Just wait a few more years if nothing changes. In a couple years, once mass production of the military's XM5 is well on its way it will drive the semi-auto civilian version, the Sig MCX Spear and its ammo prices way down to something more affordable and ammo widely available than the $8,000 the rifle currently is and when that happens it'll be only a matter of time until some shit stain kills a whole bunch of innocent people with it and people are going to see the level of destruction a real high power semi-auto rifle/ammo is capable of. It's going to be too gruesome for even the GOP and NRA to sweep under the rug.
We need universal background checks (though ideally without a registry because that presents other potential problems), private seller/gunshow loopholes closed with requiring checks there too and red flag laws for people who have already been found guilty of violent acts, assault, domestic violence (and protection orders) or cruelty to animals, ect. We're not going to be stop the sale of firearms and what we can do needs to be done in a way that it can't be abused to stop someone, most likely minorities from buying them either because that's traditionally how they got passed.