The San Bernardino shooters used large magazines that are illegal to sell in California.

It's not hard to buy them in some other state, perhaps Arizona or Nevada, and bring them illegally to California. However, if they were illegal in the whole US, it would really be hard to get them.

The guns were also modified illegally so they could fire faster.

Those intending to commit murder will not be deterred by fear of punishment for doing these modifications, but the law could require designs that make this more difficult to do.