So to beat this concept to death, here is another example. The VZ58 rifles are limited to 5 round magazines by law. Now if someone created a VZ58 pistol and designed a new magazine specifically for the pistol version of this model, then it could legally have a capacity of 10 rounds. So these pistol magazines could be purchased and now used in a VZ58 rifle. With this magazine in a VZ58 rifle, you could legally shoot 10 rounds. I know there was talk of someone trying to do this, but I believe the RCMP was or did put a stop to it. Hope that explanation was better. So that is why you can use 10 round pistol magazines in the Ruger PC9 , and keltec Sub2000 pistol calibre carbines.