You now own something. It's not a bottle of Glenlivet that you will consume while cursing officials for having made it illegal. This is something of enduring physicality that cost hundreds or perhaps thousands of dollars. You bought this item from a dealer licensed by the government. Months or years pass with you owning this item without incident. The government, in its odd wisdom, one day passes a law that says that the type of item you own is now no longer legal to buy, sell or own. Since we have constitutional constraints, the government gives you a particular length of time apply for your exemption as a current lawful owner to continue owning your item without penalty. This exemption is the part of the law that mitigates the ex post facto characterization. However, if your application process doesn't work, the effect is that the law is ex post facto.
All this is over a style of rifle. Rifles are not a prevalent murder weapon. About 1 in 40 murders are committed by persons using a rifle. Not all rifles are "assault weapons". (374 out of 15,070 nationwide in 2016) https://ucr.fbi.gov/crime-in-the-u.s/2016/crime-in-the-u.s.-2016/tables/expanded-homicide-data-table-4.xls
I have yet to find any compelling reason for any particular interest by the government in regulating "assault weapons". Extendable stocks, bayonet compatibility or any of the other fluid attributes do not make a firearm more or less deadly and these laws serve only undermine the credibility of those politicians clinging to them religiously. Even in the list (https://www.democraticunderground.com/1172202237) of most deadly mass shooting, less than half were committed using weapons that would qualify as "assault weapons".
Gun-"control" is a myth. The only real control is self-control. Empower the individual to help the overall situation and he/she will reward you much more than will a ban.
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