There's one tiny loophole in all this left. You buy a percussion revolver...
There's also one other little-bitty loophole. Firearms manufactured before January 1, 1899 are considered "antiques" under the 1968 Gun Control Act and aren't regulated.
Most of the guns covered under this loophole are black-powder arms such as flintlock muskets, early breechloading single-shot rifles, and percussion revolvers, but savvy gun historians will recognize that it encompasses a few high-powered metallic cartridge revolvers, such the Colt Single Action Army and S&W Model Number Three, and it also includes the first generation of easily-reloadable military bolt-action rifles, such the Mauser Model 96 and Mosin-Nagant Model 1891.
However, most of these guns are just what their title implies- antiques. The January 1st, 1899 cutoff date is static, it's never updated, so these guns get more rare, collectible, and expensive every day. Most crooks want to do things the cheap and easy way, which is why they became crooks in the first place. The cost and difficulty involved in buying a functional pre-1899 weapon- not to mention the problem of finding suitable ammo- would deter almost all of them. It's far, far easier to steal a modern gun or buy it on the black market.
While I'm confident that a few crimes are committed each year using "antique" firearms, I seriously doubt that the number even represents one tenth of one percent of gun crime. I'm also confident that it's far smaller than the number of violent crimes committed using readily available improvised weapons such as hammers, axe handles, or chains.
{EDIT} I'd also wager that, in the vast majority of gun crimes committed using "antique" firearms, the antique was used out of pure convenience- it was used simply because it was readily available, not because the crook sought it out to avoid the background check.