With all due respect to Sanity Challenged, intent is irrelevant. (With the exception of laws. We do need to know the intent of legislators in interpreting a law.)
My response to the "it was designed to X" argument: so what? Do we really need to delve off into the mind of the initial designer to determine how we'll craft legislation in regards to a specific item? No.
This argument seems to be specific to arms. When discussing automobile legislation, you won't hear anybody saying, "but cars were designed for X, Y and Z." Nobody cares why a thing was designed, or its intended purpose, except as to weapons.
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman