Run them goalposts down the field!
The reason for the drop in murder rates is demographics. Less young men means less murder.
That simple.
The point is the rise in the number of firearms has NOT raised murder rates. There is no evidence it has had any effect at all.
First there are more young men in America now than there were in 1978.
Second you would have to prove that murder rates now are the same as they would be if the number of firearms had remained the same.
Get it?
Never mind. I was just being funny.
You obviously do not get it and have no intention of ever getting it.
I might as well be arguing abut climate change - this is just that pointless.