IF you had numbers that subtracted out the minority/immigrant populations on both sides, then you would have a clearer picture of if GUNS are the problem.
I have never seen such numbers. I've seen American numbers with minorities subtracted out, and that puts US very close to EU averages.
But I have not seen it with immigrant populations subtracted out of European numbers.
Somewhere I had read that the murder rate by whites in the US is 2.2, but I couldn't find it. So I tried to reconstruct it.
The FBI collects homicide date. The last data was from 2015.
Expanded Homicide Data Table 6
If you add up that first column on the left in the first table, the total number of murders committed by whites was 2,897. The third column from the right are the murders committed by Hispanics. Let's assume that every single Hispanic is white. (They aren't, but we'll assume it.) There were 751 murders by Hispanics. Subtract that from the total white number and you get 2,146 murders by non-Hispanic whites in 2015.
However, the data collected by the FBI is incomplete. In fact, there were 15,696 murders in the US whereas the FBI data accounts for only 6,025 murders, or 38.4% of the total. That means 9,671 murders were unaccounted for.
https://www.usnews.com/news/articles/2016-09-29/race-and-homicide-in-america-by-the-numbers
Since 61.6% of the data is missing, or 9,671 murders, we have to make an estimate of the rate of white homicide for the rest of the population. Therefore, of the 9,671 unaccounted murders, whites committed somewhere between 0 and 9,671 of those murders.
The odds that whites committed either all or none of those unaccounted murders are statistically zero. But let's pretend that it is zero.
The population of non-Hispanic whites on July 1, 2016 was 197,969,608.
American FactFinder - Results
Assuming that the number of murders committed by whites was 2,146, that means the murder rate by whites would be 1.09.
2,146 / 197,969,608 X 100,000 = 1.09
That is still higher than the 0.99 homicide rate in Europe I posted earlier.
But since the probability of whites committing 0 of the 9,671 unaccounted murders is statistically 0%, we know that the rate is higher. But how much higher?
A reasonable starting point is "normalizing" the rate of murders by whites for the unaccounted population. IOW, if non-Hispanic whites accounted for 35.6% of the accounted murders
2,146 / 6,025 = 35.6%
It is a reasonable starting point to assume that non-Hispanic whites accounted for 35.6% of the 9,671 unaccounted murders, or 3,445.
9,671 X 35.6% = 3,445
So the total number of murders committed by non-Hispanic whites is estimated to be 5,991.
2,146 + 3,445 = 5,591
Using this estimate, then the rate of murders by non-Hispanic whites would be
5,591 / 197,969,608 X 100,000 = 2.83.
It is reasonable to assume that the rate of murder committed by non-white Hispanics is somewhere between 1.09 and 2.83, which is consistent with the 2.2 I recall seeing some time ago. It is also well above the 0.99 rate in Europe.
So it is safe to assume that the rate of murder by the non-Hispanic white population in the US is 2 to 3 times higher than in Europe.