Subject about says it all. Can you use it safely or not, specifically CCI Velocitor?
Thanks.

As far as I know, it should be fine. Velocitor uses normal length .22 LR cases and operates at normal .22 LR pressure.

Depends on when your 39 was made. Very early ones do not have a hardened bolt for 'high speed' ammo. You might consult the folks at marlinowners.com for a serial number range. I'm not a 39 expert so, I can't tell you how old it has to be to be ok.

Yes you can..These loads have no more pressure than any other 22 ammo, they get hyper velocity by decreasing the weight of the bullet.