Fear, wealth, opportunity, religion, and choice. Lack of one or all of those leads to drastic actions on his part; in this case the fear of not being able to purchase an assault weapon leads him to purchase them in bulk. While civilian disarmament is the goal; its not one to be accomplished openly. The more talk there is of restricting firearms, the greater the demand for them... Similar to the debacle when Hostess was closing and there were to be no more Twinkies; people swarmed the stores to stock up on Hostess products when they thought they'd never be able to buy them again.
A ban on the sale or ownership of assault weapons is the wrong approach to getting them out of the hands of the public; not til we can repeal the Second Amendment. If you want to remove firearms from the civilian population, then here is how you go about it.
Now in the BATFE, or ATF, there is an office that was created for the express purpose of helping those convicted of Federal felonies regain their right to own firearms. Well Congress passed a law stripping that office of funding; and legally bars it from ever receiving funding; thus ensuring felons cannot legally own firearms, while still allowing the process to still be in place.
To legally manufacture or sell firearms and ammunition you must apply for a certain Federal Firearms License, or FFL. Without an FFL, you cannot sell more than X amount of firearms or ammunition; lest you face Federal prosecution. There is an office within the ATF that handles this particular paperwork... Strip it of funding and bar it from ever receiving funding. Like with the office handling restoration of firearms ownership, the process is technically still there, but it'll never happen for those who apply for an FFL. See where I'm going with this?
Sure, you'll still have those millions of firearms, and billions of rounds of ammunition floating around. But unless those owners put those weapons into long term storage, eventually they will wear out, be used up, or simply rust to the point that they are no longer useable. Guns are mechanical devices, they require a certain amount of upkeep to remain functional; and ammunition does have a shelf life. If spare parts, new weapons, and new ammunition is no longer sold to civilians; then its simply a matter of time before the existing supply is no longer useable; (even taking into consideration it would be a matter of generations, perhaps even centuries). Yet we must all bear in mind that this is a long-term venture; Rome wasn't built in a day.