I see no reason for civilians to own large capacity magazine. In fact, if I saw a person buying a large capacity magazine I would probably wonder about his intentions. His intentions might not be terrorism or other evil intent, but the fact that he feels so fearful or insecure that he feels a need to possess a rifle capable of sustained, high volume fire would probably raise a red flag for me.
I also believe that the insertion of a high capacity magazine into a semi-auto rifle is in fact what transforms a legitimate hunting/target/casual pinking rifle into an "assault rifle".
Now, if you say he needs such a weapon to defend himself against a tyrannical government I say OK, but I think that is where the "a well regulated militia" comes in. Under this setting, individuals that are members of a "well regulated militia" can justify the ownership of this type of weapon.