV screen capture clip). I use compressor to convert to ProRes.
At first, I didn't realize my frame rate was that low, and so I didn't adjust the frame rate in compressor. So, when I imported it into FCP and tried to do 29.97, it shortened my clip and the audio didn't match up (obviously).
So then, I went back and reconverted the clip, this time setting the frame rate to 29.97. When I open the newly converted clip in quicktime, it reads as 29.97 in movie inspector. However, when I import into FCP, it's still reading the file as having a 15.?? frame rate and is still shortening the clip. I can import the source WMV, which does apprear to have a good frame rate, but then my OLD, OLD, OLD MacPro starts moving like an old woman.
Take that new 29.97 clip, open it with Cinema Tools, and CONFORM it to 29.97. CT will basically embed the metadata that FCP needs to tell that this is indeed 29.97. I had to do this with my SnapsPro screen captures, even though they were 29.97, FCP saw them as 10fps.