I have never used a red dot on a 39, but I have used them quite a bit and I have 2 rifles with red dots on them now. I like them for early season squirrel hunting when the shots are close. They are very quick on target. I would recommend that you look for one that has a 3 MOA dot. Quite a few of them have a 4 MOA dot which will be ok, but if you can find one with the 3 MOA get it. I think you will like it, but remember they are not for precision shooting.
I have a red dot on my AR15 for coyotes, and it works great! Fast target acqusition, but if you use one for squirrels, you might want to keep your shots within 25 yds. Beyond that a squirrel makes a pretty small target with a red dot.