I think that is more dependent upon the state in which you live than anything else. They are not illegal under Federal law.
Get on gunbroker and search ss190 $10 bucks a round? WTH?All of that steel core ammo is readily available and legal.
Although the SS109 is only a 62 grain it has the lenght of a 70 grain bullet.Bullet stabilization in regards to twist rates doesn't rely on weight. It relies on length. Length and weight are usually proportional but not always.
Longer length doesn't always mean heavier, especially with plastic tipped ammo, hollow points, etc.