I don't have any experience, but if you will allow for my chime-in...
They are an answer to to an un-asked question. Legal? Anything is legal until you are caught. That legality question should be asked of the BATF and not on some forum.
Safe? Until something breaks. Different ammo produces different pressures. How far do you want to push it?
If I desire a weapon in a specific caliber, I prefer to purchase one made specifically for that caliber.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)