Plus that is likely an underestimate.
Gun owners are known to show up at the polls to vote against candidates who advocate gun bans, restrictions on the amount of rounds a magazine can hold and federal registration of firearms. Many gun owners have a considerable amount of money invested in their collection of firearms and when you have money in the game you don't want to lose it.
Strong control is far more popular in many of the nation's big cities than in the rural areas. Coupled with other factors the gun owner vote may have helped Hillary lose the election in states like Ohio, Pennsylvania and Florida as well as many smaller states. For example Florida has well over 1,000,000 residents with concealed carry permits. Pennsylvania has somewhere over 1,000,000 issued licenses (although some may be owned by people who live out of state) and Ohio at least 500.000. It's hard to estimate just how many people in these states own firearms for sporting and home defense.
In my view it might be wise for the Democratic Party to concentrate on enforcing existing laws and improving them to keep guns out of the hands of the criminal element. Forget things like another assault weapons ban as gun owners fear that would be a step toward banning all semiautomatic weapons and eventually all handguns.
Another factor is that many citizens distrust their government and feel that civilian owned firearms allow "patriots" to be able to overthrow it if it were to turn into a oppressive dictatorship. I doubt if that will ever happen but I've talked to a number of gun owners who firmly believe it could.
Some argue that Al Gore lost to Bush the Younger because of the gun owner vote. I'm sure that made a big difference in the Florida vote totals which ended up as the state that determined the election. Like Hillary Gore was for strong gun control.
The gun control issue may well be a ball and chain on the ankle of the Democratic Party.