There's currently an amnesty order in place for the four-season firearms. That amnesty order ensures that people who are in possession of these firearms would not be subject to criminal liability. That amnesty order goes until 2021. It lasts for three years. That amnesty order is independent of this bill. If this bill never becomes law, that amnesty order still exists. If this bill doesn't come into force, the government could take steps to address that at that time, but there's currently no criminal liability for individuals who are in possession of these firearms. Regarding the other firearms that are currently not restricted, people aren't at any risk of prosecution for those either.
The June 30 date has nothing to do with the legality or criminal risk of these firearms. People who are in possession of them will not be at criminal risk for prosecution until the expiration of the amnesty order, which is roughly two and a half years from now.
The June 30 date is simply a mechanism to ensure that firearm owners are aware that they need to be in possession of the firearm at that date. That would make them eligible for grandfathering and if you were in possession of the firearm the next day, you would not be eligible for grandfathering. If the bill doesn't come into force, then these individuals would not be grandfathered through this bill and the June 30 date would mean nothing.