The strongest pro-gun argument I’ve heard (besides the fact that it’s literally the second amendment of the US Constitution) is that sure the military or police could easily kill a bunch of people trying to defend themselves with their basement arsenals, but those military and police have families and friends, and so do the gun owners.
In other words, once you declare war on your citizens, and 100M of them are armed, you are in big trouble. So an armed citizenry is an effective deterrent. If the government murders a bunch of your family and friends, protesting won’t work. They discovered this in the 20th century communist regimes where they just killed whoever they wanted, and no one could do anything about it.