If you're a devoted Potter fan like me, you will probably be aware of the wand ownership problem. If you're not, here is a quick overview (spoiler alert if you haven't finished the series): throughout most of the 7th book, Voldemort is searching for a wand that will be able to overpower Harry's, because their wands will not fight each other (although this is not, as he suspected, because of the twin cores, but because the part of soul that is in Harry that Voldemort is unaware of refuses to fight with any soul that still remains in Voldemort's body). He eventually takes the elder wand (a wand which is supposedly unbeatable that will win in any duel, though that raises other issues, like the fact that Dumbledore beat Grindelwald
in a duel to get the wand in the first place), but he finds that it doesn't work as well as he had thought that it would; since Snape killed Dumbledore, Voldemort believes that the 'true' ownership of the wand would have passed to him, and so he kills him. If you look carefully at everything that happens on top of the Astronomy Tower however, it is clear that Malfoy disarms Dumbledore using
Expelliarmus before Snape comes up and kills him, meaning that the true ownership of the elder wand passed to Malfoy, and later to Harry when he disarmed Malfoy.
Looking at this sequence of events, we should conclude that 1. The
Expelliarmus spell is enough to transfer ownership of a wand, and 2. It doesn't seem to matter when the actual disarming happens, or if the wand that is taken is the wand in question; the ownership of ALL wands owned by the disarmed person will be transferred. We know this because Malfoy disarms Dumbledore near the end of the 6th book, just before the beginning of the summer, and it is over 6 months later (sometime after Christmas) that Harry disarms Malfoy, transferring the ownership of the elder wand to him (when the elder wand itself is, in fact, hundreds of miles away in Dumbledore's tomb; Harry takes only Malfoy's original wand, but this disarming apparently transfers the ownership of all wands that he himself has disarmed). However, if this is true, this also means that half of the people in Dumbledore's Army do not have true ownership of their wands, since
Expelliarmus is the very first spell that Harry ever teaches them in the 5th book, and they then split into pairs and practice disarming. It has already been shown that just the use of the spell
Expelliarmus is enough to transfer ownership, so it follows logically that every time a member of the DA practiced the spell, they either took ownership of their partner's wand, or they took ownership of their partner's wand and they took back ownership of their own.
JK Rowling has addressed this issue by saying something like 'It was just practice, they didn't really mean it, so their wands didn't actually transfer ownership' which is, in my opinion, a rather weak defence. It is a very long, complicated, inter-related series, so a few mistakes are expected, and JKR has made surprisingly few, but this is a big one. Even if she can explain away the practice sessions as 'not really meaning it', there are numerous other examples of disarmings, the most notable being in the third book when both Harry and Hermione, at the same time, cast
Expelliarmus at Snape while in the Shrieking Shack; there is no way that you can say that they didn't mean it, or that it was only practice, which means that Severus Snape's wand never reached its full potential after that moment, because its true allegiances lay with Harry and Hermione.
