Fact: between roughly 2007 and 2017, about 17 people per year were shot and killed with AR-15 type rifles in the USA.
Source: https://fee.org/articles/are-ar-15-rifles-a-public-safety-threat-heres-what-the-data-say/ which quotes a New York Times article for that piece of data.
Fact: in roughly the same time period, lightning strikes killed, on average, about 27 people per year in the USA.
So, on the surface, it looks like you're about twice as likely to be struck by lightning and killed than you are to be shot and killed by an AR-15. But, as someone pointed out to me, don't you also have to take into account the total number of lightning strikes and the total number of AR-15s in the USA? Well, yes, I suppose you do. So back to Google, I went.
In the interest of full disclosure, I'm doing some rounding with these numbers to make them nice to work with. You can check the source articles if you want to be more precise.
Fact: the annual average number of lightning strikes that reach American soil is around 19 million.
Fact: the NSSF estimates that there are between 5 and 10 million AR-15s (not counting other semi-automatic rifles) owned by American civilians.
So, 27 deaths per year from lightning divided by 19 million strikes per year gives a death rate per strike of 0.00000142. That's about 1.42 deaths per million strikes.
17 deaths per year from being shot with an AR-15 divided by 10 million AR-15s in the USA gives 0.00000170, or about 1.70 deaths per million AR-15s. If you go with the lower estimate of 5 million, you get about twice that or 3.4 deaths per million AR-15s. Still not a lot, but perhaps more deadly than lightning? No, not really.
If you're going to compare how likely you are to get shot with an AR-15, you need to know how often those 5-10 million AR-15s are actually fired. After all, it's the firing of them that kills, not their mere existence, right? Come on, you know that's right.
Unfortunately for our little exercise here, that number is nigh unto impossible to know. On one hand, you have guys like me who are raising 4 kids and holding down a full-time job and into other hobbies, and .. well, you get the idea. I maybe make it to the range every couple of years with my AR-15. However, on those trips to the range, I easily expend a few hundred rounds of ammunition. There are guys who burn a few thousand rounds every weekend on the range like it's no big deal. So can we guestimate a good average? Maybe. If we assume that most guys make it to the range once a year and fire 100 rounds out of each of their AR-15s, that seems reasonable. Maybe on the low end. Now, if you multiply that by the number of AR-15s, you get a low estimate of 500 million rounds fired per year, up to 1 billion (with a B) if there are 10 million AR-15s in America. Now, I understand that this is only guesswork at this point, but I hope you're still with me in accepting that it's a fairly reasonable guess.
So, let's look again at how deadly gunfire from AR-15s is versus lightning:
17 deaths per year divided by 500 million rounds fired gives a shockingly low number of 0.000000034 deaths per round fired. That's orders of magnitude lower than the number of deaths per lightning strike. Look at the numbers on top of each other to see what I mean:
0.000001420 lightning deaths per strike
0.000000034 AR-15 deaths per round fired
So let's figure this out in percentages, shall we? We can forget about the decimal points for this and just do 100 - (34 / (1420+34)) to get a rough percentage of how much more likely you are to be killed by lightning than by someone shooting you with an AR-15. That number comes out to 99.97 percent, meaning you are 99.97% more likely to be killed by lightning than by an AR-15.
So maybe the time you spend worrying about people having AR-15s would be better spent worrying about the weather. Just sayin' ...