What you and your fellow liberals fail to acknowledgeis that we already have laws pertaining to just these things.
Fact: It is illegal for most law-abiding citizens to own fully-automatic weapons or "destructive devices". You need a special license to own such a weapon. How many licensed owners of fully-automatic weapons have committed murder?
Oh, but what about those awful "assault" weapons? Tell me what the difference is, besides cosmetic features, between an "assault" weapon and my father's Browning semi-automatic 30-06 rifle he uses for deer hunting? I'm sure you know, the Browning semi-automatic is based off the Browning Automatic Rifle (BAR) originally designed for the military?
Fact: It is illegal for the law-abiding citizen to buy "armor piercing" bullets. Armor piercing purchase is restricted to law enforcement and the military.
Cop-killer bullets are a myth. It's a made-up name born from media hype and nurtured by unrealistic Hollywood portrayals and the deliberately misleading claims of the anti-gun lobby. It's the name given to teflon covered bullets that supposedly have better penetration than standard bullets. In reality, the teflon is an outer coating for the brass penetrator and usually peels off during its trajectory through the air.
Also, as already mentioned, almost every common hunting rifle cartridge can pierce body armor. I own several guns of different calibers that could penetrate body armor. I've never felt the need to go and try and test them though.
Fact: Dealers are already required to perform background checks on law-abiding citizens that wish to purchase a gun. A citizen may be denied purchase. A purchase may also be delayed while the ATF performs a more comprehensive background check.
If you didn't notice, there's a key phrase in each of the paragraphs above - "law-abiding". I don't care how many laws you pass. It won't make a bit of difference to the criminals. But that's something else you and your ilk choose to ignore.