Sadly, I doubt our leaving the EU will make much difference to Firearm regulations already subsumed into UK law. There is very little will within the powers to be to relax anything gun related and we simply do not have the clout to make the authorities do anything that does not suit their own agenda. Once we are out of the EU, there will be far more 'important' issues to deal with.
Also, if we want to be in anyway involved with the EU Single Market, much of our import/export law will be obliged to be in sync with EU.
I may not like it but it will be the situation whatever I voted back in June 2016.