President can't create law. That power lies in Congress. But fear mongers will keep spreading fear.
If the law is sufficiently broad-reaching and vague, a President/executive branch can creatively reinterpret existing law to criminalize things that were intended to remain legal. There are multiple avenues, under the existing National Firearms Act as amended by the GCA, by which semiautos, pump/semiauto shotguns, etc. could be shifted to Title 2 instead of Title 1 by executive order. Clinton the First did exactly that with revolving-cylinder shotguns. Obama floated a trial balloon of a similar approach with ammunition under the 1986/1994 AP bullet ban, by creatively redefining ".22 caliber" to mean "exactly 0.2200000 inch" instead of the common .22 caliber bore size (0.224) it was intended to exempt, and redefining the word "core" to refer to tiny pieces of the core instead of the actual core. There are also any number of approaches that could be used to harass/shut down shooting ranges or restrict lead ammunition under existing environmental law, restrict concealed carry in workplaces under existing workplace-safety law, restrict .50 BMG's, etc. etc. etc. Obama has also started restricting home gunsmithing via existing ITAR regulations, and threatened some gun-related online speech with ITAR, and could conceivably use some facets of ITAR creatively to restrict high-end optics, magazines, or other items. The political backlash would be staggering (1994 times ten), but that didn't stop Hickenlooper in CO and it may not stop an authoritarian zealot like Clinton seems to be.
The recent sweeping ban on the majority of detachable-magazine semiautos in Massachusetts, not by a new law being passed but rather by a middle-of-the-night edict by the Attorney General, is a cautionary example, and the same billionaire control freak that caused the Colorado debacle and helped bring AG Healey to power in MA is heavily backing Hillary.
She is evil and corrupt, but I think a panic is unneeded.
I don't think "panic" is justified, but I do think reasonable hedging is wise. The *only* people who can now legally own even ban-compliant AR-15's, etc. in Massachusetts are those who went out and bought one prior to AG Healey's dictatorial edict.
I *lived* through the first Clinton administration, and had come of age as a gun owner fairly recently when the Clintons were elected. I went ten years without the rifle stock I wanted, and paid way more than I should have for magazines, because I foolishly chose not to hedge against the 1994 Feinstein law (my dad said "that won't pass; this is America"; he was wrong). I certainly hope that wiser heads prevail this time and that Clinton drops the prohibitionist shtick after the election if she wins, but reasonable hedging against the possibility that she won't is not "panic", it is prudence.