I think you concede the point that an inexpert gunman could get off a lot more shots with the magazine than with a clip. The only disadvantage to him would be the possibility of jamming the gun.
It certainly is a matter of practice but haven't we seen the last few of these shooters that the shooters did go out and practice? How many less deaths do you think there would be if the shooter only had 10 round magazines?
I got an idea why don't we limit the size of vehicles aloud in United States, a small vehicle like a KIA could never pull off a bombing as disastrous as the Oklahoma bombing because it cannot hold him on explosives needed.