The guns are obviously a huge part of it, but there are enough data points within the US and when compared to other countries with high gun ownership (Switzerland being the favourite example of the gun lobby) that it's clearly not just the guns. Even if the US didn't have a problem with mass shootings it would still appear to the rest of the developed world as a country with major issues that don't typically occur elsewhere (Trump, covid, supreme court rulings etc. etc.). But the guns is probably the easiest problem to solve physically, even if it's politically impossible.