He didn't have access to a machine gun, grenades or body armor did he? He also tried to buy more fire power at a gun store but was denied, right? There is a real life example for you of how limited access to high power weapons may have saved lives
I'm not sure that part of the fact that millions of millions of these weapons still in circulation you do not understand, but it means that all he needs to LEGALLY get one is money. During the 1994-2004 'assault weapon' ban, any and every gun show and gun dealer had 'assault weapons' to sell. Why? A huge number of existing weapons.
Are there millions upon millions of existing machineguns on the market? Grenades? No? Your point, mooted.
Unless you plan to confiscate existing weapons, access to those existing weapons is unfettered by a ban on manufacture and sale of new weapons.
Also.... body armor is cheap, legal, and easy to buy on the internet.