"If you want to play with guns, join the military or police."
First, this statement is ableist in the extreme.  Not everyone can meet the standards the police or military require. So if you're saying it, you are not the slightest bit "liberal." You are a bigot.
Second, you're also ignorant.  Police typically qualify with a sidearm once or twice a year.  The goal is for them NOT to be shooting people. Police are supposed to be filing incident reports, investigating crime, resolving conflicts.  The goal isn't to have them show up and shoot people. Since that's obviously where your brain went, once again, you are not "liberal." You are a fascist who wants a police state.
Third, most of the military similarly qualifies with weapons once a year, and sometimes less in the Navy and Air Force.  There just isn't that much close combat aboard ship or aircraft. Security and military police personnel, engineers and some support elements train more often, but certainly not with any even monthly schedule. Nor, for that matter, do Combat Arms branches fire more than a few times a year. With the exception of a handful of very elite units that you cannot possibly qualify for, "playing with guns" is not a thing in the military. Also, the purpose of the armed elements is to kill people in combat. If you are endorsing this you're not "liberal." You're imperialistic.
So that's three reasons we should maintain private weapons.  Because if you bigoted fascist imperialists get your way, we're going to need to stop you.