There have already been some very good responses posted. I especially like the advice to soak the front end of the BT with a few drops of CA. I do this in general, but if you need to tighten the NC fit (a little) you can also add more CA to the inside of the BT (swab/smear with a disposable Qtip). Make sure to sand lightly after the CA sets up, so the inside surface will be smooth. Fin sandpaper is OK, but I like fine sandpaper better (sorry Lee, I couldn't resist).
My rule of thumb is that if I hang the rocket by the NC (holding only the NC) and give it two or three sharp upward jerks, if the NC separates then the fit is probably loose enough. And if I grab the other end of the rocket and hang it nose down and give it two or three sharp upward jerks, if the NC stays put then the fit is probably tight enough. Then again, I try to design and build with minimal ballast weight in the NC, and if you have very much ballast there then the NC might fall off more easily.
This approach leaves a pretty wide middle ground where the NC is fitted more snugly than I like, but I have rarely seen this result in a stuck NC (and subsequent lawn dart). If the kit comes with a plastic NC, they almost always release cleanly at ejection. If the NC was balsa, and made by one of our outstanding NC vendors (Sandman, BMS, others), they are checked for fit pretty well when they are made, and they also almost always release cleanly.
When it comes down to it, it's pretty hard to make a NC stick too hard in a BT unless you have some BT crimps or damage, or a loop of shock cord that has pushed out and gotten stuck between the NC and BT, or if the NC itself has some damage on the shoulder. A little common sense and a minute of time for inspection and field testing will work for you 99 percent of the time---just about anyone will get that nervous feeling that tells them something might be wrong when the NC sticks too hard.
