The same way that 50% of the voters in a state can end up with 23% of the seats from their state in the HoR represented by their party.
Sure ... but the majority of voters accross all of the the USA? Well, OK, maybe still a small majority might not be enough to win the most seats (that's how the electoral system works).
But I expect you understand the point - if enough voters in the US really insist that their politicians pass truly effective changes to the gun laws, then the politicians will have to do that ... otherwise they will be voted out of office and out of a job. And new politicians who will follow the demands of the voters will be in power.
But up until now, it seems like not enough voters in the USA really want strict enough restrictions on their desire to own guns.