Guns are certainly an issue but not the issue. If you cut American gun ownership by 75% you'd still have more per capita gun crime than Switzerland, why?
[0] Homicides, not all gun deaths. It's important to compare apples to apples.
Looking at the numbers alone is not enough. Just like looking at the US numbers alone doesn't really tell the real story. Guns are definitely part of the problem, but it's the rest of the culture and economic factors that tells the story of why the US numbers are so high.
Government-issued ammunition is difficult to take home, but there's plenty of commercial ammunition that will work in the government-issued rifles.
Not every relationship has to be linear. Also there might be other factors. They might be easier to spot if you take away 3/4 of all guns in US.
From wiki about gun laws there:
In 2016, the defence ministry estimated that 2 million privately owned guns are in circulation, which given a population of 8.3 million corresponds to a gun ownership rate of around 24 guns per 100 residents. This is roughly a quarter of the rate in the United States, and lower than that in the neighbouring countries of Germany, and Austria, but about the double of Italy and France.
Quarter of guns per person US has. If US taxed gun manufacturers an used that money to buy back 3/4 of guns off the market it would surely have bit less gun related problems.