It's not the magazine. It's more than likely a worn out Bolt Stop Assembly (part no. A037-02).
Call Ruger and tell them what it's doing, they'll send you the part free. It' not hard to put in, or you could send it to them to put in.
My 678GC was doing the same thing. Called them, they sent the part. I took the old one out and compared them. I couldn't see a difference with the naked eye, but once the new one was installed I could that it made better contact with the bolt face and the gun fires flawlessly now.
Takes 10 minutes to put it in. You have to push the front pin about half way out so that you can release the bolt stop assembly from being lodged behind/under the plunger (A043). With a little wiggling the bolt stop assembly will come out. It's kinda locked in by the bolt stop thumb piece (A047).
The bold stop assembly is worn by not pulling back on the bolt to lower the bolt when it is held back by the bolt stop assembly. May not appear worn but if you lock the bolt back without a magazine inserted and look at the connection made on the bolt face by the bolt stop assembly, the stop assembly should be catching the center portion of the Bolt catch surface.
If it is catching on the inner edge of the catch surface, what will happen is, after the concusion of the last round being fired, the assembly won't catch the bolt and it will either slam on forward, or get jammed on top of the magazine.